What is Cell Cycle? Explain its various phases in details.
Cell cycle refers to the sequential events during which a cell undergoes stages until it completes its mitosis and generates new cells. It is also thought of as the life cycle of a cell.
Stages of the cell cycle
Cell cycle consists of two major phases i.e: interphase (set of phases) and mitotic phase (M phase). Mitotic phase is a short span period of cell cycle. M phase alternates in different phases with interphase, where the cell prepares itself for division.
Interphase is the time when a cell’s metabolic activity is very high, as it performs its various functions. In eukaryotic cells, or cells with a nucleus, the cell must have to follow on to these two major phases, i.e: interphase and the mitotic (M) phase.
Interphase
Interphase is divided into three sub-phases:
- G1 phase (first gap)
- S phase (synthesis)
- G2 (second gap)
Now we are going to discuss each phase with details.
G1 phase (first gap)
The first phase after the formation of cell, named as G1 phase. In this gap the cell increases its supply of proteins, raises the number of its corresponding organelles (mitochondria, ribosomes, endoplasmic reticulum), and grows in size. Various enzymes are synthesised in this phase which are required in next phase of cell cycle for the replication of chromosomes.
S phase (synthesis)
The second phase after G1 phase is S phase in which duplication of chromosomes occurs. Every chromosome includes two sister chromatids.
In this phase, cell synthesised a complete copy of its DNA present in the nucleus.
G2 phase (second gap)
The phase after which a cell is able to go mitosis. It is the phase of rapid growth of a cell with the formation of proteins. Sometimes, for some cells (cancer cell) this phase is not necessary. These cell directly go to mitosis phase for replication.
G0 phase
G0 phase is named as the resting phase in which cell is neither dividing nor preparing for division. This phase is often thought as extension of interphase.
Many cell types in an early embryo divide rapidly and so do cells in a tumor while other types of cells divide slowly or not at all and undergo phase G0.
Why does the cell enter G0 phase?
Cells enter the G0 phase when they don’t need longer to divide (when they finish mitosis). This also happens when the cell doesn't have sufficient nutrients or energy to continue division. Cells enter this phase temporarily or permanently.
What is the longest stage of the cell cycle?
Interphase is the longest phase of the cell cycle. This takes much time because in this phase the cell grows and copies its DNA before moving into mitosis.
M Phase (Mitotic Phase)
During the M phase, the cell divides itself into two new cells by copied DNA and cytoplasm.
M phase involves two distinct division-related processes:
Karyokinesis (division of nucleus) and Cytokinesis (division of cytoplsam).
In mitosis, the nuclear DNA of the cell condenses thread like structures i.e: chromosomes and is pulled apart by the mitotic spindle.
Mitosis takes place in four stages:
- Prophase
- Metaphase
- Anaphase
- Telophase
In cytokinesis, the cytoplasm split in two, making two new daughter cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. Cytokinesis takes place differently in different cells (animal and plant cells).
How long does the cell complete its cycle?
Different cells take different lengths of time to complete the cell cycle.
- A typical human cell might take about 24 hours to divide
- Fast-cycling mammalian cells, like the ones that line the intestine, can complete a cycle every 9-10 hours when they're grown in culture.
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