10.6
Mitosis and Cytokinesis
In eukaryotes, the cell division cycle is divided into distinct, coordinated cellular processes that include cell growth, DNA replication/chromosome d…
Comprised of five unique stages, mitosis is a form of division where a cells genetic material is partitioned between two daughter cells. First, during prophase in humans, nucleic chromatin condenses into x-shaped chromosomes composed of sister chromatid pairs attached at centromere junctions.
Concurrently, outside the nucleus, two l-shaped organelles called centrosomes migrate to opposite cell sides. As they do so, microtubule rods begin to grow from each, either towards the cell's interior or exterior forming a web-like spindle apparatus.
Next, the nuclear envelope dissolves during prometaphase, exposing the chromosomes to the cells other contents. Protein structures also 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.
Mitosis then progresses to metaphase where the spindle apparatus rearranges the chromosomes so that they are similarly oriented in a fixed row along the cell's equator.
With anaphase, kinetochore afixed microtubules shorten and sister chromatids, now individually referred to as chromosomes, are dragged apart. These and other microtubule dynamics also elongate the cell.
Finally, the chromosomes land at opposite cell sides during telophase, where upon the spindle apparatus disbands. The genetic material loosens, and two nuclear envelopes, one around each chromosome set, arise.
During telophase, in a distinct process called cytokinesis, the cell is cytoplasmically divided. Thus, the end result of mitosis is a cell pair genetically identical to their precursor.
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Q1: What are the five stages of mitosis?
Mitosis consists of prophase, prometaphase, metaphase, anaphase, and telophase. During prophase, chromosomes condense and the spindle apparatus forms. Prometaphase involves nuclear envelope breakdown and kinetochore attachment. Metaphase aligns chromosomes at the cell's equator. Anaphase separates sister chromatids, and telophase reforms nuclear envelopes around each chromosome set.
Q2: How do kinetochores function during mitosis?
Kinetochores are protein structures that form on centromeres during prometaphase. Spindle microtubules attach to kinetochores on opposite poles, tethering each sister chromatid to a different cell pole. During anaphase, kinetochore-attached microtubules shorten, pulling the separated chromatids toward opposite ends of the cell.
Q3: What is the role of the spindle apparatus in cell division?
The spindle apparatus is a web-like structure made of microtubules that forms between centrosomes during prophase. It attaches to kinetochores, aligns chromosomes at the metaphase plate, and pulls sister chromatids apart during anaphase. Polar and astral microtubules also push spindle poles apart and anchor them to the cell membrane.
Q4: How does cytokinesis differ between animal and plant cells?
In animal cells, actin filaments form a contractile ring that creates a cleavage furrow, pinching the cell into two. In plant cells, vesicles from the Golgi apparatus carrying structural proteins form a cell plate at the former metaphase plate location. This cell plate eventually fuses with plasma membranes, forming a new cell wall that divides the cell.
Q5: Why do chromosomes condense during mitosis?
Chromosomes condense during prophase through the action of proteins called condensins, transforming replicated chromatin into visible x-shaped structures. Condensation allows the cell to organize and efficiently distribute genetic material to daughter cells. Condensed chromosomes remain organized throughout mitosis until they decondense back into chromatin during telophase.
Q6: What happens to the nuclear envelope during mitosis?
The nuclear envelope dissolves during prometaphase, exposing chromosomes to the cell's contents and allowing spindle microtubules to attach to kinetochores. During telophase, two new nuclear envelopes reassemble around each set of chromosomes at opposite poles. This reformation protects the genetic material in each daughter cell.
Q7: How long does mitosis take in human cells?
Mitosis accounts for approximately one hour of the roughly 24-hour cell cycle in typical human cells. The remaining time is spent in interphase, which includes DNA replication and cell growth phases. This timing ensures cells divide at a controlled rate while maintaining genetic stability.