A single cell divides many times and forms a multi-celled organism. Unicellular bacteria and protozoa divide and increase in number. The injured tissues are replaced by new cells through cell division. Thus, cell division is one of the most important activities in all organisms.
Majority of cells in a multicellular organism grow and then can divide. However, the cells like the nerve and muscle cells of animals and guard cells of plants do not divide.
The process of cell division is almost same in all organisms. A cell passes through phases of growth after which are able to duplicate their chromosomes before they divide. These phases in the life of a cell constitute the cell cycle.
The term mother or parent cell is used for the cell that undergoes division and the daughter cells for the ones that are the result of the division. Before each daughter cell undergoes division, it must grow to the same size as its mother cell.
There are two main phases in the life of a cell:
- Interphase - Non-dividing period (Growth phase)
- Dividing phase or M-phase (M for mitosis or meiosis)
The interval between two successive cell divisions is termed interphase (phase in 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.
G1 (Gap-1) Phase - First phase of growth: This is the longest phase. Lot of protein and RNA are synthesized during this phase.
S or Synthetic Phase: It comes next. Lot of DNA is synthesized. A chromosome contains a single double helical strand of DNA molecule. After S-phase each chromosome becomes longitudinally double except at centromere, and thus, it has two molecules of DNA and two chromatids. Thus each chromatid contains one molecule of DNA. The two chromatids are joined by a centromere (which does not divide at this stage) to form a single chromosome.
G2 (Gap-2) phase: More protein including the histones are synthesized in this phase. Cytoplasmic organelles such as mitochondria and golgi bodies get duplicated. Centriole also divides into two centrioles contained in a single centrosome.
M-phase or Dividing Phase
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.
Kinds of Cell Division
There are two kinds of cell division:
- Mitotic cell division
- Meiotic cell division
Mitotic Cell Division
Mitotic cell division is for growth and replacement of older cells by new cells wherein the two daughter cells are identical and similar to mother cell in all respects. Mitotic cell division occurs in haploid as well as diploid cells.
Meiotic Cell Division
Meiotic cell division 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 that present in the parent cell. Meiotic cell division takes place only in diploid cells responsible for production of haploid spores or gametes.