Cell Cycle and Cell Division
The cell cycle is a sequence of events that prepares a cell for division. Cell division is the process by which a cell divides into two or more cells.
Cell cycle includes two phases, Interphase and M phase.
In interphase, a cell prepares itself for cell division by undergoing growth and replication of its DNA.It is divided into three phases – G1 phase, S phase and G2 phase.
Mitosis (or meiosis) and cytokinesis together define the mitotic (M) phase of a cell cycle. The mitosis is a process by which a somatic cell duplicates the chromosomes in its cell nucleus to generate two identical daughter nuclei. It consists of five phases – Prophase, Metaphase, Anaphase, Telophase and Cytokinesis.
During prophase, chromosome condensation occurs and the centrioles move to the opposite poles. The nuclear envelope and the nucleolus disappear.The spindle fibres start appearing.
In metaphase, the chromosomes aligned at equatorial plate.
During anaphase, the centromeres divide and thechromatids start moving towards the two opposite poles. Once the chromatidsreach the two poles, during telophase, the chromosomal elongation starts, nucleolus and thenuclear membrane reappear.
The nucleardivision is then followed by the cytoplasmic division and is called cytokinesis.
The meiosis occurs in the diploid cells and gametes. It is also known as the reduction division, as the number of chromosome reduces to half during gametes formation. Meiosis is divided into two phases – meiosis I and meiosis II.
Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids in S phase. It consists of – prophase I, metaphase I, anaphase I and telophase I.
The Prophase I stage is a longest and more complex phase than prophase of mitosis. It has been further subdivided into following five phases- leptotene, zygotene, pachytene, diplotene and diakinesis
Leptotene is the first stage of prophase I. In this stage, the two sister chromatids are still indistinguishable from one another.
The homologous chromosomes start pairing in the process of synapsis and the formation of synaptonemal complex occurs in zygotene stage.
In pachytene stage, thecrossing over occurs at the Chaismata.
In diplotene phase, dissolution of synaptonemal complex occurs and homologous chromosomes of the bivalents start separating from each other except at chaismata.
During diakinesis, terminalisation of chaismata occurs and the chromosomes are fully condensed.
During metaphase I, the bivalents arrange on the equatorial plate. This stage is then followed by anaphase I in which homologous chromosomes move to the opposite poles with both their chromatids. During telophase I, the centromeres arrive at the poles and each daughter cell has half the number of chromosomes.
Meiosis II is the second half of meiotic division and is similar to mitosis.
Meiosis provides opportunities for new genetic combinations through crossing over and ensuring heritable variation.