We are searching data for your request:
Upon completion, a link will appear to access the found materials.
Neurons and muscle cells in adult humans do not have the ability to divide by mitosis, so they can not repair themselves and their cell cycle remains in the interphase. I'm looking for more cells with this feature. Are there other cells with this inability to undergo mitosis in humans or in plants?
In an adult human:
Do nervous cells ever undergo mitosis?
I wonder if all muscle cells do not undergo mitosis or if this inability only applies to cardiac and skeletal muscle cells.
To answer the numbered questions:
- In general, neurons never divide by mitosis. However, I believe you may have unintentionally misphrased your question; there are functional neural stem cells in the adult human brain as well, and these are believed to give rise to new neurons throughout the lifespan of an individual. They have only been found in specific parts of the brain, though, such as the SVZ and the subgranular zone of the hippocampus (Gonzales-Perez 2012; Behnan et al. 2017). It also bears noting that following an external insult (such as DNA damage, oxidative stress, hypoxia), even terminally differentiated neurons can re-enter the cell cycle and go as far as to replicate their DNA, but this tends to result in apoptosis or arrest at the G2/M phase; such aberrant re-entry into the cell cycle is more frequent in brains afflicted by neurodegenerative diseases (Frade & Ovejero-Benito, 2015). In addition, the possibility to induce neurons to proliferate and form cancers has been demonstrated in retinal cells in mice (Ajioka et al. 2007).
- Correct; as with neurons, muscle cells (myocytes) lack the ability to undergo mitosis. But again, I believe what you really want to ask is whether or not muscles maintain a pool of stem cells that can replace lost cells. The answer to this is yes-there are stem cells within skeletal (Wang, Dumont & Rudnicki 2014) and smooth muscles (Majesky et al. 2011) that proliferate in response to damage. According to some studies there are stem cells in the adult heart as well, although their regenerative capacity appears to be very low (Bergmann et al. 2009).
If your intention was indeed to ask about which cells are unable to undergo mitosis, then the answer will likely be a very long list, as no terminally differentiated cell type undergoes division under normal circumstances. This would, for example, include all different blood cell types (erythrocytes, megakaryocytes, neutrophils, eosinophils, basophils, B-cells, T-cells, natural killer cells, mast cells, macrophages), most skin cells (e.g. keratinocytes, melanocytes), epithelial cells (such as those that form the intestinal lining) and such. This would likely include most if not all of the cell types listed here that do not have the "(stem cell)" note next to the links (although I did not confirm this by checking the literature on them).
- Ajioka, Itsuki, et al. "Differentiated horizontal interneurons clonally expand to form metastatic retinoblastoma in mice." Cell 131.2 (2007): 378-390.
- Behnan, Jinan, et al. "Identification and characterization of a new source of adult human neural progenitors." Cell death & disease 8.8 (2017): e2991.
- Bergmann, Olaf, et al. "Evidence for cardiomyocyte renewal in humans." Science 324.5923 (2009): 98-102.
- Frade, José M., and María C. Ovejero-Benito. "Neuronal cell cycle: the neuron itself and its circumstances." Cell cycle 14.5 (2015): 712-720.
- Gonzalez-Perez, Oscar. "Neural stem cells in the adult human brain." Biological and biomedical reports 2.1 (2012): 59.
- Guo, Dayong, and Lynda F. Bonewald. "Advancing our understanding of osteocyte cell biology." Therapeutic advances in musculoskeletal disease 1.2 (2009): 87-96.
- Majesky, Mark W., et al. "Vascular smooth muscle progenitor cells: building and repairing blood vessels." Circulation research 108.3 (2011): 365-377.
- Wang, Yu Xin, Nicolas A. Dumont, and Michael A. Rudnicki. "Muscle stem cells at a glance." (2014): 4543-4548.
Yes, nervous cells never undergo mitosis coz they lose their centriole thereby they are present permanently in G0 phase of cell cycle in which a cell doesn't divide,rather it just carries out it's day to day metabolism.
As far as plants are concerned,they have meristematic cells which are capable of regeneration.But if you know a little bit about plant anatomy then you can say that primary xylem and primary phloem do not divide throughout their lives.
I hope this helps.
- Though neurons do not possess the ability to divide in adult humans, glial cells do, and in case of injury or disease, tend to fill up the gaps created by loss of neurons.
