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Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. The fact that most eukaryotes reproduce sexually is evidence of its evolutionary success. In many animals, it is the only mode of reproduction. And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to be more advantageous. If the parent organism is successfully occupying a habitat, offspring with the same traits would be similarly successful. There is also the obvious benefit to an organism that can produce offspring by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. There is no need to expend energy finding or attracting a mate. That energy can be spent on producing more offspring. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, in asexual populations every individual is capable of reproduction. In contrast, the males in sexual populations (half the population) are not producing offspring themselves. Because of this, an asexual population can in theory grow twice as fast as a sexual population. This means that in competition, the asexual population would have the advantage. All of these advantages to asexual reproduction, which are also disadvantages to sexual reproduction, should mean that the number of species with asexual reproduction should be more common.
However, multicellular organisms that exclusively depend on asexual reproduction are rare.
So why is sexual reproduction so common?
This is one of the important questions in biology and has been the focus of much research from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of genetic variation in asexual organisms is mutation. In sexually reproducing organisms, mutations are continually reshuffled between generations when parents combine their unique genomes, and the genes are mixed into different combinations by the process of meiosis.
The Red Queen Hypothesis:
There is no question that sexual reproduction provides evolutionary advantages to organisms that employ this mechanism to produce offspring. The problematic question is why, even in the face of seemingly stable conditions, sexual reproduction persists when it is more difficult and produces fewer offspring for individual organisms? Variation is the outcome of sexual reproduction, but why is ongoing variation necessary? Enter the Red Queen hypothesis, first proposed by Leigh Van Valen in 1973.1 The concept was named in reference to the Red Queen's race in Lewis Carroll's book, Through the Looking-Glass, in which the Red Queen says one must run at full speed just to stay where one is.
All species coevolve with other organisms. For example, predators coevolve with their prey, and parasites coevolve with their hosts. A remarkable example of coevolution between predators and their prey is the unique coadaptation of night flying bats and their moth prey. Bats find their prey by emitting high-pitched clicks, but moths have evolved simple ears to hear these clicks so they can avoid the bats. The moths have also adapted behaviors, such as flying away from the bat when they first hear it, or dropping suddenly to the ground when the bat is upon them. Bats have evolved “quiet” clicks in an attempt to evade the moth’s hearing. Some moths have evolved the ability to respond to the bats’ clicks with their own clicks as a strategy to confuse the bats echolocation abilities.
Each tiny advantage gained by favorable variation gives a species an edge over close competitors, predators, parasites, or even prey. The only method that will allow a coevolving species to keep its own share of the resources is also to continually improve its ability to survive and produce offspring. As one species gains an advantage, other species must also develop an advantage or they will be outcompeted. No single species progresses too far ahead because genetic variation among progeny of sexual reproduction provides all species with a mechanism to produce adapted individuals. Species whose individuals cannot keep up become extinct. The Red Queen’s catchphrase was, “It takes all the running you can do to stay in the same place.” This is an apt description of coevolution between competing species.
Sexual reproduction requires fertilization, the union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. Haploid cells contain one set of chromosomes, diploid cells contain two sets of chromosomes. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive cycle is to continue, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. So, in addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.
The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for most plants and animals. 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.
Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis.
Early in prophase I, before the chromosomes can be seen clearly microscopically, homologous chromosomes are attached at their tips to the nuclear envelope by proteins. Homologous chromosomes are similar but not identical chromosomes. For example, chromosome 12 from your mother and chromosome 12 from your father will both be present inside each of your cells. Each chromosome 12 contains the same genes, usually in the same locations, however, each gene can be a different allele. Gene A on chromosome 12 from your mother may be allele R' and gene A on chromosome 12 from your father may be allele r. In species such as humans, even though the X and Y sex chromosomes are not homologous (most of their genes differ), they have a small region of homology that allows the X and Y chromosomes to pair up during prophase I. A partial synaptonemal complex develops only between the regions of homology. It will be very important to understand what homologous chromosomes are when following the process of meiosis.
