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When do the spindle fibers attach to the chromosome

When do the spindle fibers attach to the chromosome


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At what phase does this occur in mitosis (or even meiosis); some text books say prophase while others say metaphase.


Your confusion comes from the inconsistent terminology in the literature. The attachement of the spindles to the chromosomes happens in the Prometaphase [1, 2]. The name already suggests that this is not one of the classical phases is cell division, but lies somewhere between Pro- and metaphase. Wikipedia states:

Prometaphase is sometimes simply included as part of the end of prophase and early metaphase.

If you really need to stick to the classical scheme of pro-, meta-, ana- and telophase, I would personally suggest to treat the attachement (not the formation!) of spindles as a part of the metaphase.


What would happens if chromosomes did not attach to spindle fibers?

This is answered comprehensively here. In this regard, what stage do spindle fibers attach to chromosomes?

Mitosis: In Summary In prometaphase, kinetochores appear at the centromeres and mitotic spindle microtubules attach to kinetochores. In metaphase, chromosomes are lined up and each sister chromatid is attached to a spindle fiber. In anaphase, sister chromatids (now called chromosomes) are pulled toward opposite poles.

One may also ask, where do spindle microtubules attach to chromosomes? Centrioles begin moving to opposite ends of the cell, and microtubules extend from the centrioles and begin to attach to the centromeres of chromosomes. Eventually, the microtubules extending from centrioles on opposite poles of the cell attach to every centromere and develop into spindle fibers.

Consequently, what would happen if a cell had faulty spindle fibers?

Predict what would happen if an individual had faulty spindle fibers. Cells would have the wrong number of chromsomes (wouldn't be separated during anaphase). Cells would have too many chromosomes b/c they would not be divided into 2 cells.

What are spindle fibers attach to?

Spindle fibers move chromosomes during cell division by attaching to chromosome arms and centromeres. A centromere is the specific region of a chromosome where duplicates are linked. Identical, joined copies of a single chromosome are known as sister chromatids.


Spindle Fibers and Chromosome Movement

Spindle fiber and cell movement occur when microtubules and motor proteins interact. Motor proteins, which are powered by ATP, are specialized proteins that actively move microtubules. Motor proteins such as dyneins and kinesins move along microtubules whose fibers either lengthen or shorten. The disassembly and reassembly of microtubules produces the movement needed for chromosome movement and cell division to occur.

Spindle fibers move chromosomes during cell division by attaching to chromosome arms and centromeres. A centromere is the specific region of a chromosome where duplicates are linked. Identical, joined copies of a single chromosome are known as sister chromatids. The centromere is also where protein complexes called kinetochores are found.

Kinetochores generate fibers that attach sister chromatids to spindle fibers. Kinetochore fibers and spindle polar fibers work together to separate chromosomes during mitosis and meiosis. Spindle fibers that don't contact chromosomes during cell division extend from one cell pole to the other. These fibers overlap and push cell poles away from one another in preparation for cytokinesis.


When do the spindle fibers attach to the chromosome - Biology

M phase is relatively brief and consists of nuclear division (mitosis) and cytoplasmic division (cytokinesis). In mitosis, replicated chromosomes divide, leading to identical daughter nuclei with the same number of chromosomes and the same genetic composition as the parents. In human cells, which have 23 pairs of chromosomes, the number of chromosomes (2n = 46) remains unchanged from the beginning till the end of mitosis.

The phases are in the following sequence: prophase, metaphase, anaphase, and telophase (see figure below). The movement of chromosomes is facilitated by a structure called the mitotic spindle, which consists of microtubules and associated proteins. Spindles extend from centrioles on each of the two sides (or poles) of the cell, attach to the chromosomes and align them, and pull the sister chromatids apart.

Chromosomes are usually visible under light microscope. We can determine the number of chromosomes [and chromatid(s) per chromosome] for each stage in mitosis.

Chromatin in the nucleus begins to condense and becomes visible under the light microscope as chromosomes, each with two chromatids that are held together at the centromere. The nuclear envelope breaks down. Centrioles begin moving to opposite ends of the cell, and microtubules extend from the centrioles and begin to attach to the centromeres of chromosomes. Eventually, the microtubules extending from centrioles on opposite poles of the cell attach to every centromere and develop into spindle fibers.