- Most muscle cells lose their ability to divide after maturation. There are a few cases however, like those in disease or damage, when cells that are lost may be replaced. These are discussed below:
Skeletal muscles: Each skeletal muscle fiber arises by fusion of multiple myoblasts during embryonic development. Once fusion occurs, the ability to divide is lost. However, a few myoblasts do persist in mature skeletal muscle as satellite cells, which retain the capacity to fuse with one another or with damaged muscle fibres to regenerate functional muscle fibres. So even though our muscle cells cannot divide, they can, in some cases, be regenerated.
Smooth Muscles: Certain smooth muscle fibers, like those in the uterus retain their capacity for division. Also, new smooth fibers can arise from pericytes, stem cells present in association with small veins and blood capillaries.
Cardiac Muscle: These fibers are unable to divide on their own. However, there have been studies related to this, and it was shown that bone marrow stem cells could be used to regenerate damaged heart tissue.
(Refer: Cell transplantation for cardiac regeneration: where do we stand? Neth Heart J. 2008 Mar; 16(3): 88-95.)
I hope this answers the question.
I read this in Principles of Anatomy and Physiology, 12th Edition, Tortora, G. J., Derrickson, B.
Somatic cells are all cells of the body apart from gamete (sperm cells and egg cells). As such, they include cells that make up different parts of the body including liver cells, skin cells, and bone cells among others.
Mature somatic cells are highly specialized and therefore perform very specific functions.
* The word somatic is derived from the Greek word "Soma" which means body.
The following are some of the most common types:
The cell division process of prokaryotes, called binary fission, is a less complicated and much quicker process than cell division in eukaryotes. Because of the speed of bacterial cell division, populations of bacteria can grow very rapidly. The single, circular DNA chromosome of bacteria is not enclosed in a nucleus, but instead occupies a specific location, the nucleoid, within the cell. As in eukaryotes, the DNA of the nucleoid is associated with proteins that aid in packaging the molecule into a compact size. The packing proteins of bacteria are, however, related to some of the proteins involved in the chromosome compaction of eukaryotes.
The starting point of replication, the origin, is close to the binding site of the chromosome to the plasma membrane (Figure 6.9). Replication of the DNA is bidirectional—moving away from the origin on both strands of the DNA loop simultaneously. As the new double strands are formed, each origin point moves away from the cell-wall attachment toward opposite ends of the cell. As the cell elongates, the growing membrane aids in the transport of the chromosomes. After the chromosomes have cleared the midpoint of the elongated cell, cytoplasmic separation begins. A septum is formed between the nucleoids from the periphery toward the center of the cell. When the new cell walls are in place, the daughter cells separate.
Figure 6.9 The binary fission of a bacterium is outlined in five steps. (credit: modification of work by “Mcstrother”/Wikimedia Commons)
As if following a neatly choreographed dance, the sister chromatids separate, rapidly moving toward the pole to which their microtubule is attached. The cell appears “ stretched ” as the spindle fibers slide past one another, elongating the spindle apparatus and further separating the poles. Shortening of the microtubules by removal of tubulin units pulls the chromosomes closer and closer to the pole. The movement of sister chromatids to opposite sides of the cell completes the equal division and distribution of genetic material.
During telophase, or the “ cleanup ” stage of mitosis, the spindle apparatus is broken down, and the tubulin subunits stand ready to form the cytoskeleton of a new cell. The chromosomes, now clustered in two groups around the poles, uncoil into tangled threads again, and a new nuclear envelope forms around them.
Centriole — An arrangement of microtubules found in most animal cells and in cells of some lower plants and fungi.
Centromere — A constricted region of the chromosome joining two sister chromatids. The centromere is composed of highly repeated DNA sequences approximately 220 units in length.
Chromatin — The name given to loose tangle of DNA strands in the nuclei of cells during periods when they are not dividing. As a cell prepares to divide, chromatin strands condense into compact chromosomes.
Chromosomes — Structures in the eukaryotic cell nucleus consisting of heavily coiled DNA and proteins and carrying genetic information.
Cytokinesis — The physical division of the cytoplasm of a eukaryotic cell to form two daughter cells, each housing a newly formed nuclei.
Cytoskeleton — A network of assorted protein filaments attached to the cell membrane and to various organelles that makes up the framework for cell shape and movement.
DNA — Strands of DNA, or deoxyribonucleic acid, are like long sentences of words composed of a four letter alphabet of nucleotide base pairs: A (adenine) T (thymine) G (guanine) and C (cytosine). The “ words ” contain the instructions for sequences of amino acids making up proteins.
