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Lecture 29: Meiosis - Biology

Lecture 29:  Meiosis - Biology


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Lecture 29: Meiosis

Chapter 13 – Meiosis and Sexual Life Cycles – Lecture Outline

· Living organisms are distinguished by their ability to reproduce their own kind.

· Offspring resemble their parents more than they do less closely related individuals of the same species.

· The transmission of traits from one generation to the next is called heredity or inheritance.

· However, offspring differ somewhat from parents and siblings, demonstrating variation.

· Farmers have bred plants and animals for desired traits for thousands of years, but the mechanisms of heredity and variation eluded biologists until the development of genetics in the 20th century.

· Genetics is the scientific study of heredity and variation.

1. Offspring acquire genes from parents by inheriting chromosomes.

· Parents endow their offspring with coded information in the form of genes.

° Your genome is comprosed of the tens of thousands of genes that you inherited from your mother and your father.

· Genes program specific traits that emerge as we develop from fertilized eggs into adults.

· Genes are segments of DNA. Genetic information is transmitted as specific sequences of the four deoxyribonucleotides in DNA.

° This is analogous to the symbolic information of language in which words and sentences are translated into mental images.

° Cells translate genetic “sentences” into freckles and other features with no resemblance to genes.

· Most genes program cells to synthesize specific enzymes and other proteins whose cumulative action produces an organism’s inherited traits.

· The transmission of hereditary traits has its molecular basis in the precise replication of DNA.

° This produces copies of genes that can be passed from parents to offspring.

· In plants and animals, sperm and ova (unfertilized eggs) transmit genes from one generation to the next.

· After fertilization (fusion of a sperm cell and an ovum), genes from both parents are present in the nucleus of the fertilized egg, or zygote.

· Almost all the DNA in a eukaryotic cell is subdivided into chromosomes in the nucleus.

° Tiny amounts of DNA are also found in mitochondria and chloroplasts.

· Every living species has a characteristic number of chromosomes.

° Humans have 46 chromosomes in almost all of their cells.

· Each chromosome consists of a single DNA molecule associated with various proteins.

· Each chromosome has hundreds or thousands of genes, each at a specific location, its locus.

2. Like begets like, more or less: a comparison of asexual and sexual reproduction.

· Only organisms that reproduce asexually can produce offspring that are exact copies of themselves.

· In asexual reproduction, a single individual is the sole parent to donate genes to its offspring.

° Single-celled eukaryotes can reproduce asexually by mitotic cell division to produce two genetically identical daughter cells.

° Some multicellular eukaryotes, like Hydra, can reproduce by budding, producing a mass of cells by mitosis.

· An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals.

° Members of a clone may be genetically different as a result of mutation.

· In sexual reproduction, two parents produce offspring that have unique combinations of genes inherited from the two parents.

· Unlike a clone, offspring produced by sexual reproduction vary genetically from their siblings and their parents.

B. The Role of Meiosis in Sexual Life Cycles

· A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism.

° It starts at the conception of an organism and continues until the organism produces its own offspring.

1. Human cells contain sets of chromosomes.

· In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.

° Each chromosome can be distinguished by size, position of the centromere, and pattern of staining with certain dyes.

· Images of the 46 human chromosomes can be arranged in pairs in order of size to produce a karyotype display.

° The two chromosomes comprising a pair have the same length, centromere position, and staining pattern.

° These homologous chromosome pairs carry genes that control the same inherited characters.

· Two distinct sex chromosomes, the X and the Y, are an exception to the general pattern of homologous chromosomes in human somatic cells.

· The other 22 pairs are called autosomes.

· The pattern of inheritance of the sex chromosomes determines an individual’s sex.

° Human females have a homologous pair of X chromosomes (XX).

° Human males have an X and a Y chromosome (XY).

· Only small parts of the X and Y are homologous.

° Most of the genes carried on the X chromosome do not have counterparts on the tiny Y.

° The Y chromosome also has genes not present on the X.

· The occurrence of homologous pairs of chromosomes is a consequence of sexual reproduction.

· We inherit one chromosome of each homologous pair from each parent.

° The 46 chromosomes in each somatic cell are two sets of 23, a maternal set (from your mother) and a paternal set (from your father).

· The number of chromosomes in a single set is represented by n.

· Any cell with two sets of chromosomes is called a diploid cell and has a diploid number of chromosomes, abbreviated as 2n.

· Sperm cells or ova (gametes) have only one set of chromosomes—22 autosomes and an X (in an ovum) and 22 autosomes and an X or a Y (in a sperm cell).

· A gamete with a single chromosome set is haploid, abbreviated as n.

· Any sexually reproducing species has a characteristic haploid and diploid number of chromosomes.

° For humans, the haploid number of chromosomes is 23 (n = 23), and the diploid number is 46 (2n = 46).

2. Let’s discuss the role of meiosis in the human life cycle.

· The human life cycle begins when a haploid sperm cell fuses with a haploid ovum.

· These cells fuse (syngamy), resulting in fertilization.

· The fertilized egg (zygote) is diploid because it contains two haploid sets of chromosomes bearing genes from the maternal and paternal family lines.

· As an organism develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of the body.

° Each somatic cell contains a full diploid set of chromosomes.

· Gametes, which develop in the gonads (testes or ovaries), are not produced by mitosis.

° If gametes were produced by mitosis, the fusion of gametes would produce offspring with four sets of chromosomes after one generation, eight after a second, and so on.

· Instead, gametes undergo the process of meiosis in which the chromosome number is halved.

° Human sperm or ova have a haploid set of 23 different chromosomes, one from each homologous pair.

· Fertilization restores the diploid condition by combining two haploid sets of chromosomes.

3. Organisms display a variety of sexual life cycles.

· Fertilization and meiosis alternate in all sexual life cycles.

· However, the timing of meiosis and fertilization does vary among species.

· These variations can be grouped into three main types of life cycles.

· In most animals, including humans, gametes are the only haploid cells.

° Gametes do not divide but fuse to form a diploid zygote that divides by mitosis to produce a multicellular organism.

· Plants and some algae have a second type of life cycle called alternation of generations.

