• Cell division:- DNA replication & Cell growth• They takes place in a coordinated way to ensure correct division & formation of progeny cells which contains intact genomes.• Cell cycle- the events by which a cell duplicates its genome ,synthesises the constituents of the cell divides into daughter cells.
• Cell growth is a continuous process but DNA synthesis takes only once in this stage.• DNA Daughter nuclei (complex series of event – cell division).• DNA processes are under genetic control.
• Eukaryotic cell – divide once in approx. every 24 hrs.• This varies from organism – organism & cell – cell type.• E.g. Yeast – the cell cycle of yeast progress in 90 minutes.
There are 2 basic phases:-1) M Phase(mitosis phase) – the actual cell division/mitosis occurs.2) Interphase - phase between 2 successive M phases. Duration of Interphase is 24 hrs in a cell cycle. Karyokinesis – (nuclear division) separation of daughter chromosomes. Cytokinesis – division of cytoplasm.
The Interphase is divided into 3 phases:-a) G1 Phase (Gap 1)Interval Between mitosis & initiation of DNA replication.It is also known as growth phase & the cell is metabolically active.b) S Phase (Synthesis)DNA synthesis & replication takes placec) G2 Phase (Gap 2)Proteins are synthesized in preparation for mitosis while cell growth continues.
A diagrammatic view of cell cycle indicatingformation of two cells from one cell.
• Some cells do not divide further exit G1 Phase to enter an interactive stage – Quiescent stage (G0).M phase(mitosis) involve reorganization ofvirtually all components of the cell.Mitosis is divided into four stagesa) Prophaseb) Metaphasec) Anaphased) Telophase
Equational Division The number of chromosomes in the parentand progeny cells is the same. 46 23 23
• It is marked by the initiation of condensation of chromosomal material.• Chromosomal material mitotic chromosomes.• Spindle fiber – a set of thread like structures made by an organelle called centriole.• The centriole moves towards the opposite poles of the cell.• Two (sister) chromatids attach together at the centromere.
• Disintegration(breakup) of the nuclear envelope marks the start of metaphase.• In this phase the chromosomes are made of sister chromatids held together by the centromere.• Kinetochores (site of attachment of spindle fibers)– small disc shaped structures at the surface of the centromere.
• Chromosomes moves to the centre of the cell(spindle equator).• One chromatid of each chromosome connected n kinetochore to spindle fibers from one pole• Sister chromatids connected in the opposite poles by the kinetochores.• Metaphase Plate – plane of alignment of the chromosomes at the metaphase.