7.1
Mitosis and Cytokinesis
In eukaryotes, the cell division cycle is divided into distinct, coordinated cellular processes that include cell growth, DNA replication/chromosome d…
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 main stages of mitosis and what happens during each?
Mitosis consists of prophase, metaphase, anaphase, and telophase. During prophase, chromosomes condense and the mitotic spindle forms. Metaphase aligns chromosomes at the cell's equator. Anaphase separates sister chromatids to opposite poles. Telophase reverses prophase changes as the nuclear envelope reforms around each set of chromosomes.
Q2: How do sister chromatids remain attached before separation during mitosis?
Sister chromatids are held together by protein complexes called cohesins that hold the sister chromatids together until anaphase. These ring-shaped proteins encircle both chromatids, maintaining their connection through metaphase. When the anaphase-promoting complex signals their removal, cohesins are cleaved, allowing chromatids to separate and move to opposite poles.
Q3: What role does the mitotic spindle play in chromosome movement?
The mitotic spindle is a dynamic structure composed of microtubules that organizes and moves chromosomes. Spindle fibers attach to kinetochores on chromosomes and generate forces that pull sister chromatids apart. The spindle also elongates the cell, positioning chromosomes for proper distribution to daughter cells during cell division.
Q4: How does cytokinesis differ between animal and plant cells?
Animal cells undergo cytokinesis through cytokinesis rhoa and contractile ring formation, where a protein ring pinches the cell membrane inward to divide the cytoplasm. Plant cells form a phragmoplast, a structure that guides cell wall deposition between daughter cells. This difference reflects the presence of a rigid cell wall in plants versus the flexible membrane in animals.
Q5: What is the spindle assembly checkpoint and why is it important?
The spindle assembly checkpoint is a quality control mechanism that prevents anaphase until all chromosomes are properly attached to spindle fibers. This checkpoint ensures that kinetochores cohesin rings and spindle attachment are correct before sister chromatids separate. It prevents errors that would result in unequal chromosome distribution to daughter cells.
Q6: How is the plane of cell division determined in mitosis?
The plane of cell division is determined by the position of the mitotic spindle, which is guided by the centrosome and astral microtubules. The spindle orients perpendicular to the cell's longest axis, and the contractile ring forms at the cell's equator. This ensures determining the plane of cell division results in roughly equal distribution of cytoplasm and organelles to daughter cells.
Q7: What happens to chromosomes during prophase to prepare them for segregation?
During prophase, chromosomes condense through the action of condensins and condensation of chromosomes, making them visible under a microscope. Simultaneously, the centrosome duplicates and the two copies move to opposite poles, organizing the mitotic spindle. This condensation compacts DNA and ensures chromosomes are properly structured for attachment to spindle fibers.