The various phases of the growth and reproduction of cell is called a cell cycle. The complete duration of a cell cycle constitutes one generation time. The principal event in the cell cycle is the duplication (replication) of the genetic material (DNA) in the nucleus and its distribution among the daughter cells. The growth of the cell as well as the replication of the nuclear DNA occurs in that part of the cell cycle known as the interphase. The distribution of the replicated DNA and the physical division of the parent cell into two daughter cells occurs in the 'period of division' or M phase of the cell cycle. The entire cell cycle of most higher animal and plant cells is about 10-25 hours of which only one hour is spent in M phase. Prokaryotic cells that lack a well-defined nucleus, and with a different organisation of chromatin have a cell cycle of only 20 to 30 minutes.
Fig: Diagrammatic Illustration of the Stages of Cell Cycle of a Typical Mammalian Cell Growing in Tissue Culture with a Generation time of 24 hours.
The first part of the interphase is called the G1 phase (G for 'gap'). During this phase, the cell synthesises RNA and proteins grow in size. At this phase there is no change in DNA content in the cell. DNA synthesises occurs in the next phase of the cell cycle known as S-phase (S for 'synthesis'). During this phase the DNA molecule of each chromosome replicates. Thus each chromosome at this stage is composed of two chromatids held together by their common centromere.
Each chromosome now carries a duplicate set of genes and double the amount of DNA (4n) that is present in the cell at the beginning of the cycle (2n). Completion of the S phase is usually not followed by the M phase. The gap between the end of S phase and the onset of M phase is called G2 phase. In both the G phases there is synthesis of RNA and proteins and considerable amount of cell growth. The G2 phase is followed by the mitotic division phase or M phase when the cell divides into two daughter cells which are similar to the original parent cell.
The duration of the G1, S, G2 and M phases vary among different types of cells. The greatest variation is seen in the G1 phase. When the cell cycle is very long it is the G1 phase which is prolonged. When the fertilised egg undergoes cleavage divisions the cell cycle is very short, the G1 phase is very brief or non-existent with little intervening cell growth; one division of the cell quickly follows another. The period of cell division, the M phase, of the cell cycle is, in fact, the final microscopically visible phase of the cycle which actually follows the underlying molecular events such as DNA synthesis and chromosome duplication during S phase and protein and RNA synthesis during the gap phases.
The process of cell division generally involves a sequence of three important events:
- DNA replication and the chromosomal duplication;
- nuclear division or karyokinesis and
- cytoplasmic division or cytokinesis.
- mitosis which occurs in the somatic cells and
- meiosis which occurs in germ cells.