° This life cycle includes two multicellular stages, one haploid and one diploid.

° The multicellular diploid stage is called the sporophyte.

° Meiosis in the sporophyte produces haploid spores that develop by mitosis into the haploid gametophyte stage.

° Gametes produced via mitosis by the gametophyte fuse to form the zygote, which grows into the sporophyte by mitosis.

· Most fungi and some protists have a third type of life cycle.

° Gametes fuse to form a zygote, which is the only diploid phase.

° The zygote undergoes meiosis to produce haploid cells.

° These haploid cells grow by mitosis to form the haploid multicellular adult organism.

° The haploid adult produces gametes by mitosis.

· Note that either haploid or diploid cells can divide by mitosis, depending on the type of life cycle. However, only diploid cells can undergo meiosis.

· Although the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental feature: each cycle of chromosome halving and doubling contributes to genetic variation among offspring.

4. Meiosis reduces the chromosome number from diploid to haploid.

· Many steps of meiosis resemble steps in mitosis.

° Both are preceded by the replication of chromosomes.

· However, in meiosis, there are two consecutive cell divisions, meiosis I and meiosis II, resulting in four daughter cells.

° The first division, meiosis I, separates homologous chromosomes.

° The second, meiosis II, separates sister chromatids.

· The four daughter cells have only half as many chromosomes as the parent cell.

· Meiosis I is preceded by interphase, in which the chromosomes are replicated to form sister chromatids.

° These are genetically identical and joined at the centromere.

° The single centrosome is replicated, forming two centrosomes.

· Division in meiosis I occurs in four phases: prophase I, metaphase I, anaphase I, and telophase I.

· Prophase I typically occupies more than 90% of the time required for meiosis.

· During prophase I, the chromosomes begin to condense.

· Homologous chromosomes loosely pair up along their length, precisely aligned gene for gene.

° In crossing over, DNA molecules in nonsister chromatids break at corresponding places and then rejoin the other chromatid.

° In synapsis, a protein structure called the synaptonemal complex forms between homologues, holding them tightly together along their length.

° As the synaptonemal complex disassembles in late prophase, each chromosome pair becomes visible as a tetrad, or group of four chromatids.

° Each tetrad has one or more chiasmata, sites where the chromatids of homologous chromosomes have crossed and segments of the chromatids have been traded.

° Spindle microtubules form from the centrosomes, which have moved to the poles.

° The breakdown of the nuclear envelope and nucleoli take place.

° Kinetochores of each homologue attach to microtubules from one of the poles.

· At metaphase I, the tetrads are all arranged at the metaphase plate, with one chromosome facing each pole.

° Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad, while those from the other pole are attached to the other.

· In anaphase I, the homologous chromosomes separate. One chromosome moves toward each pole, guided by the spindle apparatus.

· Sister chromatids remain attached at the centromere and move as a single unit toward the pole.

Telophase I and cytokinesis

· In telophase I, movement of homologous chromosomes continues until there is a haploid set at each pole.

° Each chromosome consists of two sister chromatids.

· Cytokinesis usually occurs simultaneously, by the same mechanisms as mitosis.

° In animal cells, a cleavage furrow forms. In plant cells, a cell plate forms.

· No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II, as the chromosomes are already replicated.

· Meiosis II is very similar to mitosis.

° During prophase II, a spindle apparatus forms and attaches to kinetochores of each sister chromatid.

§ Spindle fibers from one pole attach to the kinetochore of one sister chromatid, and those of the other pole attach to kinetochore of the other sister chromatid.

· At metaphase II, the sister chromatids are arranged at the metaphase plate.

° Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical.

° The kinetochores of sister chromatids attach to microtubules extending from opposite poles.

· At anaphase II, the centomeres of sister chromatids separate and two newly individual chromosomes travel toward opposite poles.

· In telophase II, the chromosomes arrive at opposite poles.

° Nuclei form around the chromosomes, which begin expanding, and cytokinesis separates the cytoplasm.

· At the end of meiosis, there are four haploid daughter cells.

5. There are key differences between mitosis and meiosis.

· Mitosis and meiosis have several key differences.

° The chromosome number is reduced from diploid to haploid in meiosis but is conserved in mitosis.

° Mitosis produces daughter cells that are genetically identical to the parent and to each other.

° Meiosis produces cells that are genetically distinct from the parent cell and from each other.

· Three events, unique to meiosis, occur during the first division cycle.

1. During prophase I of meiosis, replicated homologous chromosomes line up and become physically connected along their lengths by a zipperlike protein complex, the synaptonemal complex, in a process called synapsis. Genetic rearrangement between nonsister chromatids called crossing over also occurs. Once the synaptonemal complex is disassembled, the joined homologous chromosomes are visible as a tetrad. X-shaped regions called chiasmata are visible as the physical manifestation of crossing over. Synapsis and crossing over do not occur in mitosis.

2. At metaphase I of meiosis, homologous pairs of chromosomes align along the metaphase plate. In mitosis, individual replicated chromosomes line up along the metaphase plate.

3. At anaphase I of meiosis, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell. Sister chromatids of each replicated chromosome remain attached. In mitosis, sister chromatids separate to become individual chromosomes.

· Meiosis I is called the reductional division because it halves the number of chromosome sets per cell—a reduction from the diploid to the haploid state.

· The sister chromatids separate during the second meiosis division, meiosis II.

C. Origins of Genetic Variation

· What is the origin of genetic variation?

· Mutations are the original source of genetic diversity.

· Once different versions of genes arise through mutation, reshuffling during meiosis and fertilization produce offspring with their own unique set of traits.

1. Sexual life cycles produce genetic variation among offspring.

· The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation.

· Three mechanisms contribute to genetic variation:

1. Independent assortment of chromosomes.

· Independent assortment of chromosomes contributes to genetic variability due to the random orientation of homologous pairs of chromosomes at the metaphase plate during meiosis I.

° There is a fifty-fifty chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and a fifty-fifty chance that it will receive the paternal chromosome.

· Each homologous pair of chromosomes segregates independently of the other homologous pairs during metaphase I.