By growing on one end and shrinking on the other, spindle fibers align the chromosomes along the middle of the cell nucleus, approximately equidistant from the spindle poles. This imaginary line where the chromosomes are aligned is called the metaphase plate and is equidistant from the centrioles, which are on opposite poles of the cell.

After all of the chromosomes are aligned on the metaphase plate, each pair of sister chromatids splits at the centromere, separates and moves along the shortening spindle fibers to opposite sides of the cell. Now, the number of centromeres and chromosomes within the cell is doubled. Note that each separated chromosome has only one chromatid.

Chromosomes, each with one chromatid, arrive at opposite poles of the cell, and a new nuclear membrane forms around each of the two new daughter nuclei, which are identical to each other. The spindle fibers begin to disperse and the chromosomes decondense chromosomes are no longer visible under the light microscope. Notice that the number of centromeres and chromosomes within each of the two new cells is identical to the parent cell before it underwent mitosis.

Cytokinesis

Although the nucleus divides during mitosis, the cytoplasm does not. Following mitosis, cytokinesis is the process where the cytoplasm of a eukaryotic cell is divided to form two identical daughter cells. In animal cells, the center of the cell contracts, pinching the cell into two daughter cells, each of which contains a nucleus with a complete genome.


Domains and Higher Order Structures

Early electron micrograph images of eukaryotic metaphase chromosomes gave the impression of looped fibers extending out from the central axis of each chromatid. Subsequent analysis by microscopic and biochemical techniques suggests that stretches of chromosome approximately forty thousand to eighty thousand nucleotide pairs long may be anchored to a nuclear scaffold or matrix . These points of anchorage may serve to organize or spatially restrict chromosomes during interphase. These same anchor points may coalesce at metaphase to condense chromosomes for mitotic segregation.

Chromosomes exist to hold genes, of course, and some structural features of the chromosome may serve to separate genes from one another to help regulate transcription. Gene transcription in higher eukaryotes is controlled by regulatory elements that, in some cases, are located hundreds of thousands of nucleotides away from their target promoters. How can such elements be prevented from activating other nearby promoters? Experiments suggest that there are DNA sequences that act as boundaries or barriers to prevent the distant regulatory elements from one gene from contacting the promoters of genes located elsewhere on the same chromosome. In some cases, these genetic domain borders may be equivalent to the nuclear scaffold/matrix anchorage points, but in other cases these activities appear separable.


Contents

Attachment of microtubules to chromosomes is mediated by kinetochores, which actively monitor spindle formation and prevent premature anaphase onset. Microtubule polymerization and depolymerization dynamic drive chromosome congression. Depolymerization of microtubules generates tension at kinetochores [3] bipolar attachment of sister kinetochores to microtubules emanating from opposite cell poles couples opposing tension forces, aligning chromosomes at the cell equator and poising them for segregation to daughter cells. Once every chromosome is bi-oriented, anaphase commences and cohesin, which couples sister chromatids, is severed, permitting the transit of the sister chromatids to opposite poles.

The cellular spindle apparatus includes the spindle microtubules, associated proteins, which include kinesin and dynein molecular motors, condensed chromosomes, and any centrosomes or asters that may be present at the spindle poles depending on the cell type. [4] The spindle apparatus is vaguely ellipsoid in cross section and tapers at the ends. In the wide middle portion, known as the spindle midzone, antiparallel microtubules are bundled by kinesins. At the pointed ends, known as spindle poles, microtubules are nucleated by the centrosomes in most animal cells. Acentrosomal or anastral spindles lack centrosomes or asters at the spindle poles, respectively, and occur for example during female meiosis in most animals. [5] In this instance, a Ran GTP gradient is the main regulator of spindle microtubule organization and assembly. In fungi, spindles form between spindle pole bodies embedded in the nuclear envelope, which does not break down during mitosis.

