The Cell Cycle

The cell cycle is the life cycle of a cell, specifically the period between successive cell divisions. For example, a skin cell dividing to replace old cells.

Binary fission is the normal life cycle of a bacterial cell. For example, E. coli reproduces through binary fission.
It comprises three phases:
- Replication phase (R-phase = C-period)
- Division phase (D-phase = D-period)
- Interval phase (I-phase = B-period)

This is the duration required to replicate the bacterial genome (e.g., 40 minutes for E. coli). Imagine copying a computer's hard drive; this is the time it takes to copy all the genetic information.
It results in the formation of a new chromosome, which has an independent point of attachment to the membrane.

This phase involves the segregation of daughter chromosomes and other cellular components into daughter cells. Think of this as splitting the copied hard drive and other computer parts into two new computers.
The D-phase is initiated by FtsZ proteins.
FtsZ proteins assemble into a ring (Z-ring) at the midpoint of the cell, leading to septum formation. The Z-ring acts like a drawstring, pinching the cell in half.

This is the period between cell division and the initiation of chromosome replication. It's the 'rest' time before the cell kicks off another division cycle.

Some highly specialized cells lack the ability to divide (e.g., red blood cells, neurons). Red blood cells need to maintain their shape for oxygen transport, and neurons are vital for signal transmission.
Certain cells can induce cell division through a specific stimulus (e.g., liver cells, lymphocytes). For example, liver cells can regenerate after damage, and lymphocytes multiply during an immune response.
Other cells have a relatively high level of mitotic activity (e.g., hematopoietic stem cells, epithelial cells). Hematopoietic stem cells constantly divide to produce new blood cells, while epithelial cells divide to repair skin.

Interphase includes G1, S, and G2 phases.
For a typical rapidly proliferating human cell with a 24-hour cycle time:
- G1 phase lasts about 11 hours.
- S phase lasts about 8 hours.
- G2 phase lasts about 4 hours.
- M phase lasts about 1 hour.

G1 phase is the gap phase after cell division and the first phase in interphase. It's a period of growth and preparation.
It is a phase of growth and biosynthesis activity. For instance, the cell synthesizes proteins and organelles.
The duration of the G1 phase is highly variable, even among different cells of the same species.
The cell conducts a series of checks before entering the S phase. It's like a pre-flight checklist to ensure everything is ready for DNA replication.

S phase is the DNA synthesis phase.
It starts with the replication of DNA, finishing when the amount of DNA in the cell has doubled. Imagine making a complete copy of all the genetic instructions.
Duplication of centrioles also takes place during this phase.

G2 is the gap phase after DNA synthesis and before cell division. It’s another preparation phase.
The cell conducts a series of checks before entering the M phase. It's a final check to make sure DNA replication was successful.

M phase is when the cell actually divides. This involves mitosis and cytokinesis.

G0 is the "resting phase."
In multicellular organisms, most differentiated cells exit the cell cycle and survive for varying durations (days, weeks, or even a lifetime for nerve cells and eye lens cells) without dividing again. For example, nerve cells and eye lens cells remain in G0 for an entire lifetime.
Some G0 cells can return to the cell cycle and resume replicating. Liver cells, for example, can re-enter the cell cycle to repair damage.
Cells in G0 may be quiescent (dormant) or senescent (aging or deteriorating).
Quiescent cells can re-enter the cell cycle, while senescent cells cannot. Quiescent cells are like paused players, while senescent cells are permanently retired.
Most somatic cells of an organism are differentiated and quiescent, residing in the G0 phase of the cell cycle. These cells perform specific functions without actively dividing.

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