· Therefore, the first meiotic division results in independent assortment of maternal and paternal chromosomes into daughter cells.

· The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number of the organism.

° If n = 3, there are 23 = 8 possible combinations.

° For humans with n = 23, there are 223, or more than 8 million possible combinations of chromosomes.

· Crossing over produces recombinant chromosomes, which combine genes inherited from each parent.

· Crossing over begins very early in prophase I as homologous chromosomes pair up gene by gene.

· In crossing over, homologous portions of two nonsister chromatids trade places.

° For humans, this occurs an average of one to three times per chromosome pair.

· Recent research suggests that, in some organisms, crossing over may be essential for synapsis and the proper assortment of chromosomes in meiosis I.

· Crossing over, by combining DNA inherited from two parents into a single chromosome, is an important source of genetic variation.

· At metaphase II, nonidentical sister chromatids sort independently from one another, increasing by even more the number of genetic types of daughter cells that are formed by meiosis.

· The random nature of fertilization adds to the genetic variation arising from meiosis.

· Any sperm can fuse with any egg.

° The ovum is one of more than 8 million possible chromosome combinations.

° The successful sperm is one of more than 8 million possibilities.

° The resulting zygote could contain any one of more than 70 trillion possible combinations of chromosomes.

° Crossing over adds even more variation to this.

· Each zygote has a unique genetic identity.

· The three sources of genetic variability in a sexually reproducing organism are:

1. Independent assortment of homologous chromosomes during meiosis I and of nonidentical sister chromatids during meiosis II.

2. Crossing over between homologous chromosomes during prophase I.

3. Random fertilization of an ovum by a sperm.

· All three mechanisms reshuffle the various genes carried by individual members of a population.

2. Evolutionary adaptation depends on a population’s genetic variation.

· Darwin recognized the importance of genetic variation in evolution.

° A population evolves through the differential reproductive success of its variant members.

° Those individuals best suited to the local environment leave the most offspring, transmitting their genes in the process.

· This natural selection results in adaptation, the accumulation of favorable genetic variations.

· If the environment changes or a population moves to a new environment, new genetic combinations that work best in the new conditions will produce more offspring, and these genes will increase.

° The formerly favored genes will decrease.

· Sex and mutation continually generate new genetic variability.

· Although Darwin realized that heritable variation makes evolution possible, he did not have a theory of inheritance.

· Gregor Mendel, a contemporary of Darwin ’s, published a theory of inheritance that supported Darwin ’s theory.

° However, this work was largely unknown until 1900, after Darwin and Mendel had both been dead for more than 15 years.


Phases of Meiosis

Meiosis involves two rounds of cell division giving rise to four daughter cells. these two rounds of cell division are called

Meiosis I

It is the first round of division. The process of meiosis begins with the diploid cells having double the number of chromosomes. This is because the cells have undergone DNA replication before entering the Meiosis I. Thus, chromosomes are present in the form of homologous pairs, each cell having 46 pairs of chromosomes at the beginning of Meiosis I. It should be kept in mind that normal cells have only 23 pairs of chromosomes as they have not undergone DNA replication.

Meiosis I is divided into the following four phases

Meiosis I is preceded by an interphase during which the cell prepares itself for meiosis. The detail of all these phases is given below.

Interphase

It is the phase during which a cell prepares itself for division. It occurs only before Meiosis I. there is no interphase between Meiosis I and Meiosis II. The interphase is divided into three phases

  1. G1 phase, the cell grows in size and makes necessary proteins in this phase
  2. S phase, the cell undergoes DNA replication
  3. G2 phase, the cell makes proteins that are needed for meiosis

After the G2 phase is complete, the cells enter the Prophase I.

Prophase I

It is the longest phase of meiosis I during which the nuclear envelope disappears, and genetic exchange takes place. It is further divided into five phases.

During this phase, the chromosomes start appearing as thin threads within the nucleus. Each chromosome consists of two sister chromatids. A total of 46 chromosomes, each having 2 chromatids can be seen in the nucleus towards the end of this phase.

During this phase, the homologous chromosomes come close to each other to form homologous pairs. This pairing process is called synapsis.

The pairing of homologous chromosomes is completed during this phase. It results in the formation of tetrad chromosomes (called so because of four sister chromatids) also known as bivalent (two chromosomes).

Once the pairing process is complete, the homologous recombination takes place. It is the process during which the non-sister chromatids can exchange their segments resulting in genetic variations. This process is called crossing-over.

A physical link is formed between the non-sister chromatids through which the crossing-over takes place. This is known as chiasmata.

During this phase, the homologous chromosomes undergo uncoiling and are visible as two threads. However, the bivalent structure is nor broken as the two chromosomes remain tightly linked at chiasmata points. The chiasmata are broken only during the anaphase I.

This the last stage of prophase I during which chromosomes undergo further condensation. All the four parts of the bivalent are visible at the end of diakinesis.

During diakinesis, the nuclear envelope is disentangled, the nucleoli disappear, and the mitotic apparatus starts forming.

This completes the prophase I of Meiosis I. The cell now enters the metaphase I.

Metaphase I

During this phase, the spindle fibers are formed among the centrosomes that have already migrated to the opposite poles of the cell. These centrosomes also give rise to kinetochore microtubules that attach the bivalent of homologous chromosomes at the kinetochores from each side. A tension is generated in these microtubules that arranges the bivalents along the metaphase plate in the center of the cell.

The attachment of microtubules at the kinetochores is called bivalent attachment as they are attached to the entire bivalent, not the individual chromosomes.

Anaphase I

during the anaphase I, the microtubules start shortening, pulling the bivalent towards the opposite poles. As a result, the chiasma break and the bivalent structure is lost. The individual chromosome consisting of sister chromatids, having crossed segments, are pulled towards the respective pole. The sister chromatids are not separated during this process as the centromeres holding them are supported by some guarding proteins.

The cell also elongates for division into two daughter cells.

Telophase I

This is the final stage of Meiosis I. During this phase, the mitotic apparatus disappears while the new nuclear membrane is formed around the daughter chromosomes present at each pole of the cell.