Eukaryotic cell — A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape. In contrast, the more primitive prokaryotic cells are smaller than eukaryotes, and have no nucleus, distinct organelles, or cytoskeleton.
Kinetochore — A disk of protein bound to the centromere to which microtubules attach during mitosis, linking each chromatid to the spindle.
Microtubule — A hollow protein cylinder, about 25 nanometers in diameter, composed of subunits of the protein tubulin. Microtubules grow in length by the addition of tubulin subunits at the end and are shortened by their removal.
Nucleotide — The “ letters ” or basic units of DNA, containing a phosphate group, a 5-carbon sugar, and a ring-shaped nitrogenous base.
Spindle apparatus — An axis of microtubules formed between centrioles in animal cells that aids the equal distribution of chromosomes to new cells being formed.
At this point, each new nucleus contains one copy of each chromosome. Mitosis is complete.
Non-myogenic cell types in the satellite cell niche
An interesting scenario with respect to the satellite cell niche concept is that non-myogenic cell types could accompany activated satellite cells and contribute to the maintenance of their microenvironment. Fibroblasts which are well known to express high levels of ECM molecules and have no direct myogenic potential, are required for efficient muscle regeneration 40 . Therefore, similar to other stem cell niches such as the ones found in the skin, the hematopoietic system, the central nervous system, or the intestine crypt, the satellite cell microenvironment is influenced by distinct tissue resident cell types 41 . Whether fibroblasts engage in cell-cell contact, release tropic signaling molecules for satellite cells or whether they provide adhesive ECM substrates remains to be determined.
In 2005, Collins et al. demonstrated that transplantation of single fibers with their niche associated satellite cells leads to an outstandingly efficient engraftment into immunocompromised mdx recipient mice 25 . Subsequently, the Olwin group modified this approach and isolated single muscle fibers, treated them with basic fibroblast growth factor (bFGF), suspended them in myotoxic solution and then transplanted them into non-immunocompromised wild-type mice 36 . The authors reported that this kind of transplantation led to a highly efficient population of the satellite cell compartment as well as to a dramatic age-persistent increase in muscle size. These results strikingly illustrate that an intact niche allows for the maintenance and manipulation of satellite cells retaining a high stemness ex vivo. An interpretation of the outstanding engraftment efficiency reported by Hall et al. is that the bFGF in the culture medium altered the function of the fiber-associated satellite cells toward a more proliferative phenotype. However, other cell types such as fibroblasts that are attached to muscle fibers must also be taken into consideration. bFGF strongly promotes the proliferation and migration of ECM producing fibroblasts 42 . The stimulation of these cells by the bFGF treatment could increase the availability of survival cues or preserves the structural microenvironment of satellite cells during transplantation. Moreover, bFGF can be sequestered on either cell surface heparan sulfate (HS) or matrix glycosaminoglycans. It is therefore also possible that exposure of single fibers to bFGF-rich culture medium saturates the ECM around the transplanted fibers. This could dramatically prolong its effect on the fiber-associated cells and thereby promote the engraftment of the transplanted satellite cells.
Next to fibroblasts, other non-myogenic cells, including endothelial cells and FAPs, have been implicated in the regulation of myogenesis 17 , 43 . In their niche, satellite cells are closely associated with blood vessels and the number of satellite cells in a given muscle type correlates positively with the abundance of capillaries 19 . It has been proposed that vascular endothelial growth factor (VEGF) signaling from endothelial cells stimulates the proliferation of satellite cells during muscle regeneration while Angiopoietin 1 (Ang1) signaling from periendothelial cells, such as smooth muscle cells and fibroblastic cells, instructs their return to quiescence in later stages of myogenesis 43 , 44 .
Taken together, a variety of non-myogenic cell types contribute to the microenvironment of satellite cells and are likely to play a role in the preservation of their stem cell properties. This knowledge has important implications for future cell therapy. The concept of supportive cell types for the maintenance of stemness has long been applied in case of embryonic stem (ES) cells which are routinely cultured on a supportive layer of feeder fibroblasts 45 . Such a co-culture system, involving key muscle resident non-myogenic cell types, would eventually allow for the ex vivo maintenance and genetic correction of isolated satellite cells without the dramatic loss of stemness that is observed in conventional culture systems.
