7.2 Mitosis and Cytokinesis
Key Takeaways
Mitosis in a diploid animal cell produces two genetically identical diploid nuclei. Cells use it for growth and repair, not for making gametes.
In prophase, chromosomes condense and the spindle forms. The nuclear envelope breaks down in late prophase or prometaphase, and kinetochores attach to microtubules from opposite poles.
At metaphase, sister chromatids line up at the metaphase plate. In anaphase they separate and each becomes its own chromosome. In telophase, envelopes reform and chromosomes decondense.
A cell with 8 duplicated chromosomes at mitotic metaphase has 16 chromatids. Its daughter cells each receive 8 unduplicated chromosomes.
Animal cytokinesis uses a cleavage furrow made by a contractile ring of actin. Plant cytokinesis builds a cell plate that becomes the new wall. Plant cells do not use a cleavage furrow.
7.2 Mitosis and Cytokinesis
Mitosis divides the nucleus. In a diploid animal cell it produces two nuclei that are genetically identical to each other and to the parent nucleus, and both daughter nuclei remain diploid. Mitosis does not reduce chromosome number, and it does not make gametes. Organisms use it for growth, repair, and replacement of body cells. If the starting nucleus is haploid, as in a plant gametophyte, mitosis keeps that haploid number. The rule is that mitosis preserves the chromosome set it starts with. Cytokinesis, division of the cytoplasm, is a separate event that usually overlaps the end of mitosis.
Prophase and the spindle
In prophase, chromatin condenses into visible chromosomes. Each chromosome was duplicated in the preceding S phase, so each one is two sister chromatids joined at the centromere. The mitotic spindle assembles from microtubules. In animal cells the spindle grows from centrosomes, which are organelles, not parts of the chromosome. The centrosomes move apart as the spindle forms. The nucleolus disappears. Flowering-plant cells also build a spindle, but they do it without the centrosome pair typical of animal cells. Do not confuse the centrosome with the centromere. The centromere is the region of the chromosome where the kinetochore forms.
Late prophase and the nuclear envelope
Late prophase is often called prometaphase. In that interval the nuclear envelope breaks into fragments, so microtubules can reach the chromosomes. Each sister chromatid has a kinetochore at its centromere. Microtubules from opposite poles attach to opposite kinetochores of the same chromosome. The two chromatids are therefore connected toward opposite poles. Homologous chromosomes do not find each other or pair during this attachment. Each duplicated chromosome is handled on its own.
Metaphase
In metaphase, the chromosomes are moved until their centromeres lie on the metaphase plate, the plane across the middle of the spindle. Sister chromatids face opposite poles. A spindle checkpoint delays anaphase if a kinetochore is not attached in that bipolar way. Attachment is about which pole holds which chromatid. It is not a pairing of maternal and paternal homologs.
A metaphase count of 8
A cell with 8 duplicated chromosomes at metaphase has 8 centromeres and 16 chromatids. It does not have 16 chromosomes yet. If the cell was diploid with 8 chromosomes before division, the diploid number is still 8 at metaphase.
Anaphase
In anaphase, the connection between sister chromatids is released. The sister chromatids separate and move toward opposite poles. Motor proteins and changes in microtubule length do the work. The moment the chromatids separate, each former chromatid is its own chromosome. The cell that had 8 duplicated chromosomes now has 16 chromosomes in one shared cytoplasm, with 8 moving toward each pole. Each of those chromosomes is unduplicated: one DNA molecule and one centromere.
What separates is the important identity. Anaphase of mitosis separates sister chromatids. It does not separate homologous chromosomes from their partners. Homologs never paired in mitosis, so there is no homolog-from-homolog split. That split is meiosis I.
Telophase
In telophase, the separated chromosomes arrive at the poles. A nuclear envelope reforms around each group. The chromosomes decondense toward chromatin, and nucleoli return. The spindle breaks down. The cell now has two genetically identical nuclei. If the parent was a diploid animal cell with 8 chromosomes, each new nucleus has 8 unduplicated chromosomes and is still diploid.
