Wednesday, August 12, 2020

Cell cycle and cell division - Notes | Class 11 | Part 1: Cell Cycle

10. CELL CYCLE AND CELL DIVISION 

The growth and reproduction of all organisms depend on the division and enlargement of cells.

The mechanisms of division and multiplication of cells together constitute cell reproduction.

CELL CYCLE


It is the life period of a cell during which a cell synthesizes DNA (replication), grows & divides into 2 daughter cells.

Cell growth (cytoplasmic increase) is a continuous process but DNA synthesis occurs only at a specific stage.

Duration of cell cycle varies in each organism and each cell type. E.g. Duration of a typical eukaryotic cell cycle (e.g. human cell) is about 24 hrs. In Yeasts, it is 90 minutes.

Phases of Cell Cycle


Cell cycle includes 2 basic phases: Interphase & M Phase.


1. Interphase (resting phase) 

It is the phase between two successive M phases.

It includes cell growth and DNA synthesis.

It lasts more than 95% of the duration of cell cycle.

Interphase has 3 phases:

a. G1 phase (Gap 1 or Antephase):
  • First growth phase. 
  • It is the interval between mitosis and DNA replication.
     Main events:
  • Continuous growth of cell.
  • Cell becomes metabolically active.
  • Prepares machinery for the DNA replication.
  • Synthesizes RNA and proteins.
b. S (Synthetic) phase:
  • In this, DNA replication takes place.
  • Amount of DNA per cell doubles. But chromosome number is not increased.
  • In animal cells, replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
c. G2 phase (Gap 2):
  • Second growth phase. 
  • Cell growth continues.
  • Synthesis of RNA and proteins continues.
  • Cell is prepared for mitosis.
2. M Phase (Mitosis phase) 

It represents the actual cell division (mitosis).

In human cell cycle, it lasts for only about an hour.

M Phase includes karyokinesis (nuclear division) and cytokinesis (division of cytoplasm).

Some cells do not show division. E.g. heart cells.

Many other cells divide only occasionally to replace damaged or dead cells.

The cells that do not divide further exit G1 phase and enter an inactive stage called quiescent stage (G0). Such cells remain metabolically active but do not proliferate.
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