Microtubule-associated proteins and spindle dynamics Edit

The dynamic lengthening and shortening of spindle microtubules, through a process known as dynamic instability determines to a large extent the shape of the mitotic spindle and promotes the proper alignment of chromosomes at the spindle midzone. Microtubule-associated proteins (MAPs) associate with microtubules at the midzone and the spindle poles to regulate their dynamics. γ-tubulin is a specialized tubulin variant that assembles into a ring complex called γ-TuRC which nucleates polymerization of α/β tubulin heterodimers into microtubules. Recruitment of γ-TuRC to the pericentrosomal region stabilizes microtubule minus-ends and anchors them near the microtubule-organizing center. The microtubule-associated protein Augmin acts in conjunction with γ-TURC to nucleate new microtubules off of existing microtubules. [6]

The growing ends of microtubules are protected against catastrophe by the action of plus-end microtubule tracking proteins (+TIPs) to promote their association with kinetochores at the midzone. CLIP170 was shown to localize near microtubule plus-ends in HeLa cells [7] and to accumulate in kinetochores during prometaphase. [8] Although how CLIP170 recognizes plus-ends remains unclear, it has been shown that its homologues protect against catastrophe and promote rescue, [9] [10] suggesting a role for CLIP170 in stabilizing plus-ends and possibly mediating their direct attachment to kinetochores. [11] CLIP-associated proteins like CLASP1 in humans have also been shown to localize to plus-ends and the outer kinetochore as well as to modulate the dynamics of kinetochore microtubules (Maiato 2003). CLASP homologues in Drosophila, Xenopus, and yeast are required for proper spindle assembly in mammals, CLASP1 and CLASP2 both contribute to proper spindle assembly and microtubule dynamics in anaphase. [12] Plus-end polymerization may be further moderated by the EB1 protein, which directly binds the growing ends of microtubules and coordinates the binding of other +TIPs. [13] [14]

Opposing the action of these microtubule-stabilizing proteins are a number of microtubule-depolymerizing factors which permit the dynamic remodeling of the mitotic spindle to promote chromosome congression and attainment of bipolarity. The kinesin-13 superfamily of MAPs contains a class of plus-end-directed motor proteins with associated microtubule depolymerization activity including the well-studied mammalian MCAK and Xenopus XKCM1. MCAK localizes to the growing tips of microtubules at kinetochores where it can trigger catastrophe in direct competition with stabilizing +TIP activity. [15] These proteins harness the energy of ATP hydrolysis to induce destabilizing conformational changes in protofilament structure that cause kinesin release and microtubule depolymerization. [16] Loss of their activity results in numerous mitotic defects. [15] Additional microtubule destabilizing proteins include Op18/stathmin and katanin which have roles in remodeling the mitotic spindle as well as promoting chromosome segregation during anaphase. [17]

The activities of these MAPs are carefully regulated to maintain proper microtubule dynamics during spindle assembly, with many of these proteins serving as Aurora and Polo-like kinase substrates. [17] [18]

In a properly formed mitotic spindle, bi-oriented chromosomes are aligned along the equator of the cell with spindle microtubules oriented roughly perpendicular to the chromosomes, their plus-ends embedded in kinetochores and their minus-ends anchored at the cell poles. The precise orientation of this complex is required to ensure accurate chromosome segregation and to specify the cell division plane. However, it remains unclear how the spindle becomes organized. Two models predominate the field, which are synergistic and not mutually exclusive. In the search-and-capture model, the spindle is predominantly organized by the poleward separation of centrosomal microtubule organizing centers (MTOCs). Spindle microtubules emanate from centrosomes and 'seek' out kinetochores when they bind a kinetochore they become stabilized and exert tension on the chromosomes. In an alternative self assembly model, microtubules undergo acentrosomal nucleation among the condensed chromosomes. Constrained by cellular dimensions, lateral associations with antiparallel microtubules via motor proteins, and end-on attachments to kinetochores, microtubules naturally adopt a spindle-like structure with chromosomes aligned along the cell equator.

Centrosome-mediated "search-and-capture" model Edit

In this model, microtubules are nucleated at microtubule organizing centers and undergo rapid growth and catastrophe to 'search' the cytoplasm for kinetochores. Once they bind a kinetochore, they are stabilized and their dynamics are reduced. The newly mono-oriented chromosome oscillates in space near the pole to which it is attached until a microtubule from the opposite pole binds the sister kinetochore. This second attachment further stabilizes kinetochore attachment to the mitotic spindle. Gradually, the bi-oriented chromosome is pulled towards the center of the cell until microtubule tension is balanced on both sides of the centromere the congressed chromosome then oscillates at the metaphase plate until anaphase onset releases cohesion of the sister chromatids.

