Part 29 : Cellular Level of Organization | Cell Cycle (Hindi) | B. Pharm | Nursing | Medical

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3.1 The cell cycle
You can use the term mother or parent cell for the cell that undergoes division and the daughter cells for the ones that are the result of this division. Before each daughter cell undergoes division, it must grow to the same size as its mother cell. We can distinguish two main phases in the life of a cell.
(i) Interphase - Non-dividing period (Growth phase)
(ii) M-phase - Dividing phase (M for Mitosis or Meiosis)
i. Interphase - (Inter = in between)
The interval between two successive cell divisions is termed interphase (phase at which the cell is not dividing). It is the longest period in the cell cycle. The interphase is subdivided into three main periods - G1, S and G2.

a) G1 (Growth Phase 1 ) Phase i.e. First phase of growth – This is the longest phase. The first phase of interphase and the cell cycle is called G1. During G1, the cell is preparing to replicate DNA by synthesising the mRNAs and proteins required to execute the future steps. The cell usually grows larger, and some organelles are copied.
b) S or synthetic Phase - It comes next. Lot of DNA is (synthesised). During the S phase, all the genetic information in the cell is copied by the process of DNA replication. This process of replication generates sister chromatids, which are identical pairs of chromosomes. These sister chromatids are attached to each other by a centromere. A centromere is a specialised sequence of DNA that links the sister chromatids and is important throughout mitosis.

c) G2 (Growth Phase 2) phase - More protein is synthesised in this phase. Cytoplasmic organelles such as mitochondria, golgi bodies get duplicated. Centriole also divides into two centrioles contained in a single centrosome.

ii. M-phase or Dividing phase - Represented by the symbol M (Mitosis or meiosis). Mitosis occurs so that during this period the chromatids separate and form daughter chromosomes. The daughter chromosomes go to daughter nuclei and cytoplasm divides forming two identical daughter cells.

Types of cell division
There are two kinds of cell division- mitosis and meiosis.
1. Mitosis : Cell division for growth and replacement wherein the two daugher cells are identical and similar to mother cell in all respects.
2. Meiosis : It occurs in the gonads for sexual reproduction to produce gametes. The resultant cells, egg (in female) and sperms (in male), possess half the chromosome number of the parent cell.

1. Mitosis (mitos = thread)
Mitosis is divided into 4 phases or stages termed as
(i) Prophase
(ii) Metaphase
(iii) Anaphase
(iv) Telophase
(These phase refer to the changes taking place in the nucleus.)
Prophase
a) Chromosomes are visible
b) Nucleolus and Nuclear envelop disappears.
c) Each centromere mpves to an opposite pole of the cell.
Metaphase
a) chromosomes move, towards the equator of the cell.
b) Each chromosome becomes attached to the spindle fibre by centromere.
c) The sister chromatids are not yet separated.
Anaphase
a) Centromeres divide
b) Two daughter chromatids separate
c) Each chromatid now contains a centromere and is now termed a chromosome.
d) Half the number of now chromosomes (daugher chromatids) move toward one pole and the other half to the other pole.
e) Cytokinesis begins as the cleavage furrow starts in animal cells.
Telophase
a) Chromosomes begin to form a chromatin network as in a nucleus.
b) New nuclear membrane is formed around each daughter nucleus.
c) Nucleolus becomes visible again.
Cytokinesis
It is the process of the division of cytoplasm into two. It is initiated in the beginning of telophase and is completed by the end of telophase. In an animal cell, invagination of plasma membrane proceeds from the periphery of the cell towards the interior.
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