Cytokinesis in animals and in plants
Mitosis can leave two nuclei in one cell if cytokinesis fails. The usual outcome is two separate cells.
In animal cells, cytokinesis uses a cleavage furrow. A contractile ring of actin filaments, working with myosin, tightens in the plane of the old metaphase plate. The ring pulls the plasma membrane inward until the cytoplasm is pinched in two. Animal cells do not build a new wall across the middle of the cell.
Plant cells have a rigid wall, so they do not pinch. Vesicles derived from the Golgi apparatus move to the midline along a phragmoplast of microtubules and fuse to form a cell plate. The plate grows outward until it joins the existing membrane and wall. It becomes the new stretch of wall and membrane between the daughter cells. Plant cells do not use a cleavage furrow.
| Event | Animal cell | Plant cell |
|---|---|---|
| Nuclear sorting | Mitosis separates sister chromatids | Mitosis separates sister chromatids |
| Cytoplasmic division | Cleavage furrow | Cell plate |
| Structure that does the work | Contractile ring of actin | Golgi-derived vesicles guided by a phragmoplast |
| What this cell type does not use | A cell plate | A cleavage furrow |
Warning
Homologous chromosomes do not pair and do not separate during mitosis. Pairing of homologs, and separation of those homologs, is meiosis I. Mitosis separates sister chromatids.
Why cells use mitosis
A diploid animal zygote becomes an embryo by repeated mitosis. A cell that replaces a lost neighbor uses mitosis. In each case the point is a genetic copy of the nucleus, not a haploid gamete. Sperm and eggs are produced through meiosis. If a question says mitosis reduces the chromosome number by half, or says mitosis separates homologs, it is describing meiosis instead.
Daughter nuclei of mitosis match the parent nucleus because sister chromatids were copies made in S phase and because homologs were not reshuffled by crossing over. The process is a copying division. It is the way a multicellular body grows and repairs tissue without changing ploidy.
The arithmetic is the same for any diploid number the exam chooses. At metaphase, the chromatid count is twice the chromosome count. After anaphase and cytokinesis, each daughter cell is back to the original chromosome count, with one DNA molecule per chromosome. Nothing in that sequence builds a tetrad, and nothing produces four haploid cells.
A diploid animal cell has 8 duplicated chromosomes at metaphase of mitosis. Which description matches that cell and the cells produced after mitosis and cytokinesis?
The metaphase cell has 16 chromatids, and each daughter cell has 8 unduplicated chromosomes.
Homologous chromosomes separate at anaphase, so the division produces four haploid cells.
The metaphase cell already has 16 chromosomes, because each chromatid counts as a chromosome before anaphase.
The metaphase cell has 8 chromatids, and each daughter cell has 4 unduplicated chromosomes.
How does cytokinesis of a typical animal cell differ from cytokinesis of a typical plant cell?
Both cells use a cleavage furrow, and only the animal cell reforms a nuclear envelope after telophase.
The animal cell deposits a cell plate of Golgi vesicles, and the plant cell pinches inward with a contractile ring of actin.
The animal cell divides by a cleavage furrow formed by a contractile ring of actin, and the plant cell builds a cell plate that becomes the new wall.
The plant cell skips cytokinesis, because the existing wall already splits the cytoplasm into two cells.
Which statement correctly describes mitosis in a diploid animal cell?
Mitosis runs without a preceding S phase and yields four haploid nuclei used for tissue repair.
Mitosis pairs homologous chromosomes and separates the partners so the daughter cells can become gametes.
Mitosis separates sister chromatids and produces two genetically identical diploid nuclei. It does not pair homologous chromosomes.
Mitosis replicates the nuclear DNA during anaphase, which is why each daughter nucleus remains diploid.
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