- Slides: 29
CELL CYCLE AND CELL DIVISION
CELL CYCLE � The sequence of events by which a cell duplicates its genome synthesis the other constituents of the cell and eventually divides into two daughter cells. � Phases � The of Cell Cycle cell cycle is divided into two basic phases: 1. Interphase 2. M Phase (Mitosis phase)
� Interphase: The phase between subsequent cell divisions is called the interphase. The interphase lasts for more than 95% of the cell cycle. �M Phase (Mitosis phase): The actual cell division takes place in the M phase. The M phase lasts for less than 5% of the cell cycle. The M phase is composed of two major steps, -karyokinesis and cytokinesis. Division of nucleus happens during karyokinesis. Division of cytoplasm happens
� The interphase is further divided into three phases, which are as follows: � G 1 phase (Gap 1): During this phase, the cell is metabolically active and continuously grows. �S phase (Synthesis): During this phase, DNA synthesis or replication takes place. The amount of DNA becomes double during this phase, but the number of chromosomes remains the same. � G 2 phase (Gap 2): During this phase, protein synthesis takes place.
� Quiescent Stage (G 0): � Cells which do not divide further, exit G 1 phase to enter an inactive stage. This stage is called quiescent stage (G 0) of the cell cycle. The cells in this stage remain metabolically active but do not undergo division. But these cells can resume division as and when required.
�M PHASE: � Mitosis is divided into four stages, 1. Prophase, 2. Metaphase, 3. Anaphase and 4. Telophase 1. Prophase � Condensation of chromosomal material takes place. A chromosome is seen to be composed of two chromatids. The chromatids are attached together at the centromere. � Spindle fibres are formed.
� Various cell organelles; like golgi bodies and ER cannot be seen during this staged. Nucleolus and nuclear envelope also disappear. 2. Metaphase � All the chromosomes come to lie at the equator. � In each chromosome, one chromatid is connected to the spindle fibre from one pole and another chromatid is connected to the spindle fibre from another pole.
� The plane of alignment of chromosomes during this phase is called metaphase plate. 3. Anaphase � Centromeres split which results in separation of chromatids. � After that, chromatids move to opposite poles. 4. Telophase � The chromosomes form clusters at opposite poles. They become inconspicuous.
� Nuclear envelope is formed around the chromosome clusters. � Nucleolus, golgi complex and ER are also formed Cytokinesis � Division of cytoplasm is achieved by cytokinesis. In animal cell, a furrow appears in the plasma membrane. The furrow gradually deepens and finally joins in the centre. �
� Thus, the cytoplasm is divided into two parts. � In plant cells, cell wall formation begins in the centre. This grows outwards to meet the existing lateral walls and thus, the cytoplasm is divided into two parts.
� Onion Root Cell
�MEIOSIS: � Meiosis involves two sequential cycles of nuclear and cell division, but only a single cycle of DNA replication. Meiosis is divided into meiosis I and meiosis II. � Meiosis I begins after the S phase, and meiosis II follows meiosis I. � Pairing of homologous chromosomes happens during meiosis which results in recombination of genes. � Four haploid daughter cells are formed at the end of meiosis.
�MEIOSIS � Prophase I I: � Prophase in meiosis I is typically longer and more complex than the prophase in meiosis II. Prophase I is subdivided into five phases, 1. Leptotene 2. Zygotene 3. Pachytene, 4. Diplotene and 5. Diakinesis. 1. Leptotene: � During this stage, the chromosomes become gradually visible under light microscope. Compaction of chromosomes continues throughout this phase.
2. Zygotene: 2. Zygotene � Chromosomes start pairing together. This process is called synapsis. The paired chromosomes are called homologous chromosomes. � Formation of synapsis is accompanied by the formation of synaptonemal complex. � The synaptonemal complex by a pair of homologous chromosomes is called a bivalent or a tetrad.
3. Pachytene: � Bivalent chromosomes clearly appear as tetrads, at this stage. � Recombination nodules appear. These nodules are the sites at which crossing over takes place between non-sister chromatids of the homologous chromosomes. � Exchange of genetic materials between two homologous chromosomes takes place during crossing over. This leads to recombination of genetic materials on the two chromosomes.
4. Diplotene: � Synapotnemal complex is dissolved at this stage. � The recombined homologous chromosomes of the bivalent separate from each other; except at the site of crossing over. � The X-shaped structures; thus formed; are called chiasmata.
5. Diakinesis: � Meiotic spindles are formed to prepare the homologous chromosomes for separation. � Nucleolus disappears and nuclear envelope breaks down by the end of diakinesis. � Metaphase I: � The bivalent chromosomes are aligned on the equatorial plate. � Spindle fibres from opposite poles attach to the pair of homologous chromosomes.
� Anaphase I: � Homologous chromosomes separate, but sister chromatids remain attached at their centromeres. � Telophase I: � Nuclear membrane and nucleolus reappear. � This is followed by cytokinesis and this stage is called the diad of cells.
� The stage between the two meiosis divisions is called interkinesis. Interkinesis is usually short live �MEIOSIS � Prophase II II: II Meiosis II resembles the mitotic cell division. It begins immediately after cytokinesis. Nuclear membrane disappears. Chromosomes again become compact.
� Metaphase II: II The chromosomes align at the equator. Spindle fibres from the opposite poles get attached to the kinetochores of sister chromatids. � Anaphase II: II Centromeres split and sister chromatids move towards the opposite poles. � Telophase II: II The two groups of chromosomes get enclosed by nuclear envelope. This is followed by cytokinesis; resulting in the formation of four daughter cells.
� Significance of Meiosis: � Conservation of specific chromosome number of each species is achieved across successive generations in sexually reproducing organisms through meiosis. � Meiosis helps in increasing the genetic variations in the population of organisms from one generation to the next.