Function of mitosis
Mitosis makes it possible to divide the genetic information contained in the chromosomes in such a way that two daughter cell nuclei receive the same genetic information again. For this to happen, the genetic material in the nucleus of a mother cell must first have been duplicated – during the preceding interphase of the cell cycle. Each chromosome, which initially consists of one chromatid after nuclear division, has two identical sister chromatids after doubling, which are connected at the centromere. In the mitosis phases, these are compressed, attached, arranged, separated and moved apart so that two spatially different – but identical in number and type of chromosomes – ordered collections are formed, between which the nucleus is then divided.
Chromosomes are connected at the centromere, shown along a dotted line.
In multicellular eukaryotes, mitosis is the prerequisite for the formation of a new cell nucleus and usually – with a few exceptions – also for the formation of new cells. In multicellular organisms such as humans, cell division does not occur in all developed cell lines during their development. Thus, nerve cells and muscle cells do not multiply once differentiation is complete. These cells leave the division cycle post-mitotic and enter the so-called G0 phase, so that the DNA is not replicated at all. Mature human red blood cells can no longer divide because they then lack their cell nucleus and thus mitosis cannot be initiated. Epithelial cells in the intestine and skin, on the other hand, multiply much more frequently than the average and thus renew the inner and outer surfaces of the body.
The actual nuclear division of human cells usually takes about one hour the interphase of the cell cycle of continuously dividing cells, which takes place between the mitosis phases, lasts considerably longer, about 12-24 hours, depending on the cell type. In other organisms, the duration of mitosis can be longer, as in the field bean with about two hours, or shorter, as in the fruit fly, where it is often only 9 minutes long.
Mitosis can be stimulated by various peptides or proteins called mitogens. An example is the maturation promoting factor (MPF), the protein structure of cyclin B with a kinase (CDK 1).
Diploid Chromosome Number
The diploid chromosome number of a cell is calculated using the number of chromosomes in a cell's nucleus. This number is abbreviated as 2n where n stands for the number of chromosomes. For humans, the diploid chromosome number equation is 2n = 46 because humans have two sets of 23 chromosomes (22 sets of two autosomal or non-sex chromosomes and one set of two sex chromosomes).
The diploid chromosome number varies by organism and ranges from 10 to 50 chromosomes per cell. See the following table for the diploid chromosome numbers of various organisms.
Diploid Chromosome Number (2n)
A muscle hernia is a protrusion of muscular tissue through a defect in the containing epimysium (i.e., fascia). 66 This commonly occurs in the anterior and lateral muscle groups of the lower leg (especially the tibialis anterior) but is also recognized in the rectus femoris and the hamstrings ( Fig. 12-17 ). 23 It is thought that the fascia overlying tibialis anterior has an area of potential weakness due to penetrating branches of the peroneal nerve and associated vasculature. 42
There may be a history of previous trauma or surgery, but this is unusual. The hernia usually manifests as a mass that may appear only after exercise or on standing. 23 The hernia may be painful on exertion, but the main problem frequently is cosmetic, and it must be remembered that surgical treatment is not without complications. 67 The clinical differential diagnosis includes an incompetent perforating vein.
Ultrasound can accurately identify the thick echogenic muscle fascia, and any defect is seen as a hypoechoic gap (see Fig. 12-17 ). 23 Dynamic maneuvers can be performed to reproduce the muscle hernia if it is reduced. In an acute herniation, the muscle may appear hyperechoic due to compression of the fascial planes within it. However, if chronic, it may appear hypoechoic due to some degree of edema or necrosis. 23 Because of its small size and variable presentation on dynamic maneuvers, MRI can be relatively ineffective in demonstrating these lesions. 9,23
Science Picture Co/Collection Mix: Subjects/Getty Images
Nerve cells or neurons are the most basic unit of the nervous system. Nerves send signals between the brain, spinal cord, and other body organs via nerve impulses. Structurally, a neuron consists of a cell body and nerve processes. The central cell body contains the neuron's nucleus, associated cytoplasm, and organelles. Nerve processes are "finger-like" projections (axons and dendrites) that extend from the cell body and transmit signals.
10.2 The Cell Cycle
In this section, you will explore the following questions:
- What processes occur during the three stages of interphase?
- How do the chromosomes behave during the mitotic phase?