Each daughter nucleus carries half the number of chromosomes (23 chromosomes, each having two sister chromatids) as compared to the diploid parent nucleus. The resulting daughter nuclei contain only one copy of each chromosome and are haploid. The two sister chromatids are not copies of the chromosome as they are only formed as a result of DNA replication.

Cytokinesis

Meiosis I is followed by cytokinesis in which a cleavage furrow is formed dividing the cell into two daughter cells.


Lecture 29: Meiosis - Biology

Chapter 9, Sexual Reproduction and Meiosis

You have open access (no log-in or password needed) to instructional materials on the Text web site. Select "Resources" from the upper left of the page and select the text chapter you want.

Moodle

You may also ask questions and see answers to your classmates' questions in Moodle in the "Talk to Ed" forum.

Objectives:

The content of this lecture will help you complete this assignment:

After studying this material you should be able to:

Discuss the relationship between sex and reproduction, and compare sexual and asexual reproduction.

Draw a diagram that illustrates the relationships among the terms: chromosome, DNA, genes, chromatids, centromeres, homologous chromosomes (homologs), and alleles.

Recognize the essential elements of the process of meiosis.

Explain the role of meiosis in an organism's sexual life cycle.

Describe your own life history in terms of a general sexual life cycle.

Indicate where and when in your body meiosis occurs and describe what is produced by the process.

Compare the process of meiosis in human females and males.

Use common objects such as paper clips or scraps of paper to model the changes in number and movement of chromosomes during meiosis.

Compare the timing, location, numbers of cells, numbers of chromosomes, and genetic outcomes of mitosis and meiosis.

Web resources:

Genes and Disease (Selected genes and their functions and locations on the chromosomes) from the National Center for Bitechnology Information

Sex - Biologically speaking:

Are sex and reproduction always linked?

Sex, reproduction, and the usefulness of genetic variability

Reproduction and sex are not necessarily linked. Many can reproduce without sex (asexual reproduction):

Microbes (bacteria, fungi): "Sex" in bacteria (Lewis, et. al. pg. 166, fig. 10.3

An egg develops into a new individual without fertilization. Parthenogenesis naturally occurs in some plants, insects, some fishes, frogs, and lizards. It does not normally occur in mammals, but has been artificially stimulated in mice.

But not mammals (and very rarely in other vertebrates)

Asexual reproduction tends to produce genetically identical individuals.

A "good idea" in a stable environment.

Maybe not a good idea if the environment is variable

Genetic changes can occur randomly by mutation

Sexual reproduction produces genetic variability within a population.

Variability is evolutionarily beneficial in a changing environment, allowing populations to adapt to changes over time (as measured in generations).

The General Sexual Life Cycle

Does meiosis occur in your body?

What is meiosis?

Human Chromosomes Hoefnagels Page 158, figure 84.5

The problem is to produce a new human being.

Both parents have to contribute genetic information.

The question is, in what form and how much?

You get one of each pair of numbered chromosomes from each parent.

The paired chromosomes are called homologous pairs.

Homologous chromosomes carry genes that control the same characteristics (traits).

Genes and Disease (Selected genes and their functions and locations on the chromosomes) from the National Center for Bitechnology Information

Preparing for Meiosis:

Cell Cycle Diagram Hoefnagels Page 160, fig. 8.5

DNA replication PRODUCES sister chromatids

How does DNA replicate? Hoefnagels, pg 144, figure 7.11

Visit cellsalive.com and select Cell Biology and then the Cell Cycle.

Watch for the DNA replication / chromosome replication / formation of sister chromatids during the S phase of the cell cycle.

Details of Meiosis Lewis, et. al. pg. 170-1, fig. 10.7

Meiosis I Hoefnagels pg. 180, fIg. 9.6.1

Meiosis II Hoefnagels pg. 180, fIg. 9.6.2

Meiosis: an illustration of the meiotic process provided by Access Excellence.

Meiosis Tutorial from the University of Arizona

Summary of the basic process of meiosis

A process including TWO cell divisions which results in FOUR daughter cells.

Meiosis occurs in the sex organs in cells destined to produce gametes.

Each daughter cell receives only half the number of chromosomes as the mother cell. That means the cells are described as haploid

Each of the resulting 4 cells is genetically unique.

If meiosis works properly, each of the 4 cells has one of each type of chromosome.

The resulting haploid cells develop into gametes (eggs or sperm).

Diploidy is restored when egg and sperm combine at fertilization. Lewis, et. al. pg. 174, fig. 10.11

Genetic Recombinations during Meiosis

The way the chromosomes are assorted during meiosis, there is no way to predict which set of chromosomes will end up in which daughter cell. It is only certain that, unless something goes wrong, each daughter cell will have one of each type (one of each numbered) chromosome.

Independent Assortment

Independent assortment Hoefnagels pg. 203, figure 10.9

In humans, because there are 23 pairs of chromosomes, the number of possible assortments is:

Any one of these assortments can combine with any one of the 8,388,608 combinations of his/her partner!

Meiosis and Crossing Over

During meiosis, chromosomes exchange parts of their genetic material with the corresponding regions on their homologous chromosome. This process is called crossing over and it makes the number of possible combinations nearly unlimited.

Gamete variation by Crossing Over depends on the relative location of linked gene loci.

Cloning - Another means of asexual reproduction

Comparison of Mitosis and Meiosis:

Hoefnagels, pg 184, fig. 9.10


Mitosis Meiosis
One division Two divisions
Homologous Chromosomes line up
independent of each other at metaphase
Homologous Chromosomes
synapse at Metaphase I
Two daughter cells per cycle Four daughter cells per cycle
Daughter cells genetically identical Daughter cells genetically different
Same chromosome no. as parents Chromosome no. half that of parents
Occurs in somatic cells Occurs in germ-line cells
Throughout life cycle Completed after sexual maturity
Used in growth, repair, asexual reproduction Sexual reproduction, new gene combinations

Glossary of terms relating to reproduction and meiosis:

Crossing over: The exchange of genetic material between homologous chromosomes during the first stage of meiosis. It results in genetic variation in populations greater than that which might result from independent assortment alone.

