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Cell cycle

This document summarizes the key stages of mitosis and meiosis. It describes that mitosis occurs in somatic cells and results in two identical daughter cells with the same number and type of chromosomes as the parent cell. Meiosis occurs in germ cells and results in four haploid daughter cells each with half the number of chromosomes as the original diploid parent cell. The document outlines the main phases of the cell cycle, including interphase and the phases of mitosis (prophase, metaphase, anaphase, telophase). It also provides details about the key events and phases of meiosis I and meiosis II, including homologous chromosome pairing, crossing over, and the formation of haploid daughter cells.

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CELL CYCLE MITOSIS AND MEIOSIS DR. DILIP V. HANDE ASSOCIATE PROFESSOR, DEPT OF BOTANY SHRI SHIVAJI SCIENCE COLLEGE, AMRAVATI MS.
INTRODUCTION:  The number of cells increases by the division as preexisting cells.  The dividing nucleate cells include two integral activities i.e. The division of nucleus (karyokinesis) and division of the cytoplasm (cytokinesis).  Usually the karyokinesis is followed by the cytokinesis.  Some times cytokinesis does not follow karyokinesis these division results in the formation of multinucleate cells.
CELL DIVISION  The cell may divide by any one of the following methods. Amitosis or direct cell division Mitosis or indirect cell division Meiosis or reduction division
AMITOSIS  It is a simple mode of cell division, generally occurs in unicellular organisms like bacteria and protozoon.  It this case the nucleus elongate first become double shaped forming middle constriction finally form two nuclei and later on by constriction the cytoplasm the two daughter cells get formed .  In the process there is complete absence of nuclear events.
AMITOSIS
MITOSIS  Mitosis generally occurs in somatic (vegetative) cell.  It is the process by which cell divides with sets of chromosome exactly similar to the parent cell.  Thus number of cells increase without any change in the genetic composition i.e. structure & number or chromosome replace and repair of cells in the theory.  The most convenient material for the study of mitosis is the root tips of onion.  The mitotic cycle or cell cycle includes two distinct phases.  Inter phase or non dividing period.  Mitotic phase or cell- division period.
INTERPHASE :  It is called resting period, but during this period the cell is metabolically very active.  In this phase the DNA content is doubled and also some proteins and enzymes are synthesized.  Cell prepare itself for the division.  It is longest phase of cell division include 3 sub- phases.
CELL CYCLE
CELL CYCLE  G1 phase (gap period):- It is growth phase during this synthesis of proteins and RNA takes place and the cell grows in volume.  S phase :- (synthetic period):- During this period DNA synthesis occurs i.e replication of chromosomal DNA takes place which result in doubling of the chromosomal threads.  G2 (Gap2):- In this part the volume of cell increases and it is pre division stage, amount of DNA and cell content becomes more synthesis of proteins and RNA takes place. In this phase chromatin fibers are seen.
MITOTIC PHASE:  Actual cell division occurs in this phase, it include karyokinesis and cytokinesis.  Karyokinesis is the division of nucleus into two daughter nuclei.  It consist of following phases
PROPHASE  It is the first phase of mitosis in which cell becomes spheroid, and viscous.  The chromatin material becomes visible as separate threads or chromosomes.  Each chromatid contains a single DNA molecule.  The two chromatids of a chromosome are connected by a centromere.  The nucleolus and nuclear membrane start disappearing.  The chromosomes remain distributed in nucleoplasm.
METAPHASE  In this phase the chromosomes reaches the central or equatorial portion of the spindle.  The chromosomes one lined up in one plane to form equatorial plate or metaphasic plate .  There are two kinds of spindle fibers.  Some fibers are long and extend from pole to pole. These are called continuous fibers.  Other get attached to the chromosomes at the centromere these are called chromosomal fibers.
ANAPHASE :  In this phase the centromers of each chromosome divides into two.  The two sister chromatids of each chromosome separate from each other these chromatids now called as a daughter chromosomes.  The two sets of daughter chromosomes migrate to the opposite poles of the spindle.  Probably this movement is caused due shorting of chromosomal fibers.  Depending to upon the position of centromere the chromosomes showing shapes like V,J,L or I (metacentric submetacentric and telocentric).