Two homologous chromsomes are shown prior to DNA replication. Each chromosome has three genes with their locus marked. Homologous chromosomes contain the same genes but are not identical. They each can contain different alleles of each gene.
As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma) (see figure below).
Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.
Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules. These assemblies mark the points of later chiasmata and mediate the multistep process of crossover—or genetic recombination—between the non-sister chromatids. Near the recombination nodule on each chromatid, the double-stranded DNA is cleaved, the cut ends are modified, and a new connection is made between the non-sister chromatids. As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata (figure below) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.
The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.
Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.
What are the major differences between Prophase I of Meiosis and Prophase of Mitosis?
The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules attach at each chromosomes' kinetochores. With each member of the homologous pair attached to opposite poles of the cell, in the next phase, the microtubules can pull the homologous pair apart. A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely.
During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b, or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. This is called Independent Assortment. Recall that homologous chromosomes are not identical, they contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup.
This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.
This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate; the possible number of alignments therefore equals 2n, where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (223) possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (see figure below).
To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.
Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2). In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With n = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes.
In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart.
What major difference occurs in Anaphase I of Meiosis compared to Anaphase of Mitosis?
Telophase I and Cytokinesis
In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.
Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.
In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II in synchrony. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes. Therefore, each cell has half the number of sister chromatids to separate out as a diploid cell undergoing mitosis.
If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed.
The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.
The sister chromatids are maximally condensed and aligned at the equator of the cell.
The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. Non-kinetochore microtubules elongate the cell.
The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the midpoint of the cells in metaphase II. In anaphase II, the sister chromatids are separated.
Telophase II and Cytokinesis
The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes (with their sets of genes) that occurs during crossover. The entire process of meiosis is outlined in the figure below.
An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form four haploid daughter cells.
Comparing Mitosis and Meiosis
Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to very different outcomes. Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells. In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is diploid.
The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together with the synaptonemal complex, develop chiasmata and undergo crossover between sister chromatids, and line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I.
When the chiasmata resolve and the tetrad is broken up with the homologs moving to one pole or another, the ploidy level—the number of sets of chromosomes in each future nucleus—has been reduced from two to one. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level during mitosis.
Meiosis II is much more analogous to a mitotic division. In this case, the duplicated chromosomes (only one set of them) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid—now referred to as a chromosome—is pulled to one pole while the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossover, the two products of each individual meiosis II division would be identical (like in mitosis). Instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.
Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.
The Mystery of the Evolution of Meiosis
Some characteristics of organisms are so widespread and fundamental that it is sometimes difficult to remember that they evolved like other simpler traits. Meiosis is such an extraordinarily complex series of cellular events that biologists have had trouble hypothesizing and testing how it may have evolved. Although meiosis is inextricably entwined with sexual reproduction and its advantages and disadvantages, it is important to separate the questions of the evolution of meiosis and the evolution of sex, because early meiosis may have been advantageous for different reasons than it is now. Thinking outside the box and imagining what the early benefits from meiosis might have been is one approach to uncovering how it may have evolved.
Meiosis and mitosis share obvious cellular processes and it makes sense that meiosis evolved from mitosis. The difficulty lies in the clear differences between meiosis I and mitosis. Adam Wilkins and Robin Holliday2 summarized the unique events that needed to occur for the evolution of meiosis from mitosis. These steps are homologous chromosome pairing, crossover exchanges, sister chromatids remaining attached during anaphase, and suppression of DNA replication in interphase. They argue that the first step is the hardest and most important, and that understanding how it evolved would make the evolutionary process clearer. They suggest genetic experiments that might shed light on the evolution of synapsis.
