Cell Cycle and Cell Division
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The cell cycle is a sequence of events occurring within a cell encompassing growth, development, and division. It consists of two primary phases:
Interphase: the cell grows, performs DNA replication and prepares for the division.
Mitotic (M) phase involves the separation of the cell's genetic material and cytoplasm to produce two daughter cells.
Interphase is further divided into three sub-phases:
G1 (Gap 1) phase: the cell grows, increases the number of organelles, and synthesises proteins.
S (Synthesis) phase: DNA replication occurs, resulting in the duplication of the genetic material.
G2 (Gap 2) phase: the cell performs additional protein synthesis and growth which prepares it for mitosis.
Many cells of a multicellular organism quit the replicative cell cycle and after the G1 phase switch to a non-proliferative G0 phase.
After DNA replication, each chromosome consists of two chromatids containing identical copies of the chromosome's DNA. During mitosis chromosomes are split and the chromatids are directed to the opposite poles of the cell.
In animal cells, mitosis is facilitated by organelles called centrosomes which serve as microtubule-organising centres. Microtubules are filaments that consist of multiple protein subunits and can move chromosomes.
The microtubules of the so-called mitotic spindle extend from the centrosomes and attach to regions of the chromosomes called centromeres. The attachment occurs via protein complexes called kinetochores, which assemble at the centromeres.
As the mitotic spindle forms and microtubules attach to kinetochores, they exert forces on the chromosomes, pulling the chromatids apart and guiding their movement to opposite poles of the cell. This ensures that each daughter cell receives an identical set of chromosomes during cell division.
Mitosis includes five stages: prophase, prometaphase, metaphase, anaphase and telophase. Cytokinesis is the process of cytoplasm division which usually starts in the late telophase.
