Question

What is cell cycle and what are its phases?

Answer 1

Cell cycle or cell-division cycle is the series of events that take place in a cell leading to its division and duplication (replication) that produces two daughter cells.

Phases of Cell Cycle:
The cell cycle is divided into two basic phases:
1. Interphase: The phase between subsequent cell divisions is called the interphase. The interphase lasts for more than 95% of the cell cycle.
2. M Phase (Mitotic phase): The actual cell division takes place in the M phase. The M phase lasts for less than 5% of the cell cycle. The M phase is composed of two major steps, viz. karyokinesis and cytokinesis. Division of nucleus happens during karyokinesis. Division of cytoplasm happens during cytokinesis.
The interphase is further divided into three phases, which are as follows:
1. $G_1$ phase (Gap 1): During this phase, the cell is metabolically active and grows continuously.
2. S phase (Synthesis): During this phase, DNA synthesis or replication takes place. The amount of DNA becomes double during this phase, but the number of chromosomes remains the same.
3. $G_2$ phase (Gap 2): During this phase, protein synthesis takes place.
Quiescent Stage $\left(G_0\right)$: Cells which do not divide further, exit G1 phase to enter an inactive stage. This stage is called quiescent stage (G0) of the cell cycle. The cells in this stage remain metabolically active but do not undergo division. But these cells can resume division as and when required.
M PHASE
Mitosis is divided into four stages, viz. Prophase, Metaphase, Anaphase and Telophase.
PROPHASE
Condensation of chromosomal material takes place. A chromosome is seen to be composed of two chromatids. The chromatids are attached together at the centromere.
Spindle fibres are formed.
Various cell organelles; like golgi bodies and ER cannot be seen during this stage. Nucleolus and nuclear envelope also disappear.
METAPHASE
All the chromosomes come to lie at the equator.
In each chromosome, one chromatid is connected to the spindle fibre from one pole and another chromatid is connected to the spindle fibre from another pole.
The plane of alignment of chromosomes during this phase is called metaphase plate.
ANAPHASE
Centromeres split which results in separation of chromatids.
After that, chromatids move to opposite poles.
TELOPHASE
The chromosomes form clusters at opposite poles. They become inconspicuous.
Nuclear envelope is formed around the chromosome clusters.
Nucleolus, golgi complex and ER are also formed.
Cytokinesis
Division of cytoplasm is achieved by cytokinesis. In animal cell, a furrow appears in the plasma membrane. The furrow gradually deepens and finally joins in the centre. Thus, the cytoplasm is divided into two parts. In plant cells, cell wall formation begins in the centre. This grows outwards to meet the existing lateral walls and thus, the cytoplasm is divided into two parts.

Significance of Mitosis
Mitosis results in the formation of new cells which are required for growth and repair.
Mitosis results in the formation of two daughter cells; which have identical genetic makeup, similar to the mother cell.