In this model, microtubule organizing centers are localized to the cell poles, their separation driven by microtubule polymerization and 'sliding' of antiparallel spindle microtubules with respect to one another at the spindle midzone mediated by bipolar, plus-end-directed kinesins. [19] [20] Such sliding forces may account not only for spindle pole separation early in mitosis, but also spindle elongation during late anaphase.

Chromatin-mediated self-organization of the mitotic spindle Edit

In contrast to the search-and-capture mechanism in which centrosomes largely dictate the organization of the mitotic spindle, this model proposes that microtubules are nucleated acentrosomally near chromosomes and spontaneously assemble into anti-parallel bundles and adopt a spindle-like structure. [21] Classic experiments by Heald and Karsenti show that functional mitotic spindles and nuclei form around DNA-coated beads incubated in Xenopus egg extracts and that bipolar arrays of microtubules are formed in the absence of centrosomes and kinetochores. [22] Indeed, it has also been shown that laser ablation of centrosomes in vertebrate cells inhibits neither spindle assembly nor chromosome segregation. [23] Under this scheme, the shape and size of the mitotic spindle are a function of the biophysical properties of the cross-linking motor proteins. [24]

The guanine nucleotide exchange factor for the small GTPase Ran (Regulator of chromosome condensation 1 or RCC1) is attached to nucleosomes via core histones H2A and H2B. [25] Thus, a gradient of GTP-bound Ran is generated around the vicinity of mitotic chromatin. Glass beads coated with RCC1 induce microtubule nucleation and bipolar spindle formation in Xenopus egg extracts, revealing that the Ran GTP gradient alone is sufficient for spindle assembly. [26] The gradient triggers release of spindle assembly factors (SAFs) from inhibitory interactions via the transport proteins importin β/α. The unbound SAFs then promote microtubule nucleation and stabilization around mitotic chromatin, and spindle bipolarity is organized by microtubule motor proteins. [27]

Spindle assembly is largely regulated by phosphorylation events catalyzed by mitotic kinases. Cyclin dependent kinase complexes (CDKs) are activated by mitotic cyclins, whose translation increases during mitosis. CDK1 (also called CDC2) is considered the main mitotic kinase in mammalian cells and is activated by Cyclin B1. Aurora kinases are required for proper spindle assembly and separation. [28] Aurora A associates with centrosomes and is believed to regulate mitotic entry. Aurora B is a member of the chromosomal passenger complex and mediates chromosome-microtubule attachment and sister chromatid cohesion. Polo-like kinase, also known as PLK, especially PLK1 has important roles in the spindle maintenance by regulating microtubule dynamics. [29]

By the end of DNA replication, sister chromatids are bound together in an amorphous mass of tangled DNA and protein that would be virtually impossible to partition into each daughter cell. To avoid this problem, mitotic entry triggers a dramatic reorganization of the duplicated genome. Sister chromatids are disentangled and resolved from one another. Chromosomes also shorten in length, up to 10,000 fold in animal cells, [30] in a process called condensation. Condensation begins in prophase and chromosomes are maximally compacted into rod-shaped structures by the time they are aligned in the middle of the spindle at metaphase. This gives mitotic chromosomes the classic “X” shape seen in karyotypes, with each condensed sister chromatid linked along their lengths by cohesin proteins and joined, often near the center, at the centromere. [30] [31] [32]

While these dynamic rearrangements are vitally important to ensure accurate and high-fidelity segregation of the genome, our understanding of mitotic chromosome structure remains largely incomplete. A few specific molecular players have been identified, however: Topoisomerase II uses ATP hydrolysis to catalyze decatenation of DNA entanglements, promoting sister chromatid resolution. [33] Condensins are 5-subunit complexes that also use ATP-hydrolysis to promote chromosome condensation. [34] Experiments in Xenopus egg extracts have also implicated linker Histone H1 as an important regulator of mitotic chromosome compaction. [35]