Connection for AP ® Courses
The cell cycle describes an orderly sequence of events that are highly regulated. In eukaryotes, the cell cycle consists of a long preparatory period (interphase) followed by mitosis and cytokinesis. Interphase is divided into three phases: Gap 1 (G1), DNA synthesis (S), and Gap 2 (G2). Interphase represents the portion of the cell cycle between nuclear divisions. During this phase, preparations are made for division that include growth, duplication of most cellular contents, and replication of DNA. The cell’s DNA is replicated during the S stage. (We will study the details of DNA replication in the chapter on DNA structure and function.) Following the G2 stage of interphase, the cell begins mitosis, the process of active division by which duplicated chromosomes (chromatids) attach to spindle fibers, align themselves along the equator of the cell, and then separate from each other.
Following mitosis, the cell undergoes cytokinesis, the splitting of the parent cell into two daughter cells, complete with a full complement of genetic material. In animal cells, daughter cells are separated by an actin ring, whereas plant cells are separated by the cell plate, which will grow into a new cell wall. Sometimes cells enter a Gap zero (G0) phase, during which they do not actively prepare to divide the G0 phase can be temporary until triggered by an external signal to enter G1, or permanent, such as mature cardiac muscle cells and nerve cells.
Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP ® Biology Curriculum Framework, as shown in the tables. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.
|Big Idea 3||Living systems store, retrieve, transmit and respond to information essential to life processes.|
|Enduring Understanding 3.A||Heritable information provides for continuity of life.|
|Essential Knowledge||3.A.2 In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.|
|Science Practice||6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.|
|Learning Objective||3.7 The student can make predictions about natural phenomena occurring during the cell cycle.|
|Essential Knowledge||3.A.2 In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.|
|Science Practice||1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain.|
|Learning Objective||3.8 The student can describe the events that occur in the cell cycle.|
|Essential Knowledge||3.A.2 In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.|
|Science Practice||5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question.|
|Learning Objective||3.11 The student is able to evaluate evidence provided by data sets to support the claim that heritable information is passed from one generation to another generation through mitosis.|
Discuss with students the difference between diploid and haploid cells. Show students a graphic of the difference.
Discuss with students how in mitosis, the ploidy of the cell remains constant. In a cell culture of human somatic cells, all of the cells will be diploid. In contrast the DNA content, the amount of DNA in a cell culture will change as the cells replicate (undergo S-phase and replicate their DNA). In relative amounts, the initial amount of DNA is considered to be 1x, after S-phase it will be 2x, and so on. More information on the methods used by scientists to track ploidy can be found here.
Introduce mitosis using visuals such as this video.
Students may think that interphase is a resting phase, where no events occur. Remind students that cells are metabolically active in this phase. Cells in G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G0 temporarily until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently.
In addition, students may not realize that the events of mitosis are continuous, and the organization into discrete stages is for convenience. Show students a time lapse video to illustrate this, such as found here.
The stages of the cell cycle can be taught using the images available here.
The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 2.35][APLO 2.15][APLO 2.19][APLO 3.11][APLO 2.33][APLO 2.36][APLO 2.37][APLO 2.31]
The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produces two identical (clone) cells. The cell cycle has two major phases: interphase and the mitotic phase (Figure 10.5). During interphase , the cell grows and DNA is replicated. During the mitotic phase , the replicated DNA and cytoplasmic contents are separated, and the cell divides.
The cell cycle consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei. The cytoplasm is usually divided as well, resulting in two daughter cells.
During interphase, the cell undergoes normal growth processes while also preparing for cell division. In order for a cell to move from interphase into the mitotic phase, many internal and external conditions must be met. The three stages of interphase are called G1, S, and G2.
G1 Phase (First Gap)
The first stage of interphase is called the G1 phase (first gap) because, from a microscopic aspect, little change is visible. However, during the G1 stage, the cell is quite active at the biochemical level. The cell is accumulating the building blocks of chromosomal DNA and the associated proteins as well as accumulating sufficient energy reserves to complete the task of replicating each chromosome in the nucleus.
S Phase (Synthesis of DNA)
Throughout interphase, nuclear DNA remains in a semi-condensed chromatin configuration. In the S phase , DNA replication can proceed through the mechanisms that result in the formation of identical pairs of DNA molecules—sister chromatids—that are firmly attached to the centromeric region. The centrosome is duplicated during the S phase. The two centrosomes will give rise to the mitotic spindle , the apparatus that orchestrates the movement of chromosomes during mitosis. At the center of each animal cell, the centrosomes of animal cells are associated with a pair of rod-like objects, the centrioles , which are at right angles to each other. Centrioles help organize cell division. Centrioles are not present in the centrosomes of other eukaryotic species, such as plants and most fungi.