Daughter cell: A cell which results from division of another cell (a mother cell), either in meiosis of mitosis.

Diploid: A cell with two copies of each of its chromosomes.

Embryo: The stage of an organism's development in which tissues and organs develop beginning with a fertilized egg.

Gamete: In animals, a haploid cell which results from the second division of meiosis. In plants, the haploid cells proceed through an intermediate, multicellular stage before producing gametes. Male gametes are sperm female gametes are eggs.

Haploid: A cell with only a single copy of each chromosome.

Homologous chromosomes: Chromosome pairs within cells which have the same sequence of gene locations (Genes for the same traits). One chromosome of each pair comes from each of the parents through the gametes.

Independent assortment: The random arrangement and partitioning of homologous chromosomes during the first cell division stage of meiosis.

Sister Chromatids: The two halves of a replicated chromosome. Each chromatid is an identical copy of the DNA of the original chromosome before DNA replication.

Zygote: The fused egg and sperm the result of fertilization. In humans, this is also called the pre-embryo and the term is applied to the dividing cells during the first two weeks of development.


Lecture 29: Meiosis - Biology

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resemble their parent or parents. Hippopotamuses give birth to hippopotamus calves, Joshua trees produce seeds from which Joshua tree seedlings emerge, and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same.

Figure 1. Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)

As you have learned, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for most plants and animals. While many unicellular organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitosis, many single-celled organisms and most multicellular organisms reproduce regularly using another method: meiosis. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms, both multicellular and unicellular, can or must employ some form of meiosis and fertilization to reproduce.

Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division.

Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on.


Chapter 13 Meiosis Objectives

1. Explain in general terms how traits are transmitted from parents to offspring.

2. Distinguish between asexual and sexual reproduction.

The Role of Meiosis in Sexual Life Cycles

3. Distinguish between the following pairs of terms:

a. somatic cell and gamete

b. autosome and sex chromosome

4. Explain how haploid and diploid cells differ from each other. State which cells in the human body are diploid and which are haploid.

5. Explain why fertilization and meiosis must alternate in all sexual life cycles.

6. Distinguish among the three life-cycle patterns characteristic of eukaryotes, and name one organism that displays each pattern.

7. List the phases of meiosis I and meiosis II and describe the events characteristic of each phase.

8. Recognize the phases of meiosis from diagrams or micrographs.

9. Describe the process of synapsis during prophase I and explain how genetic recombination occurs.

10. Describe three events that occur during meiosis I but not during mitosis.

Origins of Genetic Variation

11. Explain how independent assortment, crossing over, and random fertilization contribute to genetic variation in sexually reproducing organisms.

12. Explain why heritable variation is crucial to Darwin ’s theory of evolution by natural selection


STUDY DESIGN

Establishment of Ploidy as a Key Conceptual Issue

Biology, biotechnology, bioinformatics, and biomedical science majors receive instruction in the topic of meiosis in at least three separate courses: Introductory Biology (freshman year), Cell Biology (sophomore year), and Genetics (junior or senior year). Typically, they have already been exposed to the subject more than once prior to college. We administered the same open-ended test question to students enrolled in a large, sophomore level Cell Biology course that was held in the fall of 2010 (mostly sophomores, enrolled in two sections, combined N = 131). The question was given before the topic of meiosis was introduced. We also tested a Genetics course that was held for a small number of students over the summer of 2010 (N = 13, mostly juniors), where the question was given as a bonus on their final exam. Thus, the Cell Bio group was assumed to have been exposed to the topic once and the Genetics group three times during their postsecondary education.

Students were given a diagram to fill in and asked to identify the ploidy of the cell at each stage. The initial cell was depicted as a diploid cell containing three pairs of unreplicated chromosomes, shown in the upper left corner of Fig. 1. An expert would recognize three maternal and three paternal chromosomes, represented by the black and white colors, and homologous pairs would be matched by size and shape. They would also understand that these structures represent chromosomes prior to replication because they only have one chromatid each.

The open-ended test question administered to students in two majors-level courses. Instructions described the first diagram as a precursor germ cell with three pairs of chromosomes. This is an example of a student's drawing with all the steps depicted essentially correctly, along with the common mistake of assigning cells between anaphase I and anaphase II as diploid.

We found very little difference between the classes, as shown in Fig. 2, thus confirming that misconceptions persist in our student population, despite repeated exposure to the topic. The more advanced group was slightly better at identifying the point at which the cell becomes haploid (i.e. after the first division see Fig. 2), but none of them drew the process fully correctly (note that N for this group was small, though). Interestingly, few students considered the process of crossing over to be important to a description of meiosis, even in a Genetics course. Figures 3 and 4 illustrate the different types of models that students constructed, showing that our population of students is fairly typical in the types of mistakes that they make. Also, although nearly one in five students was able to draw an essentially correct diagram of the process, most still retained misconceptions about ploidy. This leads to the disturbing conclusion that 96% of intermediate-level college students do not understand the fundamentals of meiosis.

Students' test results, coding for features of students' drawings. Students were asked to draw the steps of meiosis starting with the cell depicted in Fig. 1. This was a pretest for students in both Cell Biology sections (experimental group, N = 68 control group, N = 63) and a bonus question on the final exam of the Genetics class (N = 13). Drawings of meiosis from the three groups of students showed no significant differences using chi-square analysis.

Students' test results categorized into different conceptual issues. Data from all three classes were combined in this graph (N = 144). As multiple mistakes may have been made, models were categorized by their first mistake, as depicted by the clockwise order of the pie chart, starting at the top.

Representative drawings from students' worksheets demonstrating some of the incorrect models of meiosis. Although students were asked to draw the entire process, only the beginning steps of the worksheets are shown here. (a) Pairing homologous chromosomes like sister chromatids: in this very common model, the student joins the unreplicated homologous chromosomes together at their centromeres as if they are sister chromatids rather than separate chromosomes. (b) Failure to replicate: in this model, the student does not replicate the chromosomes before lining them up. (c) Incorrect segregation: in this drawing, the student did not pair homologous chromosomes and depicted a mitosis-like metaphase. (d) Failure to replicate along with incorrect segregation. (e) Unclear logic: in this drawing, the individual chromosomes are not distinct enough to determine whether or not homologous chromosomes are paired or what happened when the total number changed. (f) Incorrect segregation: although pairs of replicated chromosomes are lined up in the center at metaphase I, homologous chromosomes were not paired with each other. Note that the pairs appear to be separated by a physical barrier and do not touch each other. Thus, there is no apparent mechanism for ensuring that one of each type of chromosome is aligned on each side of the dividing cell.