The completion of spindle formation is a crucial transition point in the cell cycle called the spindle assembly checkpoint. If chromosomes are not properly attached to the mitotic spindle by the time of this checkpoint, the onset of anaphase will be delayed. [36] Failure of this spindle assembly checkpoint can result in aneuploidy and may be involved in aging and the formation of cancer. [37]

Cell division orientation is of major importance for tissue architecture, cell fates and morphogenesis. Cells tend to divide along their long axis according to the so-called Hertwig rule. The axis of cell division is determined by the orientation of the spindle apparatus. Cells divide along the line connecting two centrosomes of the spindle apparatus. After formation, the spindle apparatus undergoes rotation inside the cell. The astral microtubules originating from centrosomes reach the cell membrane where they are pulled towards specific cortical clues. In vitro, the distribution of cortical clues is set up by the adhesive pattern. [38] In vivo polarity cues are determined by localization of Tricellular junctions localized at cell vertices. [39] The spatial distribution of cortical clues leads to the force field that determine final spindle apparatus orientation and the subsequent orientation of cell division.


Control of the Cell Cycle

The length of the cell cycle is highly variable even within the cells of an individual organism. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development to an average of two to five days for epithelial cells, or to an entire human lifetime spent in G0 by specialized cells such as cortical neurons or cardiac muscle cells. There is also variation in the time that a cell spends in each phase of the cell cycle. When fast-dividing mammalian cells are grown in culture (outside the body under optimal growing conditions), the length of the cycle is approximately 24 hours. In rapidly dividing human cells with a 24-hour cell cycle, the G1 phase lasts approximately 11 hours. The timing of events in the cell cycle is controlled by mechanisms that are both internal and external to the cell.


The Cell Cycle and Cancer

  1. Cancer is a cellular growth disorder that occurs when cells divide uncontrollably i.e., cancer results from the loss of control and a disruption of the cell cycle.
  2. Most cancers begin as abnormal cell growth that is benign, or not cancerous
  3. When additional mutations occur, the growth becomes malignant, or cancerous, and possesses the ability to spread.
  4. Characteristics of Cancer Cells
  5. Cancer cells lack differentiation.
  6. Unlike normal cells that differentiate into muscle or nerves cells, cancer cells have an abnormal form and are nonspecialized.
  7. Normal cells enter the cell cycle only about 50 times cancer cells are immortal in that they can enter the cell cycle repeatedly.
  8. Cancer cells have abnormal nuclei.
  9. The nuclei may be enlarged and may have an abnormal number of chromosomes.
  10. The chromosomes have mutated some chromosomes may be duplicated or deleted.
  11. Cancer cells do not undergo apoptosis
  12. Whereas ordinary cells with DNA damage undergo apoptosis, cancer cells do not.
  13. Cancer cells form tumors.
  14. Normal cells are anchored and stop dividing when in contact with other cells i.e., they exhibit contact inhibition.
  15. Cancer cells invade and destroy normal tissue and their growth is not inhibited.
  16. Cancer cells pile on top of each other to form a tumor.
  17. Cancer cells undergo metastasis and angiogenesis.
  18. A benign tumor is encapsulated and does not invade adjacent tissue.
  19. Many types of cancer can undergo metastasis, in which new tumors form which are distant from the primary tumor.
  20. Angiogenesis, the formation of new blood vessels, is required to bring nutrients and oxygen to the tumor.