G2 Phase (Second Gap)
In the G2 phase , the cell replenishes its energy stores and synthesizes proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources for the mitotic phase. There may be additional cell growth during G2. The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis.
The Mitotic Phase
The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells. The first portion of the mitotic phase is called karyokinesis , or nuclear division. The second portion of the mitotic phase, called cytokinesis, is the physical separation of the cytoplasmic components into the two daughter cells.
Link to Learning
Revisit the stages of mitosis at this site.
- Colchicine increases inflammation by inhibiting mitosis. Inhibition of mitosis results in decreased white blood count.
- Colchicine decreases inflammation by inhibiting mitosis. Inhibition of mitosis results in decreased white blood count.
- Colchicine increases inflammation by inhibiting mitosis. Inhibition of mitosis results in increased white blood count.
- Colchicine decreases inflammation by inhibiting mitosis. Inhibition of mitosis results in increased white blood count.
Karyokinesis, also known as mitosis , is divided into a series of phases—prophase, prometaphase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus (Figure 10.7).
Everyday Connection for AP® Courses
These budding plants demonstrate asexual reproduction, one of the main purposes of mitosis. The other two purposes are growth and repair.
Which of the following statements best describes the relationship between mitosis and asexual reproduction?
- Mitosis is a process that can result in asexual reproduction.
- Mitosis is a process that always results in asexual reproduction.
- Asexual reproduction is a process that always results in mitosis.
- Asexual reproduction is a process that can result in mitosis.
- Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus reforms and the cell divide. Cohesin proteins break down and the sister chromatids separate.
- The kinetochore becomes attached to the mitotic spindle. Cohesin proteins break down and the sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus reforms and the cell divides.
- The kinetochore becomes attached to the cohesin proteins. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus reforms and the cell divides.
- The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the cell divide.
During prophase , the “first phase,” the nuclear envelope starts to dissociate into small vesicles, and the membranous organelles (such as the Golgi complex or Golgi apparatus, and endoplasmic reticulum), fragment and disperse toward the periphery of the cell. The nucleolus disappears (disperses). The centrosomes begin to move to opposite poles of the cell. Microtubules that will form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen. The sister chromatids begin to coil more tightly with the aid of condensin proteins and become visible under a light microscope.
During prometaphase , the “first change phase,” many processes that were begun in prophase continue to advance. The remnants of the nuclear envelope fragment. The mitotic spindle continues to develop as more microtubules assemble and stretch across the length of the former nuclear area. Chromosomes become more condensed and discrete. Each sister chromatid develops a protein structure called a kinetochore in the centromeric region (Figure 10.8). The proteins of the kinetochore attract and bind mitotic spindle microtubules. As the spindle microtubules extend from the centrosomes, some of these microtubules come into contact with and firmly bind to the kinetochores. Once a mitotic fiber attaches to a chromosome, the chromosome will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister chromatids will be attached via their kinetochores to microtubules from opposing poles. Spindle microtubules that do not engage the chromosomes are called polar microtubules. These microtubules overlap each other midway between the two poles and contribute to cell elongation. Astral microtubules are located near the poles, aid in spindle orientation, and are required for the regulation of mitosis.
During metaphase , the “change phase,” all the chromosomes are aligned in a plane called the metaphase plate , or the equatorial plane, midway between the two poles of the cell. The sister chromatids are still tightly attached to each other by cohesin proteins. At this time, the chromosomes are maximally condensed.
During anaphase , the “upward phase,” the cohesin proteins degrade, and the sister chromatids separate at the centromere. Each chromatid, now called a chromosome, is pulled rapidly toward the centrosome to which its microtubule is attached. The cell becomes visibly elongated (oval shaped) as the polar microtubules slide against each other at the metaphase plate where they overlap.
During telophase , the “distance phase,” the chromosomes reach the opposite poles and begin to decondense (unravel), relaxing into a chromatin configuration. The mitotic spindles are depolymerized into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell. Nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area.
Cytokinesis , or “cell motion,” is the second main stage of the mitotic phase, during which cell division is completed via the physical separation of the cytoplasmic components into two daughter cells. Division is not complete until the cell components have been apportioned and completely separated into the two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.
In cells such as animal cells that lack cell walls, cytokinesis starts during late anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward, forming a fissure. This fissure, or “crack,” is called the cleavage furrow . The furrow deepens as the actin ring contracts, and eventually the membrane is cleaved in two (Figure 10.9).