Figure 5 shows more detail of students' ideas about ploidy. When asked to label the major stages of meiosis as “haploid” or “diploid,” many students showed signs of confusion on the pretest. Thirty-six percent of students left one or more of these questions blank, and answers were often scratched out and revised (data not shown). Although 87% of all students knew that the starting cell was diploid and the end product was haploid, only 11% correctly identified all the steps in between. The most common mistake is to assign the first incidence of haploidy after the second division rather than after the first. As in the example shown in Fig. 1, many students who could diagram the segregation correctly could not answer the ploidy question correctly. Overall, it is clear that few students knew how to determine ploidy.

Students' ideas about ploidy before the lesson. Drawings from both Cell Biology sections are combined in the graph: 131 total, 84 of which had “haploid” or “diploid” circled at every stage where the question was asked and were therefore considered to be complete answers.

The Interactive Lesson: Overview

We designed and carried out a novel lesson plan for teaching the concept in the same Fall 2010 Cell Biology class. A different instructor taught a second, similar-sized class using his traditional lecture materials, so they acted as a control for this experiment. The students in the control section were given the same pretest and gave similar responses to the test group (Fig. 2). In the new lesson, we attempted to correct the misconception of haploidy being derived at the final step of meiosis with an intervention that focused on the differences between chromosomes and chromatids, and how to count “N” in a cell. The lesson took place in a large lecture hall for ∼80 students. The lesson was designed to be interactive and participatory (see Table I).

Tactic Primary point Secondary point
Use of socks to represent DNA, students to represent chromosomes Replication of DNA does not affect number of chromosomes Chromosomes are more than just DNA
Students start with a single sock that “replicates” to a pair of socks Emphasize importance of DNA replication to the process One chromosome may contain one or two chromatids
Male and female students as maternal and paternal chromosomes, different-sized socks for chromatids Homologous pairs are different from sister chromatids Maternal and paternal of same kind of chromosome always pair together
Use of three pairs of chromosomes Emphasize need for replication (three cannot be evenly divided) Homologous chromosomes, not chromatids, define pairs in this context
Repeated counting of chromosomes at every step Understanding ploidy and how/when it changes during meiosis Chromosomes rather than chromatids determine ploidy
Avoid mentioning phase names Cell division is a fluid process, not a series of disjointed steps De-emphasis of rote memorization of labels
Show a short piece of DNA sequence to illustrate homology Make a connection between different levels of representations Homologous chromosomes are almost completely identical—genetic differences are a tiny fraction of the sequence
Projection of short DNA sequence lined up to illustrate simplistic view of crossing over Homologous pairs find each other via DNA sequence homology Don't overwhelm with molecular details of crossing over and recombination, but focus on the ”big picture”
Multiple striped and patterned socks to show results of crossing over Crossing over happens along entire chromosome, not just once or twice Both chromatids on both chromosomes are involved in crossing over and results are different
Students link arms before spindles (ropes) attach Physical linkage (synapsis) is essential to proper segregation Tension from spindles causes chromosomes to line up in the center
Joke that students would have to be torn in half during meiosis II Emphasize difference between kinetochore structure/behavior in the two divisions Emphasize that meiosis II is not simply a repeat of meiosis I

Student volunteers acted as “chromosomes” and “centrosomes.” Different-colored and -sized socks were used to represent DNA, and “chromosomes” were counted at every stage. The important point illustrated by the socks was to differentiate between a strand of DNA (one sock) and a chromosome (a student's hand holding one or two socks) this helped to make the connection of the number of chromosomes rather than copies of DNA with the concept of ploidy. The actors and observers were constantly questioned to elicit cognitive dissonance and to lead students to confront and resolve their mistakes appropriately. When students volunteered different answers, we encouraged debate and pointed out features that led them to derive their own correct conclusions instead of “giving” the answers to them. Using this constructivist pedagogy, students were answering questions about chromosome number correctly by the end of the lesson.

A secondary point of the lesson was the molecular mechanism behind metaphase. Discussions with students along with their in-class responses to questioning revealed that metaphase was a mysterious process to them. They could not explain how homologous pairs “find” each other, nor how they “find” the center of the cell. Therefore, we particularly emphasized the importance of homologous DNA sequence and the physical interaction of the strands of DNA for matching (this was accomplished primarily through a sidebar discussion of homology). We also discussed the importance of the spindle fibers in setting up the alignment of chromosomes during metaphase and used ropes held at one end by the human “chromosomes” and pulled by human “centromeres” at opposite ends of the “cell” to demonstrate how the spindle fibers actually cause the chromosomes to line up in the center. This appeared to be a novel concept to most students—they previously believed that the spindle was only actively involved in pulling the chromosomes apart during anaphase.

In the last few minutes of class, we asked students to reflect on the lesson. They discussed how this exercise developed new ideas for them and pointed out mistakes they had made previously.

Specific Details of the Lesson

The following sequence of steps explains how the demonstration worked in a large lecture hall. See Table II for prepared questions posed to the class by the instructor during the demonstration. Six volunteers were solicited to come to the front of the room. The instructor discreetly chose three males and three females to represent maternal and paternal homologous chromosomes (later in the lesson, students were able to draw the correct conclusion about why we chose these particular individuals). Each student was given a unique solid-colored sock (with its mate hidden inside) in one of three sizes (adult, child, or infant) the instructor was careful to give the same-sized socks to one male and one female for each set. Students were asked to hold up the sock in one hand. Student volunteers “replicated their DNA” by pulling the hidden second sock out and were instructed to now hold both socks in the same hand. When the class was asked to count chromosomes, the majority of students responded with “twelve.” The instructor then asked a series of questions (Table II) to remind or teach students how to correctly count ploidy.