B.Origin of Cancer

  1. Proto-oncogenes code for proteins that stimulate the cell cycle and prevent apoptosis.
  2. Tumor-suppressor genes code for proteins that inhibit the cell cycle and promote apoptosis.
  3. Mutations of either of these genes can cause cancer.
  4. Proto-oncogenes are at the end of a stimulatory pathway from the plasma membrane to the nucleus a growth factor binding at the plasma membrane can result in turning on an oncogene.
  5. Proto-oncogenes can undergo mutation to become oncogenes (cancer-causing genes).
  6. An oncogene may code for a faulty receptor in the stimulatory pathway.
  7. Or an oncogene can specify an abnormal protein product or abnormally high levels of a normal product that stimulates the cell cycle.
  8. Tumor-suppressor genes are at the end of an inhibitory pathway a growth-inhibitory factor can result in turning on a tumor suppressor gene that inhibits the cell cycle.
  9. The balance between stimulatory and inhibitory signals determines whether proto-oncogenes or tumor-suppressor genes are active, and therefore whether or not cell division occurs.
  10. Researchers have identified about a half dozen tumor-suppressor genes.
  11. The RB tumor-suppressor gene prevents retinoblastoma, a cancer of the retina, and has been found to malfunction in cancers of the breast, prostate, bladder, and small-cell lung carcinoma.
  12. The p53 tumor-suppressor gene is more frequently mutated in human cancers than any other known gene it normally functions to trigger cell cycle inhibitors and stimulate apoptosis.
  13. In some cancer cells, mutation of an enzyme that regulates the length of telomeres causes the telomeres to remain at a constant length, which allows the cancer cells to continue dividing.

Prokaryotic Cell Division

  1. Unicellular organisms reproduce via asexual reproduction, in which the offspring are genetically identical to the parent.
  2. The Prokaryotic Chromosome
  3. Prokaryotic cells (bacteria and archaea) lack a nucleus and other membranous organelles.
  4. The prokaryotic chromosome is composed of DNA and associated proteins, but much less protein than eukaryotic chromosomes.
  5. The chromosome appears as a nucleoid, an irregular-shaped region that is not enclosed by a membrane.
  6. The chromosome is a circular loop attached to the inside of the plasma membrane it is about 1,000 times the length of the cell.
  7. Binary Fission
  8. Binary fission of prokaryotic cells produces two genetically identical daughter cells.
  9. Before cell division, DNA is replicated—both chromosomes are attached to a special site inside the plasma membrane.
  10. The two chromosomes separate as a cell lengthens and pulls them apart.
  11. When the cell is approximately twice its original length, the plasma membrane grows inward, a septum (consisting of new cell wall and plasma membrane) forms, dividing the cell into two daughter cells.
  12. The generation time of bacteria depends on the species and environmental conditions Escherichia coli’s generation time is about 20 minutes.
  13. Comparing Prokaryotes and Eukaryotes
  14. Both binary fission and mitosis ensure that each daughter cell is genetically identical to the parent.
  15. Bacteria and protists use asexual reproduction to produce identical offspring.
  16. In multicellular fungi, plants, and animals, cell division is part of the growth process that produces and repairs the organism.
  17. Prokaryotes have a single chromosome with mostly DNA and some associated protein there is no spindle apparatus.
  18. Eukaryotic cells have chromosomes with DNA and many associated proteins histone proteins organize the chromosome.
  19. The spindle is involved in distributing the daughter chromosomes to the daughter nuclei.
  • The Self-Quiz includes multiple-choice questions from the end of the textbook chapter.
  • The Practice Test will test your knowledge of the content in the textbook chapter.

Aster (cell biology)

An aster is a cellular structure shaped like a star, consisting of a centrosome and its associated microtubules during the early stages of mitosis in an animal cell. [1] : 221 Asters do not form during mitosis in plants. Astral rays, composed of microtubules, radiate from the centrosphere and look like a cloud. Astral rays are one variant of microtubule which comes out of the centrosome others include kinetochore microtubules and polar microtubules.

During mitosis, there are five stages of cell division: Prophase, Prometaphase, Metaphase, Anaphase, and Telophase. During prophase, two aster-covered centrosomes migrate to opposite sides of the nucleus in preparation of mitotic spindle formation. During prometaphase there is fragmentation of the nuclear envelope and formation of the mitotic spindles. During metaphase, the kinetochore microtubules extending from each centrosome connect to the centromeres of the chromosomes. Next, during anaphase, the kinetochore microtubules pull the sister chromatids apart into individual chromosomes and pull them towards the centrosomes, located at opposite ends of the cell. This allows the cell to divide properly with each daughter cell containing full replicas of chromosomes. In some cells, the orientation of the asters determines the plane of division upon which the cell will divide. [2]

Astral microtubules are a subpopulation of microtubules, which only exist during and immediately before mitosis. They are defined as any microtubule originating from the centrosome which does not connect to a kinetochore. [3] Astral microtubules develop in the actin skeleton and interact with the cell cortex to aid in spindle orientation. They are organized into radial arrays around the centrosomes. The turn-over rate of this population of microtubules is higher than any other population.

