Eukaryotic Cell Division (Cell Cycle)

Cell Cycle 

The increased size and more complex organization of eukaryotic genomes, compared to prokaryotes, required radical changes in how replicated genomes are partitioned into daughter cells. The cell cycle requires the duplication of the genome, its accurate segregation, and the division of cellular contents.

The Cell Cycle is Divided into Five Phases

The cell cycle is divided into phases based on the key events of genome duplication, chromosome segregation, and distribution of cytroplasm. The cell cycle is usually diagrammed using the metaphor of a clock face see figure below. 

■ G1 (gap phase 1) is the primary growth phase of the cell. The term gap phase refers to its filling the gap between cytokinesis and DNA synthesis. For most cells, this is the longest phase.

■ S (synthesis) is the phase in which the cell synthesizes a copy  of its DNA genome.

■ G2 (gap phase 2), the second growth phase, involves preparation for separation of the newly replicated genome. This phase fills the gap between DNA synthesis and the beginning of mitosis. During this phase, microtubules begin to reorganize to form a spindle.

G1, S, and G2 together constitute interphase, the portion of the cell cycle between cell divisions.

■ Mitosis is the phase of the cell cycle in which the spindle apparatus assembles, binds to the chromosomes, and moves the sister chromatids apart. Mitosis is the essential step in the separation of the two daughter genomes. It is traditionally subdivided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase.

■ Cytokinesis is the phase of the cell cycle when the cytoplasm divides, creating two daughter cells. In animal cells, the microtubule spindle helps position a contracting ring of actin that constricts like a drawstring to pinch the cell in two. In cells with a cell wall, such as plant and fungal cells, a plate forms between the dividing cells.

Mitosis and cytokinesis together are usually referred to collectively as the M phase, to distinguish the dividing phase from interphase.

Figure:The cell cycle. The cell cycle is depicted as a circle. The first gap phase, G1, involves growth and preparation for DNA synthesis. During S phase, a copy of the genome is synthesized. The second gap phase, G2, prepares the cell for mitosis. During mitosis, replicated chromosomes are partitioned. Cytokinesis divides the cell into two cells with identical genomes.

The duration of the cell cycle varies 

depending on cell type

The time it takes to complete a cell cycle varies greatly. Cells in animal embryos can complete their cell cycle in under 20 minutes. These cells simply divide their nuclei as quickly as they  can replicate their DNA, without cell growth. Half of their cycle is taken up by S, half by M, and essentially none by G1 or G2.

Because mature cells require time to grow, their cycles are much longer than those of embryonic tissue. Typically, a dividing mammalian cell completes its cell cycle in about 24 hours, but some cells, such as certain cells in the human liver, have cell cycles lasting more than a year. During the cycle, growth occurs throughout the G1 and G2 phases, as well as during the S phase. The M phase takes only about an hour, a small fraction of the entire cycle.

Most of the variation in the length of the cell cycle between organisms or cell types occurs in the G1 phase. Cells often pause in G1 before DNA replication and enter a resting state called the G0 phase; cells may remain in this phase for days to years before resuming cell division.

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1.Interphase
2.M-Phase (Mitosis &Cytokinesis)