THE CELL CYCLE
The period from the time a cell is produced until it completes mitosis is called cell cycle. Cell division occurs during growth and repair processes. Cell cycle has following basic stages:
(a) Mitosis: It is the division of the nucleus.
(b) Cytokinesis: It is the division of the cytoplasm.
(c) Interphase: The cell grows and carries out its various metabolic processes during interphase. The cell cycle is divided into following stages:
- The G1 (first growth or gap) phase: It is the early growth phase of the cell.
- S (DNA synthesis) phase: During this phase the growth continues. This phase also involves DNA replication.
- G2 (second growth or gap): This phase prepares the cell for division It includes replication of the mitochondria and other organelles, synthesis of microtubules and protein. These microtubules and proteins will make up mitotic spindle fibers, and chromosome condensation.
- The M (Mitotic) phase: Partitioning of chromosomes and daughter cells take
place in M phase. 7
INTERPHASE: REPLICATING THE HEREDITARY MATERIAL Interphase occupies 90% of the total cell cycle. It includes the G1 S, and G2 phases. Normal activities of the cell take place during this phase. DNA replication is completed during the .S phase of interphase. Thus Interphase also prepares the cell for division.
A exact copy of the DNA is formed before the cell division. This process is called replication. The double stranded DNA makes a replica, or duplicate, of it elf. Each daughter cell receives the same genetic material as the parent cell. Therefore, replication is essential. Thus a pair of sister chromatids is formed. A chromatid is a copy of a chromosome produced by replication. Sister chromatids a e attached by centromere. The centromere is a specific DNA sequence of a out 220 nucleotides. The centromere has a specific location on any given chromosome. A disk of protein called a kinetochore is attached to each centromere . It forms an attachment site for the micro-tubules of the mitotic
The cell cycle moves into G2 phase, the condensation of chromosomes begins. he cell also begins to assemble different structures during G2 phase. The cell I ter uses .these structures to move the chromosomes to opposite poles of the ell. For example, centrioles replicate, and there is extensive synthesis of the roteins that make up the microtubules.