In plant cells, a new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking into vesicles and dispersing throughout the dividing cell. During telophase, these Golgi vesicles are transported on microtubules to form a phragmoplast (a vesicular structure) at the metaphase plate. There, the vesicles fuse and coalesce from the center toward the cell walls this structure is called a cell plate . As more vesicles fuse, the cell plate enlarges until it merges with the cell walls at the periphery of the cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall. The Golgi membranes become parts of the plasma membrane on either side of the new cell wall (Figure 10.9).
Science Practice Connection for AP® Courses
- Use a set of pipe cleaners (or other materials as directed by your teacher) that you can use to model chromosomes during mitosis and meiosis:
- Each of the pipe cleaners represents a single, unreplicated chromosome. Each chromosome should differ in size, as they do in most organisms. Assume that your dividing cell contains 3 chromosomes: numbered chromosome 1, 2, and 3.
- Using both members of each homologous pair for chromosomes 1–3, model how the chromosomes would appear in a cell that had just finished the S phase of the cell cycle. Once your teacher has approved your model, have one member of your group document the model by photographing or drawing it.
- Now, repeat step 2 but show the cell at metaphase during mitosis.
- Finally, model the two daughter cells that will result from mitosis. Again, have one member of your group document the model.
- Repeat steps 2–5 for both meiosis I and meiosis II. Remember that you should have four daughter cells at the end of meiosis II. Also remember to ask your teacher for approval and document your model before moving on to the next phase of meiosis.
- Exchange/ copy all of the drawings or photographs that your group took of your models. As a group or individually (as directed by your teacher) create a report to turn in that labels and explain each picture of your model.
- An organism’s ploidy count is the total number of chromosome sets contained in each body cell. Most organisms have a ploidy level of 2, meaning that they have two sets of chromosomes due to presence of homologous pairs. However, some plants are multiploid, meaning they can have ploidy levels greater than 2. The table shows possible multiploid levels of some common crop plants.
|Common name||Multiploid chromosome count||Normal chromosome count|
Analyze the data with a partner of in a group as directed by your teacher. On a separate sheet of paper, answer the following questions.
- How does the multiploid count of the crop plants relate to their normal chromosome count?
- Explain the basis for the relationship you described in part a, in terms of what occurs to chromosomes during replication and meiosis.
- Give one additional example of a possible multiploid chromosome count for each species in the table above.
A. A comparison of the relative time intervals of mitotic stages can be made by completing the task described. In evaluating each time interval, the problem suggests that you assume that the length of time to complete one cell cycle is 24 hours. How can that assumption be tested?
Suppose that you have a growth chamber in which roots of a newly germinated plant can be examined visually with a lens that provides a magnification from which lengths can be determined with a precision of ± 0.05 mm. The field of view can be rotated so that measurements can be made of both the length and diameter of the growing tip. A large number of growing roots can be studied. Tips can be sampled, sectioned, and examined microscopically with a 25× magnification so that estimates of the diameter and length of cells can be made.
Cells in the growing tip of the root rapidly undergo mitosis, just as the whitefish blastula described in Figure 10.10. With increasing distance from the growing tip, the rate at which mitosis occurs slows until tissue is reached in which the initiation of the cell cycle is delayed.
A. Describe a sequence of measurements that could be used to test the assumption that the cell cycle, once started, has a total time interval of 24 hours. Hint: Rather than counting cells, it might be useful to measure the length of the root tip and the average length of a cell.
B. Using the data obtained from your measurements described in part A, how can the rate of cell division be calculated?
An experiment that is perhaps similar to one you have proposed was conducted previously (Beemster and Baxter, 1998), and the results are shown in the table.
|Distance (mm)||Per hour|
|0||0.035 ± 0.01|
|0.1||0.047 ± 0.005|
|0.2||0.044 ± 0.01|
|0.3||0.039 ± 0.01|
|0.4||0.042 ± 0.01|
|0.5||0.031 ± 0.005|
C. Using these data, estimate the length of time of the cell cycle, including an estimate of precision by calculating the standard deviation.
Growth factors are signals that initiate cell division in eukaryotes. (The data in the table above show that cells in the plant root less than a mm from the root tip are showing a reduction of growth rate.) The interaction of two plant hormones, auxin and brassinosteroids, have been shown [Chaiwanon and Wang, Cell, 164(6), 1257, 2016] to regulate cell division in root tips. Auxin concentrations are higher near the root tip and decrease with distance from the tip. Brassinosteroids decrease in concentration near the root tip. Auxin is actively transported between cells, whereas brassinosteroids have limited transport between cells.
