CBSE Class 11-science Answered

• The sequence of events by which a cell duplicates its genome and synthesizes all other cell contents and eventually divides into two daughter cells is called the cell cycle.
• A typical eukaryotic somatic cell has two main stages—along undividing state called the interphase also known as the intermitosis or I phase and a shorter phase of nuclear division called the mitotic or M phase. These main phases are followed by a still shorter phase of cytoplasmic division known
  as cytokinesis.

Interphase

• Interphase involves a series of changes which take place in a newly formed cell and its nucleus before it gets ready for division again. It is also called intermitosis.
• During this phase, DNA replication, synthesis of nuclear histones, division of centrioles to form a new pair of centrioles, synthesis of energy-rich compounds and synthesis of RNA and proteins take place.
• Interphase is further divided into three periods—first gap or G₁ phase, synthetic or S phase and second gap or G₂ phase. Duration of these phases is different in different organisms.
• G₁ Phase (Post-mitotic Gap Phase):
• The G₁ phase is the interval phase between mitosis and the initiation of DNA replication.
• The cell grows to its maximum size to prepare for DNA replication.
• The duration of the G₁ phase is highly variable. The cells which divide frequently have a shorter G₁ phase, while the cells which divide less frequently have a longer G₁ phase.

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  • During the S phase, synthesis or replication of DNA occurs on the template of the existing DNA strand.
  • The amount of DNA per cell doubles, but there is no change in the chromosome number of the cell.
  • The S phase lasts for 6–8 hours.

• The G₂ phase is a stage of further growth of the cell and preparation for its division.
• Synthesis of RNA and proteins continues.
• Spindle protein synthesis and aster formation take place.
• The G₂ phase lasts for 2–5 hours.

• The mitotic or M phase follows interphase. It starts with nuclear division, corresponding to the separation of daughter.
• The M phase lasts for only about an hour.
• After completion of mitosis, the daughter cells may enter the G₁ phase of the next cell cycle or the
  G₀ phase.