The role of astral microtubules is assisted by dyneins specific to this role. These dyneins have their light chains (static portion) attached to the cell membrane, and their globular parts (dynamic portions) attached to the microtubules. The globular chains attempt to move towards the centrosome, but as they are bound to the cell membrane, this results in pulling the centrosomes towards the membrane, thus assisting cytokinesis.

Astral microtubules are not required for the progression of mitosis, but they are required to ensure the fidelity of the process. The function of astral microtubules can be generally considered as determination of cell geometry. They are absolutely required for correct positioning and orientation of the mitotic spindle apparatus, and are thus involved in determining the cell division site based on the geometry and polarity of the cells.

The maintenance of astral microtubules is dependent on the integrity of centrosome. It is also dependent on several microtubule-associated proteins such as EB1 and adenomatous polyposis coli (APC).

Growth of Microtubules

Polymerization and nucleation are the two microscopic processes in with the growth of asters occur. At the negative ends of the aster , centrosomes will nucleate (form a nucleus) and anchor to the microtubules. At the positive end, polymerization of the aster will occur towards the outer. Cortical Dyenein, a motor protein moves along the microtubules of the cell and plays a key role in the growth and inhibition of aster microtubules. A dyenein that is barrier-attached can inhibit and trigger growth.

  1. ^Campbell NA, Reece JB (2005). Biology (7th ed.). San Francisco, CA: Benjamin Cummings. ISBN0-8053-7171-0 .
  2. ^
  3. Lodish HF, Darnell DE (2008). Molecular Cell Biology (6th ed.). New York: W. H. Freeman and Company. pp. 782–783. ISBN978-0-7167-7601-7 .
  4. ^Mitosis, Molecular Biology of the Cell, Albert et al 4th Edition.
  • Ishihara, Keisuke, et al. "Physical basis of large microtubule aster growth." eLife, vol. 5, 2016. Gale OneFile: Health and Medicine, link.gale.com/apps/doc/A476395269/HRCA?u=cuny_hunter&sid=HRCA&xid=5e6ad228. Accessed 28 Apr. 2021.
  • Laan, Liedewij et al. “Cortical Dynein Controls Microtubule Dynamics to Generate Pulling Forces That Position Microtubule Asters.” Cell (Cambridge) 148.3 (2012): 502–514. Web.

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MEIOSIS

  • Observed in sexually reproducing organisms, meiosis is the process of cell division that leads to the formation of reproductive cells or gametes in higher organisms.
  • A somatic cell undergoes two successive divisions to produce four gametocyte cells, each having half the number of chromosomes as the parent cell.

What are somatic cells?

Any cell other than a reproductive cell is known as a somatic cell (soma = body).

What are gametocytes?

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A cell that leads to the formation of a gamete upon division is a gametocyte.

In meiosis, cell division stages can be broken down into two subcategories: Meiosis I and Meiosis II

  • Meiosis I has 5 different sub-stages, known as Prophase I, Metaphase I, Anaphase I, Telophase I, and Cytokinesis I
  • Meiosis II has 5 different sub-stages, known as Prophase II, Metaphase II, Anaphase II, Telophase II, and Cytokinesis II

Interphase I

  • Before these four stages of cell division begin, the initial process is similar to that of Interphase in mitosis, except the substage G2.
  • Cell growth and development are still the key factors initially, and once the cell is prepared for division, chromosome replication begins. Each chromosome, thus, replicates its genetic material, forming an identical twin.
  • Microscopically, chromosomes now appear in the form of two sister chromatids, joint together at the centromere.

Prophase I

At this stage, the cell has replicated its DNA material and organelles. The physical division begins with many changes occurring at the same time. Thus, for better understanding, Prophase I is demarcated into the following five sub-stages:

Leptonema

  • The chromosomes begin to condense and are now microscopically visible.
  • It is important to note that they are still condensing and not condensed at this point.
  • At this stage, each chromosome (consisting its identical chromatids) starts looking for its homogeneous pair-mate.