TELOPHASE  This is the last phase in karyokinesis.  The two sets of daughter chromosomes reach the opposite poles.  The chromosomes begin to uncoil and form chromatin networks.  Nuclear envelope formed around each set of the chromosomes.  The nucleoli appear at the site of nuclear organizer.  The after telophase two daughter nuclei are formed due to karyokinesis.  Karyokinesis is then followed by cytokinesis.
CYTOKINESIS  The division of cytoplasm into two daughter cells is called cytokinesis.  It is start simultaneously with telophase stage.  In plant cells it is accomplished by the formation of phragmoplast and cell plate.  The tubular elements of E.R. , vesicles of Golgi complex reach the equator forming cell plate.  This plate grow and join the cell wall. This plate later convert into the middle lamella.
SIGNIFICANCE OF MITOSIS :-  Mitosis ensures equal distribution of the nucleus and cytoplasm between the daughter cells.  The hereditary material (DNA)is also equally distributed.  The constant number of chromosomes in all cells of the body is because of mitosis.  Mitosis helps in the growth and development of the organs and the body of organisms.  Mitosis help in the repair of damaged tissues or organs by producing new cells.  Mitosis helps in the asexual reproduction in some organisms.
MEIOSIS  Introduction  The term meiosis was coined by Former in 1905.  It occurs only in the reproductive cells which results in the formation of haploid gametes.  During meiosis the chromosomes divide once and the nucleus and cytoplasm divide twice.  Due to this four haploid cells are formed from diploid cell.  Hence it is also called reduction division.  In this case the haploid daughter cells differ from each other as well as from mother cell.
PROCESS OF MEIOSIS  The process of meiosis shows a sequences of events similar to those of mitosis but these events are repeated twice with or without a short interphase between them.  In the first meiotic division the diploid parent cell divide into two haploid daughter cells, so this division is also known as Heterotypic division.  The second meiotic division is a simple mitotic division.  This division is also known as the homeotypic division.  Each of the two meiotic cell division is further distinguished into sub stages.  They are as follows.
MEIOSIS-I  It is the first meiotic division during which the diploid parent cell gives rise to two haploid daughter cells.  It includes the further phases.  Prophases –I :  This phase has longer duration. It is having about 5 phases.
LEPTOTENE / LEPTONEMA :  In this stage the volume of the nucleus increases.  Chromosome becomes uncoiled and long thread like in shape.  The chromosomes look beaded in appearance i.e. each chromosome has one centromere and numerous nucleosomes (beads).  The nucleolus and nuclear envelop is visible.
ZYGOTENE / ZYGONEMA :  In this stage the chromosomes become shorter and thicker and pairing of homologous chromosomes takes place.  The homologous chromosomes (one paternal and other maternal) from the two sets are attracted towards each other and form pairs.  In each pair, the two homologous lie parallel to each other all along their lengths.  This pairing is called synapsis and the paired chromosomes are called bivalent.  At this stage each chromosome appear to have only chromatid.  Thus each pair has in all two chromatids.  Hence each pair is in the dyad stage.
PACHYTENE / PACHYNEMA  In this stage the recombination of characters (genes) takes place through a phenomenon called crossing over.  The chromosome becomes shorter, thicker, distinct now each chromosome has two sister chromatids joined by a centromere.  Thus each pair of the homologous chromosomes (bivalent ) at this stage consist of four chromatids (
PACHYTENE / PACHYNEMA  The non sister chromatids are twisted round each other and may take part in crossing over.  The chromatid is the unit of crossing over.  Two chromatid belonging to two different homologues (non- sister chromatids) undergo one or more breaks at the same level.  After the break inter change of chromatid segments takes place between the non- sister chromatids.  The broken chromatid segments unit with other chromatids due to presence of an enzyme ligase.  This process inter change of chromatin
D) DIPLOTENE/ DIPLONEMA :  In this stage the homologous chromosomes repel each other as the force of attraction between them decreases.  The separating chromosomes are held together at one or more points, where crossing over occurred.  These points are called chiasmata. The number of chiasmata varies with the length of the chromosomes.  Generally shorter chromosomes have few chiasmata than the longer ones.  By the end of diplotene, the chiasmata begin to move from the centromere towards the end.  This displacement of chiasmata is termed as terminalization.  When terminalization is complete the homologous chromosomes are held together by the terminal chiasmata.
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