35.1
Mitosis and Cytokinesis
In eukaryotes, the cell division cycle is divided into distinct, coordinated cellular processes that include cell growth, DNA replication/chromosome d…
Mitosis is a form of cell division where a cell's genetic material is partitioned between two daughter cells.
First, during prophase, nucleic chromatin condenses into x-shaped chromosomes composed of sister chromatid pairs attached at centromere junctions.
Concurrently, the centrosomes migrate to opposite cell sides. As they do so, microtubule rods begin to grow from each, forming a web-like spindle apparatus.
Next, the nuclear envelope dissolves during prometaphase and the protein structures, termed kinetochores, appear on both sides of the centromeres, one for every chromatin.
Once these kinetochores form, extending interior microtubules fasten to them, with each sister chromatid being tethered to a different cell pole.
In metaphase, the spindle apparatus rearranges the chromosomes so that they are oriented in a fixed row along the cell's equator.
With anaphase, kinetochore afixed microtubules shorten and sister chromatids are dragged apart. These and other microtubule dynamics also elongate the cell.
During telophase, the spindle apparatus disbands, chromatids loosen, and nuclear envelopes are reformed.
Finally, the cell is cytoplasmically divided by cytokinesis, forming a pair of cells genetically identical to their precursor.
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Q1: What happens to chromosomes during prophase of mitosis?
During prophase, replicated chromosomes condense into visible threadlike structures with the aid of condensin proteins. Simultaneously, centrosomes migrate to opposite cell poles and begin forming the mitotic spindle apparatus from microtubules. The nucleolus disappears, signaling the impending breakdown of the nuclear envelope.
Q2: How do kinetochores function during chromosome separation?
Kinetochores are protein structures on centromeres where spindle microtubules attach during prometaphase. During anaphase, kinetochore-attached microtubules shorten, pulling sister chromatids apart toward opposite cell poles. This separation is enabled by the breakdown of cohesion proteins holding the chromatid pairs together.
Q3: What is the role of the metaphase plate in mitosis?
The metaphase plate is the cell's equatorial region where spindle microtubules align all condensed sister chromatid pairs in a fixed row during metaphase. This alignment ensures that each daughter cell receives an identical set of chromosomes. The metaphase plate marks the transition point before sister chromatids separate during anaphase.
Q4: How does cytokinesis differ between animal and plant cells?
In animal cells, actin filaments form a contractile ring that pinches the cell into two, creating a cleavage furrow. In plant cells, Golgi vesicles carrying structural proteins form a cell plate at the former metaphase plate location, which fuses with plasma membranes to create a new cell wall dividing the cell.
Q5: What occurs during telophase to prepare for cytokinesis?
During telophase, chromosomes decondense into chromatin at opposite cell poles, and spindle microtubules depolymerize into tubulin monomers for reuse in daughter cells. Nuclear envelopes reassemble around each chromosome set, restoring two distinct nuclei. These events prepare the cell for cytokinesis and the formation of two genetically identical daughter cells.
Q6: What is the relationship between the spindle apparatus and chromosome movement?
The spindle apparatus is a web-like structure of microtubules that extends from centrosomes at opposite cell poles. Kinetochore microtubules attach to chromosomes and shorten during anaphase, pulling sister chromatids apart. Polar and astral microtubules push spindle poles apart, coordinating chromosome segregation and cell elongation throughout mitosis.
Q7: How long does mitosis typically take in human cells?
Mitosis accounts for approximately one hour of the 24-hour cell cycle in most human cells. The complete cell cycle includes DNA replication during S phase and gap periods for cell preparation. This timing ensures coordinated, regulated progression through all mitotic stages from prophase through cytokinesis.