There are other approaches to understanding the evolution of meiosis in progress. Different forms of meiosis exist in single-celled protists. Some appear to be simpler or more “primitive” forms of meiosis. Comparing the meiotic divisions of different protists may shed light on the evolution of meiosis. Marilee Ramesh and colleagues 3 compared the genes involved in meiosis in protists to understand when and where meiosis might have evolved. Although research is still ongoing, recent scholarship into meiosis in protists suggests that some aspects of meiosis may have evolved later than others. This kind of genetic comparison can tell us what aspects of meiosis are the oldest and what cellular processes they may have borrowed from in earlier cells.
Link to Learning
Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: How Cells Divide.
- Leigh Van Valen, “A new evolutionary law,” Evolutionary Theory 1 (1973): 1–30.
- Adam S. Wilkins and Robin Holliday, “The Evolution of Meiosis from Mitosis,” Genetics 181 (2009): 3–12.
- Marilee A. Ramesh, Shehre-Banoo Malik and John M. Logsdon, Jr, “A Phylogenetic Inventory of Meiotic Genes: Evidence for Sex in Giardia and an Early Eukaryotic Origin of Meiosis,” Current Biology 15 (2005):185–91.
Lecture 29 and 30: Meiosis - Biology
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- The CosmoLearning Team
- Overview of Module 1
- How different are we?
- Properties of DNA
- Properties of genes
- Why molecular biology is confusing
- Properties of chromosomes
- Life cycles and ploidy
- Comparing DNA sequences
- Genetic and evolutionary relationships of human populations
- Overview of Module 2
- Fidelity of DNA replication
- Why most mutations are harmless
- Types of mutations and their consequences
- Germline and Somatic Mutations
- Mutagens (what should we worry about)
- Mutations, selection and evolvability
- Origins and evolution of new genes and gene families
- Comparing DNA sequences reveals evolutionary history
- Overview of Module 3
- Protein basics
- Catalytic proteins (enzymes)
- Structural, transport and carrier proteins
- Regulatory proteins and RNAs
- Homozygous phenotypes
- Diploids: Heterozygous phenotypes
- All about dominance
- How genes are named
- Gene interaction in biochemical pathways
- Regulatory interactions
- How somatic mutations cause cancer
- Frameworks for predicting the phenotypic effects of mutation
- Overview of Module 4
- Sex chromosomes and sex determination
- Expression of X‐linked genes in females
- Expression of X‐linked genes in males
- Can natural genetic variation explain natural phenotypic variation?
- Most natural variation has very small effects
- Many natural genetic variants affect multiple traits
- Effects of natural genetic variation depend on the environment
- Effects of natural genetic variation depend on chance
- How natural genetic variation affects the risk of cancer
- Integrating new understanding into old concepts
- Overview of Module 5
- DNA fingerprinting
- Analyzing a single gene or gene 'panel'
- SNP-typing the genome, Part 1
- SNP-typing the genome, Part 2
- SNP-typing services
- Exome sequencing
- Ethical and social issues surrounding personal genomics
- Not-so-personal genomics
- Overview of Module 6
- Sexual life cycles
- Meiosis, the basics
- Following genotypes through meiosis, part 1
- More about meiosis: homolog pairing and crossing-over
- Following genotypes through meiosis (this time with crossovers)
- Following alleles through generations
- Sex chromosomes in meiosis
- Overview of Module 7
- Genetic analysis began with Mendel
- Mendels findings and what we now know
- How to do genetic analysis
- Mendel's genetic analysis
- Detecting sex-linkage, predicting outcomes
- Using crosses to investigate locations of autosomal genes
- Using pedigrees to investigate family inheritance
- Using crosses to investigate gene function
- Genetic analysis: numbers matter
- Overview of Module 8
- GWAS redux
- Inbreeding in livestock and pets
- Inbreeding and genetic variation in evolution and conservation
- Plant Breeding & Transgenics
- Overview of Module 9
- Aneuploidy for sex chromosomes
- Chromosomes rearrangements
- Consequences of chromosome rearrangements
- Small changes (structural variation)
- Junk and selfish DNA
- Genome evolution
- Overview of Module 10
- Origin of Life
- Mitochondrial genetics
- Mosaics and chimeras
- Fetal DNA
- Genetics of aging
This college-level course gives students a thorough understanding of gene function, and enables them to apply this understanding to real-world issues, both personal and societal.
