SUMMARY
The cell hypothesis states that cells develop from already existing cells. Cell division is the process through which this takes place. Every sexually reproducing creature begins its life cycle as a zygote, which is a single cell. The development of the adult organism does not mark the end of cell division; rather, it continues throughout the organism's life cycle. The cell cycle refers to the phases that a cell goes through as it progresses from one division to the next. The cell cycle is split into two phases: (0) Interphase, which is the period of cell division preparation, and (ii) Mitosis (M phase), which is the actual cell division period. The G, 5 and G, phases of interphase are the times when the cell develops and performs regular metabolism. Most of the organelle duplication also occurs during this phase. S phase marks the phase of DNA replication and chromosome duplication G, phase is the period of cytoplasmic growth. Mitosis is also divided into four stages namely prophase, metaphase, anaphase and telophase. Chromosome condensation occurs during prophase. Simultaneously, the centrioles move to the opposite poles. The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing. Metaphase is marked by the alignment of chromosomes at the equatorial plate. During anaphase the centromeres divide and the chromatids start moving towards the two opposite poles. Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear. This stage is called the telophase. Nuclear division is then followed by the cytoplasmic division and is called cytokinesis. Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell.
In contrast to mitosis, melosis occurs in the diploid cells, which are destined to form gametes. It is called the reduction division since it reduces the chromosome number by half while making the gametes. In sexual reproduction when the two gametes fuse the chromosome number is restored to the value in the parent. Melosis is divided into two phases-melosis I and metosis II. In the first meiotic division the homologous chromosomes pair to form bivalents, and undergo crossing over. Melosis I has a long prophase, which is divided further into five phases. These are leptotene, zygotene. pachytene. diplotene and diakinests. During metaphase I the bivalents arrange on the equatorial plate. This is followed by anaphase I in which homologous chromosomes move to the opposite poles with both their chromatids. Each pole receives half the chromosome number of the parent cell. In telophase 1. the nuclear membrane and nucleolus reappear. Melosis Il is similar to mitosis. During anaphase II the sister chromatids separate. Thus at the end of meiosis four haploid cells are formed.
