Cell Cycle And Cell Division Revision Notes For NEET

CELL CYCLE AND CELL DIVISION

 

        All multicellular organisms start life as a single cell, formed after fertilisation and further grow to give rise to new population. The phenomenon of production of daughter cells from the parent cells is known as cell division.

        Cell cycle is the sequence of events through which a cell duplicates its content and divided into daughter cell. It is comprised of two main stages, i.e. interphase and M-phase or dividing phase.

        The interphase is the longest phase (about 96%) of the cell cycle during which cell prepares itself for both cell growth and DNA replication. It consists of four phases, i.e. G1-phase, G0-phase, S-phase and G2-phase.

        In G1-phase cell remains metabolically active and acquires nutrients. Synthesis of cell organelles, RNA, ribosomes, protein, etc. takes place.

        G0-phase known as quiescent stage during this phase cell neither divides nor synthesises organelles. Some cells such as nerve cells and erythrocytes that do not divide further during cell cycle exit from G1 phase to enter into an inactive phase called G0-phase.

        S-phase also called as synthesis phase. DNA replication occur in this phase.

        In G2-phase preparations are made for genomic separation. Mitochondria and other organelles replicate, chromosomes condense, etc.

        M-phase is the phase of actual division. It consists of major events, karyokinesis (division of nucleus) and cytokinesis (division of cell cytoplasm).

        The karyokinesis of cell division occurs via two processes known as mitosis and meiosis.

        Mitosis is an equational division in which parent cell (2n) divides into two daughter cells (2n).

        The various stages of mitosis are prophase, metaphase, anaphase and telophase.

        During prophase the chromatin fibres become shorten and thicken. Each chromosome consists of two chromatids. Nucleoli and nuclear envelope is no longer visible.

        During metaphase the chromosomes lines up at the equator of the spindle to form metaphase plate. The kinetochores of sister chromatids get joined to spindle fibres from opposite poles.

        During anaphase the sister chromatids of each chromosomes starts moving to the opposite poles of the spindle. Splitting of chromosomes takes places due to the division of centromere which attaches two chromatids.

        During telophase nuclear envelope is reformed around the chromatids, nucleolus reforms in each nucleus, chromosomes decondense into long fine filaments and mitotic spindle disappears.

        Ribonuclease, azide and cyanide are some mitotic poisons that interfere with the normal miotic division.

        Auxin, gibberellins and insulin are some mitogens that promote mitosis.

        Amitosis is also called direct cell division. It occurs without formation of spindle and appearance of chromosomes.

        Meiosis is reductional division in which the parent cell (2n) divides into four daughter cells (n). Meiosis comprises of two substages meiosis-I and meiosis-II.

        Meiosis-I is further divided into prophase-I (with stage like leptotene, zygotene, pachytene, diplotene and diakinesis), metaphase-I, anaphase-I and telophase-I.

        At leptotene stage, chromosomes shorten and become visible as single structures and appear as beaded structure.

        At zygotene stage, homologous chromosomes pair up. This pairing is called synapsis.

        At pachytene stage, non-sister chromatids of homologous chromosomes exchange segments between themselves. This exchange of segments is called crossing over.

        At diplotene stage, chiasma formation occurs. Chiasmata formation is necessary for the separation of homologous chromosomes which have undergone crossing over.

        At diakinesis chiasmata shifts towards the terminals of the chromosomes. It is known as terminalisation of chiasmata.

        Meiosis-II is similar to mitosis but only difference lies in the fact that after completion of meiosis-II four haploid (n) daughter cells are formed.

        Cytokinesis is the process of division of cytoplasm. It occurs in plant cell by cell plate formation while in animal cell by cell furrowing method.

        Cyclin-dependent protein kinases and cyclin are proteins that control cell cycle at certain points in a cell cycle.

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