Question Correct answer
Prereplication
• What does the sock represent? • DNA
• How many chromosomes are there right now? • Six
• What do the different sizes of the socks represent? • Different chromosomes
• Why are there two of each size? • Maternal and paternal chromosomes
• What is the first thing that has to occur? • DNA replication
After replication
• How many chromosomes do we now have? • Six
• How many students did we start with? • Six
• How many students do we now have? • Six
• Did the number change? • No
Re-emphasis of important concepts
• Before DNA replication what does one sock represent? • DNA in one chromosome
• After DNA replication what does one sock represent? • DNA in one chromatid
• How many chromosomes are there right now? • Six
• If socks are DNA, what does the student represent in this model? • Chromosome or more specifically, the kinetochore
• What is the ploidy of the cell before DNA replication? • Diploid
• What is the ploidy of the cell after DNA replication? • Diploid
• What can we count to determine ploidy? • Students (chromosomes, or to be accurate, kinetochores)
Mechanism of Homologous pairing:
• What makes two chromosomes homologous? • DNA sequence
• What allows homologous chromosomes to pair? • Interaction of homologous sequence
Significance of “crossing over”
• What is the process of “crossing over” essential for? • Homologous pairing of replicated chromosomes
• Does crossing over have to happen during meiosis? • Yes
• What will the socks look like after crossing over? • Mixed up (not solid colors anymore)
Separation of homologous chromosomes
• How do homologous pairs find the center of the cell to “line up”? • Spindle fibers attached to each kinetochore, tension pulls the pairs to the center
• What has to separate? • Homologous chromosomes
• What has to be broken? • Physical linkage between homologous chromosomes (synapses)
After meiosis I, establishment of haploidy
• What do we count to determine ploidy? • Chromosomes (students)
• How many people are in each cell? • Three
• How does that compare to our starting cell? • Half
• So what is the ploidy of each cell now? • Haploid
Separation of sister chromatids
If we were really going to stay true to our model, what would we have to do to our students? • Split them in half, because that's what happens to the kinetochore
After meiosis II, re-emphasis of key concepts
• How many chromosomes in each cell? • Three
• What is the ploidy of each cell? • Haploid
• How many different allele combinations do we have? • Four (each group of three students had different sock combinations)

Student “chromosomes” were then asked to find their homologous pair. As anticipated, student chromosomes all moved to the middle of the virtual “cell” before linking with their homologous chromosome. The instructor halted the lesson and did not move the action forward until the class was able to vocalize that “interaction of homologous sequences” mediated the process of crossing over. Without overwhelming the class with molecular details, a simple diagram showing short stretches of identical DNA code was used to help illustrate the general mechanism of crossing over.

Student chromosomes were asked to link arms with their homologous chromosome to demonstrate a physical linkage (but were not yet allowed to line up in the center of the “cell”). After some discussion, the class came to the realization that crossing over could occur anywhere between two chromatids, not just at one point between the chromatids. As socks are 3D and flexible (like actual DNA), the instructor and/or student volunteers could manipulate their socks in a way to demonstrate multiple crossovers. Students were asked to predict what the socks might look like after (for example) a pair of red and a pair of same-sized black socks had crossed over. The instructor now brought out adult-, child-, and infant-sized socks that were patterned (striped, spotted, argyle, and floral) with the same colors as the original pairs, and students agreed that a patterned sock was an acceptable way to represent the reshuffling and recombination of genetic material after crossing over had occurred. The instructor then switched solid-colored socks for patterned socks so that none of the four individual chromatids looked identical.

Next two students representing the centromeres were given three ropes each and asked to toss one end to each of the pairs of student chromosomes. As each student caught a rope, the entire pair would be reeled toward the corresponding centromere. When the other member of the pair caught a rope, they would be pulled in the opposite direction. In this way, tension from the “centromeres” pulling on the ropes represented the dynamics of spindle fibers during this process and allowed “chromosomes” to find a happy medium in the center of the “cell.” Students were asked to vocalize what was represented by the socks (DNA), ropes (spindle fibers), hands holding the socks and ropes (kinetochores), and linked arms (protein linkage, a.k.a. “synaptonemal complex”) to overcome any representational confusion at this point in the lesson.

After proper alignment, the instructor asked the class why the male and female student chromosomes were not segregated to one side or the other of the virtual equator, and the class immediately recognized the principle of random assortment. The student chromosomes unlinked arms and were pulled to opposite sides of the virtual cell. The instructor then asked the class a series of questions to reinforce their new conceptual understanding of ploidy. Students hesitantly identified the new cells as “haploid” and the instructor confirmed their responses. At this point there were many questions from the student audience about chromosomes, chromatids, and ploidy, suggesting that the establishment of haploidy after the first meiotic division was a novel concept and one that created cognitive dissonance. Most questions could easily be answered using the student “chromosomes” to repeat various steps of the interactive lesson through the point of the first meiotic division.

When the student chromosomes were asked to demonstrate the next division (meiosis II) there was laughter from the class when they realized student chromosomes would literally have to be split apart to accurately represent separation of sister chromatids. Six more student “chromosomes” were quickly recruited into the action and asked to hold one of the socks (chromatids) with the original student volunteers. Ropes were used again to line up and pull apart student pairs (“sister chromatids” this time). After the second meiotic division, the class had no trouble counting chromosomes, chromatids, or identifying ploidy of the four daughter cells. The instructor finally pointed out the number of different allelic combinations that were represented by the different pattern combinations of socks.


# Cytokinesis.

Division of the cytoplasm in known as cytokinesis. In plant cells, cytokinesis takes place by the formation of cell plate. Whereas, in animal cells it is by the appearance of furrow in the cytoplasm.

⇒ In plant cells, vesicles from Golgi bodies appear at the equator of the spindle. It forms a cell plate. It divides the cytoplasm into two equal halves, one around each daughter nucleus. Then primary cell wall in laid on either side of the cell plate.