D. Based on these data and the observed distribution of brassinosteroids and auxin in the growing root, predict a mechanism for their interaction and justify the claim that brassinosteroid synthesis is negatively regulated by auxin transported to the cell, and that auxin is positively regulated and amplified.
Think About It
Chemotherapy drugs such as vincristine and colchicines disrupt mitosis by binding to tubulin (the subunit of microtubules) and interfering with microtubule assembly and disassembly. What mitotic structure is targeted by these drugs, and what effect would this have on cell division?
The first activity is an application of Learning Objective 3.8 and Science Practice 1.2 because students are modelling steps of the cell cycle, including mitosis and meiosis. A variety of materials can be used to represent chromosomes in the model as long as the students can easily distinguish between the three chromosomes (such as by having different-sized pipe cleaners) as well as distinguish between homologs (such as by using two colors of pipe cleaner). Be sure to provide enough chromosomes to represent sister chromatids in both the mitosis and meiosis models. The critical point to stress is that, in modelling mitosis, students should place homologous chromosomes (each with a sister chromatid) above and below each other during metaphase, ensuring a sister chromosome from each homolog enters each daughter cell. Conversely, in metaphase I of meiosis, the homologous chromosomes (each with a sister chromatid) will pair together side-by-side so that each cell only receives one of the two homologs.
The second activity is an application of Learning Objective 3.11 and Science Practice 5.3 because students are using their knowledge of meiosis to explain and predict possible ploidy levels in crop plants. Students should work in pairs or as a group.
An expanded lab investigation for mitosis and meiosis, involving studying onion cells undergoing mitosis (part 2), and karyotype analysis (part 3) is available from the College Board’s ® AP Biology Investigative Labs: An Inquiry-Based Approach in Investigation 7.
- The multiploid count is always a whole-number multiple of the normal chromosome count.
- Before meiosis (and mitosis) all of an organism’s chromosomes are replicated before any segregation takes place. Therefore, ploidy levels will always involve whole-number multiples of the original chromosome levels.
- Answers will vary but all answers should be whole-number multiples of the normal chromosome numbers.
The Think About It question is an application of Learning Objective 3.7 and Science Practice 6.4 because the student must be able to describe the events that occur in the cell cycle before you can make a prediction about the effects of a disruption in mitosis.
The mitotic spindle is formed of microtubules. Microtubules are polymers of the protein tubulin therefore, it is the mitotic spindle that is disrupted by these drugs. Without a functional mitotic spindle, the chromosomes will not be sorted or separated during mitosis. The cell will arrest in mitosis and die.
Not all cells adhere to the classic cell cycle pattern in which a newly formed daughter cell immediately enters the preparatory phases of interphase, closely followed by the mitotic phase. Cells in G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G0 temporarily until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently.
Scientific Method Connection
Determine the Time Spent in Cell Cycle Stages
Problem: How long does a cell spend in interphase compared to each stage of mitosis?
Background: A prepared microscope slide of blastula cross-sections will show cells arrested in various stages of the cell cycle. It is not visually possible to separate the stages of interphase from each other, but the mitotic stages are readily identifiable. If 100 cells are examined, the number of cells in each identifiable cell cycle stage will give an estimate of the time it takes for the cell to complete that stage.
Problem Statement: Given the events included in all of interphase and those that take place in each stage of mitosis, estimate the length of each stage based on a 24-hour cell cycle. Before proceeding, state your hypothesis.
Test your hypothesis: Test your hypothesis by doing the following:
- Place a fixed and stained microscope slide of whitefish blastula cross-sections under the scanning objective of a light microscope.
- Locate and focus on one of the sections using the scanning objective of your microscope. Notice that the section is a circle composed of dozens of closely packed individual cells.
- Switch to the low-power objective and refocus. With this objective, individual cells are visible.
Switch to the high-power objective and slowly move the slide left to right, and up and down to view all the cells in the section (Figure 10.10). As you scan, you will notice that most of the cells are not undergoing mitosis but are in the interphase period of the cell cycle.
Record your observations: Make a table similar to Table 10.2 in which you record your observations.
|Phase or Stage||Individual Totals||Group Totals||Percent|
Analyze your data/report your results: To find the length of time whitefish blastula cells spend in each stage, multiply the percent (recorded as a decimal) by 24 hours. Make a table similar to Table 10.3 to illustrate your data.