The replication + 1 nuclear division +

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The recipe for mitotic cell division is 1 replication + 1 nuclear division + cytokinesis (1 cytoplasmic division). The purpose of mitotic cell division, in animals, is the growth and repair of somatic cells. The cells must divide, so surface area to volume ratio is optimal. This allows for differentiation. Interphase occupies about 90% of the cell cycle. It includes all cell activity between mitotic divisions during which the cell is preparing for division. These activities include growing by producing proteins and cytoplasmic organelles and copying its chromosomes.

The cell cycle has 4 parts- interphase with its 3 subphases, and the M phase (mitotic cell division). Interphase’s three subphases are: the G1 phase (“first gap”), the S phase (“synthesis”), and the G2 phase (“second gap”). G1 concerns cell growth. DNA is a single and unreplicated helix. Many biochemical activities occur here, such as the synthesis of enzymes. The S phase concerns DNA replication (genome synthesis). Each homologous chromosome replicates, resulting in 2 sister chromatids attached by the centromere. It is the most important phase.

G2 is where the cell prepares for cell division. The nucleus becomes defined, spindle proteins form, the centrioles replicate, the microtubules radiate from the centrosomes, enzymes and other proteins are synthesized, mitochondrion are replicated, and the chromosomes begin to condense. Chromosomes are duplicated only during the S phase. A typical animal cell might divide once every 24 hours. Of this time, the M phase would last less than an hour, while the S phase might take 10–12 hours, or half the cycle. The rest of the time would be divided between the G1 and G2 phases.

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The G1 phase varies most in length from cell to cell. The mitotic (M) phase consists of mitosis and cytokinesis. Mitosis is usually broken into five subphases: prophase, prometaphase, metaphase, anaphase, and telophase. During prophase, the nucleolus fades and chromatin (replicated DNA and associated proteins) condenses into chromosomes. Each replicated chromosome comprises two chromatids, both with the same genetic information. Microtubules of the cytoskeleton, responsible for cell shape, motility and attachment to other cells during interphase, disassemble.

And the building blocks of these microtubules are used to grow the mitotic spindle from the region of the centrosomes. In the stage of prometaphase (also the shortest phase), the nuclear envelope breaks down so there is no longer a recognizable nucleus. Some mitotic spindle fibers elongate from the centrosomes and attach to kinetochores, protein bundles at the centromere region on the chromosomes where sister chromatids are joined. Other spindle fibers elongate but instead of attaching to chromosomes, overlap each other at the cell center.

In metaphase, tension applied by the spindle fibers aligns all chromosomes in one plane (metaphase plate) at the center of the cell. Protein kinetochores attach to the centromere, aligning the sister chromatids. Non-kinetochore microtubules radiate from the centrosomes or across the metaphase plate without attaching to the chromosomes. In anaphase, the centromeres divide and separate. The spindle fibers shorten, the kinetochores separate, and the chromatids (daughter chromosomes) are pulled apart and begin moving to the cell poles.

In telophase, the daughter chromosomes arrive at the poles and the spindle fibers that have pulled them apart disappear. Telophase is also where cytokinesis begins. Cytokinesis is where the cytoplasm divides, forming 2 cells. The spindle fibers not attached to chromosomes begin breaking down until only that portion of overlap is left. It is in this region that a contractile ring cleaves (cleavage furrow) the cell into two daughter cells. Microtubules then reorganize into a new cytoskeleton for the return to interphase.

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