⇒ In animal cells, a constriction (Furrow) appears parallel to the equator of spindle, in the cell membrane. It deepens towards the centre of the cell, finally dividing the cytoplasm in two halves. One half of the cytoplasm is around one daughter nucleus and the other around the second daughter nucleus. It completes the formation of two daughter cells. They contain exactly similar and the same number of chromosomes as present in the parent cell.


Department of Biology

Errors in chromosome segregation can have devastating consequences. In mitosis, chromosomal instability is a hallmark of cancer. In meiosis, chromosome mis-segregation can result in trisomy conditions such as Down syndrome, the leading genetic cause of developmental disability. The goal of our research is to understand the mechanisms the cell uses to ensure faithful chromosome segregation in mitosis and meiosis. We are studying how the cell prevents errors in chromosome segregation, including how chromosomes properly attach to the spindle in both meiotic divisions and the monitoring of this attachment by the spindle checkpoint. Many of the genes involved in these processes are conserved, allowing us to use the powerful genetic tools of the budding yeast, S. cerevisiae.

Chromosomes attach to the meiotic spindle at the kinetochore, the protein complexes built on centromeric regions of DNA. We are interested in the proteins that regulate this connection in meiosis. For example, in the first meiotic division (meiosis I), the kinetochores on paired homologous chromosomes must be bound to microtubules from opposite poles of the spindle, but in meiosis II, the kinetochores of sister chromatids must be bound to opposite poles. Furthermore, to prevent chromosome missegregation, if the kinetochores on homologous chromosomes attach to microtubules from the same spindle pole, one kinetochore must release and re-attach properly. The spindle checkpoint monitors this connection and, if the attachment of microtubules to kinetochores is defective, halts the cell cycle to allow time to correct the error. We are studying the regulation of the proteins within the kinetochore to execute each of these steps: microtubule binding to homologous chromosomes in meiosis I and sister chromatids in meiosis II, sensing inappropriate microtubule attachment, signaling the checkpoint, and correcting the error.

In meiosis, the spindle checkpoint proteins not only act in a surveillance system to ensure that chromosomes are properly attached to the spindle, but they also have additional roles. Certain spindle checkpoint proteins are involved in ensuring that kinetochores can initially attach to the bipolar spindle. Other spindle checkpoint proteins are also involved in the timing of the meiotic cell cycle. We are interested in understanding how the different roles of the spindle checkpoint proteins are executed.

Publications

Cairo G and Lacefield S. Establishing correct kinetochore-microtubule attachments in mitosis and meiosis. Essays in Biochemistry (2020, online ahead of print).

Wang F, Zhang R, Feng W, Tsuchiya D, Ballew O, Li J, Denic V, Lacefield S. Autophagy of an amyloid-like translational repressor regulates meiotic exit. Developmental Cell 52:141-151 (2020).

Falk JE, Tsuchiya D, Verdaasdonk J, Lacefield S, Bloom K, Amon, A. Spatial signals link exit from mitosis to spindle position. Elife 5. pii:e14036 (2016)

Yang Y, Tsuchiya D, and Lacefield S. Bub3 promotes Cdc20-dependent activation of the APC/C in S. cerevisiae. Journal of Cell Biology 209(4): 519-527 (2015).
This paper was featured in the JCB Biobytes podcast.

Tsuchiya D, Gonzalez C, and Lacefield S. The spindle checkpoint protein Mad2 regulates APC/C activity during prometaphase and metaphase of meiosis I in S. cerevisiae. Molecular Biology of the Cell 22 (16): 2848-2861 (2011). This paper was chosen for “Highlights in MBoC” by the editorial board.
This paper was recommended by F1000.


Chromosome Structural Rearrangements

Cytologists have characterized numerous structural rearrangements in chromosomes, including partial duplications, deletions, inversions, and translocations. Duplications and deletions often produce offspring that survive but exhibit physical and mental abnormalities. Cri-du-chat (from the French for “cry of the cat”) is a syndrome associated with nervous system abnormalities and identifiable physical features that results from a deletion of most of the small arm of chromosome 5 (Figure 7). Infants with this genotype emit a characteristic high-pitched cry upon which the disorder’s name is based.

Figure 7 This individual with cri-du-chat syndrome is shown at various ages: (A) age two, (B) age four, (C) age nine, and (D) age 12. (credit: Paola Cerruti Mainardi)

Chromosome inversions and translocations can be identified by observing cells during meiosis because homologous chromosomes with a rearrangement in one of the pair must contort to maintain appropriate gene alignment and pair effectively during prophase I.

A chromosome inversion is the detachment, 180° rotation, and reinsertion of part of a chromosome (Figure 8). Unless they disrupt a gene sequence, inversions only change the orientation of genes and are likely to have more mild effects than aneuploid errors.

Figure 8 An inversion occurs when a chromosome segment breaks from the chromosome, reverses its orientation, and then reattaches in the original position.

A translocation occurs when a segment of a chromosome dissociates and reattaches to a different, nonhomologous chromosome. Translocations can be benign or have devastating effects, depending on how the positions of genes are altered with respect to regulatory sequences. Notably, specific translocations have been associated with several cancers and with schizophrenia. Reciprocal translocations result from the exchange of chromosome segments between two nonhomologous chromosomes such that there is no gain or loss of genetic information (Figure 9).

Figure 9 A reciprocal translocation occurs when a segment of DNA is transferred from one chromosome to another, nonhomologous chromosome. (credit: modification of work by National Human Genome Research/USA)

One specific example of a chromosomal translocation – the “Philadelphia chromosome” – is found in people who suffer from chronic myeloid leukemia (CML). In this translocation, a piece of chromosome 9 is swapped with a section of chromosome 22. This connects two genes on chromosome 22 one that was originally from chromosome 9 and one that was from chromosome 22. This translocation produces the BCR-ABL fusion protein, which causes white blood cells to divide out of control. BCR-ABL positive cancers can be treated with the drug Gleevac.

Figure 9 “Philadelphia chromosome” showing the location of the BCR-ABL fusion protein. Photo credit A Obeidat Wikimedia.


Watch the video: Real Microscopic Mitosis MRC (May 2022).