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Zoology 101: Animal Biology Last Lecture Outline Lecture 16 1. Finish up on cancer2. Meiosis3. Sexual ReproductionCurrent Lecture 1. Finish up of meiosis2. Genetics Finish up meiosis• Mistakes can happen during mitosis/meiosis• Nondisconjunction: homologous chromosomes in meiosis or sister chromatids is meiosis 2 don't separate from each other ◦ abnormal gametes (either 3 or 1 chromosomes)▪ Aneuploid gametes◦ n+1 sperm and and n egg = 2n+1 zygote → Trisomy. Trisomy 21: Down Syndrome ◦ As age increases in female, risk of down syndrome increases ◦ Human ogenisis (egg formation) in fetal ovary haults at the beginning of meiosis 1▪ during ovulation, meiosis resumes▪ as you age, eggs ignore metaphase checkpoint Genetics • From sexual reproduction and meiosis, gametes are variable but similar • Two Hypothesis' ◦ Blending ▪ From the two parents, offspring are intermediate ◦ Particulate ▪ Mendel and pea experiment each parent has discrete units passed through generations ▪ Pea used as a model organism → many offspring are produced, can control matings • P generation: true breeding plants (homozygus for purple flowers) ◦ Homo dominate is purple dominate allele haploid gamete is P◦ Homo recessive is white recessive allele Haploid gamete is p • F1 generation is heterozygous (different alleles: Pp) ◦ Principle of segregation: Alleles separate from each other during meiosis half have P half have p ◦ Punnett square- allows us to organize and predict the out come Gametes P pP PP Ppp Pp pp◦ Genotypic Ratio 1PP:2Pp: 1pp◦ Phenotypic ratio: 3 purple flowers to 1 white ◦ Phenotype is determined by genotype◦ Purple color= pigment → anthocyanin▪ P gene= wild type → dominant/normal (anthocyanin) ▪ p gene= mutant of anthocyanin gene, recessive ◦ In purple all normal genes are homozygous → protein P ◦ F1: heterozygous 1 chromosome has wild, 1 has mutant, meaning:▪ ½ gametes will have normal▪ ½ will have mutants▪ Normal cancels out mutants, doesn't matter mutant is present • Does inheritance influence the inheritance of other traits? ◦ Seed color: Y-yellow y-green ◦ Shape: R- round r- wrinkled ◦ Parent RRYY and rryy form RY and ry gametes ◦ Cross fertilization occurs ▪ Principle of independent assortment: random alignment of chromosomes in metaphase ▪ use every combination RY Ry rY ryRY RRYY RRYy RrYY RrYyRy RRYy RRyy RrYy RryyrY RrYY RrYy rrYY rrYyry RrYy Rryy rrYy rryy▪ 9-3-3-1 ratio• Human genetics: correlating genotype with phenotype ◦ ex. Cystic
Overview of meiosis: how meiosis
Meiosis reduces the number of chromosome sets from diploid to haploid
- A spindle apparatus forms
- In late prophase II (not shown here), chromosomes, each still composed of two chromatids associated at the centromere, move toward the metaphase II plate
- The chromosomes are positioned at the metaphase plate as in mitosis
- Because of crossing over in meiosis I, the two sister chromatids of each chromosome are not genetically identical
- The kinetochores of sister chromatids are attached to microtubules extending from opposite poles
- Breakdown of proteins holding the sister chromatids together at the centromere allow the chromatids to separate. The chromatids move toward opposite poles as individual chromosomes
Telophase II and Cytokinesis
- Nuclei form, the chromosomes begin decondensing, and cytokinesis occurs
- The meiotic division of one parent cell produces four daughter cells, each with a haploid set of (unduplicated) chromosomes
- The four daughter cells are genetically distinct from one another and from the parent cell
Meiosis permits cross-over: exchange of fragments of homologous chromosomes
- Three events are unique to meiosis o Synapsis and crossing over in prophase I: homologous chromosomes physically o At metaphase plate, there are paired homologous chromosomes (tetrads) o At anaphase I, it is homologous chromosomes that separate
Genetic variation produced in sexual life cycles contributes to evolution
4.2.4 Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).