*Note: When two chromosomes are physically same in all aspects and have similar genetic coding, then one of them is said to be the homologue of the other. In other words, they are a homologous chromosome pair.

Zygonema

  • In this stage, each chromosome finds its homologue and begins ‘rough pairing.’
  • Rough pairing is when these chromosomes align next to each other.
  • The chromosomes continue condensing throughout this stage.
  • Thereafter, the chromosomes actually begin pairing this process is known as ‘synapsis.’
  • The two chromosomes are joint throughout their length by a ‘synaptonemal complex.’ The point where pairing begins is not specific.

Pachynema

  • The sister chromatids disassociate from one another in this stage, while the homologous chromosomes remain connected.
  • Chromatids from homologous chromosomes may exchange genetic information by physically swapping their parts. This process is known as ‘crossing over’ of chromosomes, and the exact point of crossing over is called ‘chiasmata.’
  • It is the most important step because the genetic coding changes here, leading to a variation in the genetic information passed along to the daughter cells.

Diplonema

  • The synaptonemal complex that holds any two chromosomes together begins to dissolve at this stage.
  • Eventually, all the chromosome sets are completely detached from one another, other than the crossover section.
  • In short, this common portion of the chiasmata is holding all the four chromatids together.
  • Meanwhile, the chromosomes continue to condense, making them shorter.

Diakinesis

  • At this stage, the chiasma begins to shift towards the end of the chromatid, as though trying to separate the bond. This continues until they reach the very end of the chromatid.
  • Now spindle fibers (made of microtubules) begin to form from the centrioles of the cell.
  • Lastly, the nucleolus and the nuclear membrane disintegration marks the beginning of the next step―Metaphase I.

Metaphase I

  • The spindle fibers attach themselves to homologous chromosome pairs at the centromeres.
  • With the nuclear membrane disintegrated, the chromosomes are free to move.
  • The spindle fibers then align the chromosomes along the equatorial plane of the cell.

Anaphase I

  • With the spindle fibers being pulled from opposite ends of the cells, the homologous pairs split apart. This is also known as ‘disjunction.’
    Nondisjunction is when a chromosome pair does not separate, resulting in abnormal number of chromosomes and serious defects in the offspring so produced.

Telophase I

  • With chromosomes pulled to either sides of the cell, cytokinesis takes place.
  • The cell divides into two halves, each one having half the number of chromosomes as the parent cell.
  • The nuclear membrane may or may not reform, and the chromosomes decondense.

Cytokinesis I

  • As the name suggests, Cytokinesis literally means division of the cytoplasm or the cell body.
  • Once the genetic material is replicated and segregated into different parts of the cell, the cell membrane begins pinching towards the inner side.
  • A cleft is formed which gradually deepens and separates the two newly formed daughter cells.
    As discussed earlier, plant and animal cells have a slightly different phenomenon of the cell membrane dividing the cell body.

Prophase II

  • If the nuclear membrane had reformed during Telophase I, it once again starts disintegrating for cell division.
  • The chromosomes start recondensing, and the centrioles start moving towards the opposite ends of the cell.

Metaphase II

  • Once again, spindle fibers arise from the centrioles and attach themselves to the centromeres at the location of the kinetochores.
  • They start aligning the chromosomes along the equatorial plane of the cell by creating tension from opposite ends.

Anaphase II

  • With the spindle fibers being pulled, the sister chromatids finally break and start moving in the opposite direction.
  • Once the chromatids are separated from one another, they are considered as individual chromosomes.
  • With the chromosomes (earlier chromatids) concentrated in different ends of the cell, cytokinesis begins.

Telophase II

  • Nuclear membrane reappears for both the to-be-daughter cells at this stage.
  • The cells undergo cytokinesis to form four haploid daughter cells, each having half of the original parent cell’s chromosomes.
  • As the illustration shows, the daughter cell’s chromosomes carry crossovers from other chromosomes, thus making each of them unique.

Cytokinesis II

  • Once again, the cytoplasm divides post nuclear division to physically separate the new cells.
  • The cell wall again pinches inward to create a cleavage, which in turn, deepens and forms two different cell walls for each of the daughter cells.

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