A number of problems can arise during meiosis. A common problem is non-disjunction. This is when the chromosomes do not separate properly during meiosis, either in meiosis I (in anaphase I) or meiosis II (in anaphase II). This leads the production of gametes that either have a chromosome too many or too few. Gametes with a missing chromosome usually die quite fast however gametes with an extra chromosome can survive. When a zygote is formed from the fertilization of these gametes with an extra chromosome, three chromosomes of one type are present instead of two. An example of this is Down syndrome. Down syndrome is a disease in which the chromosomes failed to separate properly during meiosis leading to three chromosomes of type 21 instead of two. A person with the condition therefore has a total of 47 chromosomes instead of 46. The non-disjunction can take place either in the formation of the egg or the sperm. Down syndrome leads to many complications and also the risk of having a child with the condition increases with age.
Below is a diagram illustrating a non-disjunction:
Meiosis is a process of nuclear division that reduces the number of chromosomes in the resulting cells by half. Thus, meiosis is sometimes called “reductional division.” For many organisms the resulting cells become specialized “sex cells” or gametes . In organisms that reproduce sexually, chromosomes are typically diploid ( 2N ) or occur as double sets ( homologous pairs ) in each nucleus. Each homolog of a pair has the same sites or loci for the same genes. You might recognize that you have one set of chromosomes from your mother and the remaining set from your father. Meiosis reduces the number of chromosomes to a haploid ( 1N ) or single set. This reduction is significant because a cell with a haploid number of chromosomes can fuse with another haploid cell during sexual reproduction and restore the original, diploid number of chromosomes to the new individual. In addition to reducing the number of chromosomes, meiosis shuffles the genetic material so that each resulting cell carries a new and unique set of genes in a process of independent assortment .
As in mitosis, meiosis is preceded by replication of each chromosome to form two chromatids attached at a centromere. However, reduction of the chromosome number and production of new genetic combinations result from two events that don’t occur in mitosis. First, meiosis includes two rounds of chromosome separation. Chromosomes are replicated before the first round, but not before the second round. Thus, the genetic material is replicated once and divided twice. This produces half the original number of chromosomes.
Crossing over between chromatids of homologous chromosomes increases genetic diversity during meiosis I. Synapsis occurs during prophase I as the homologous chromosomes begin to pair up. Credit: Jeremy Seto (CC-BY-NC-SA)
Second, during an early stage of meiosis each chromosome (comprised of two chromatids) pairs along its length with its homolog. This pairing of homologous chromosomes results in a physical touching called synapsis , during which the four chromatids (a tetrad) exchange various segments of genetic material. This exchange of genetic material is called crossing-over and produces new genetic combinations. During crossing-over there is no gain or loss of genetic material. But afterward, each chromatid of the chromosomes contains different segments (alleles) that it exchanged with other chromatid.
Stages and Events of Meiosis
Stages of Meiosis. Credit: Ali Zifan (CC-BY 4.0)
Although meiosis is a continuous process, we can study it more easily by dividing it into stages just as we did for mitosis. Indeed, meiosis and mitosis are similar, and their corresponding stages of prophase, metaphase, anaphase, and telophase have much in common. However, meiosis is longer than mitosis because meiosis involves two nuclear divisions instead of one. These two divisions are called Meiosis I and Meiosis II. The chromosome number is reduced ( reductional division ) during Meiosis I, and chromatids comprising each chromosome are separated in Meiosis II. Each division involves the events of prophase, metaphase, anaphase, and telophase.
Advanced Video Overview of Meiosis
This site is a reading guide for the Molecular and Cell Biology BIO3620 Course.
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.