MITOSIS & MEIOSIS. Ms.S.ANITHA Head, Department of Zoology & Microbiology R.B.V.R.R.Women s College

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1 MITOSIS & MEIOSIS Ms.S.ANITHA Head, Department of Zoology & Microbiology R.B.V.R.R.Women s College

2 AMITOSIS / AMITOTIC DIVISION During amitosis the nucleus elongates and appears as dumb bell shaped. The constriction increases in size and divides the nucleus into two nuclei. The division of nucleus followed by the constriction of (cytoplasm); which divides the cell into two equal similar values, two daughter cells is formed.

3 PHASES OF AMITOTIC DIVISION The basic outline of mitosis is same in all living organisms. It includes four phases; G1 phase, G2 phase, S phase and mitotic phase. All the phases together cell cycle; G1, S & G2 phases collectively forming the inter phase continuously diving cells, individual cell passes through two main phases/mitotic cycle. i.e.; inter phase & mitotic phase.

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5 MITOSIS / MITOTIC DIVISION A. Interphase: The resting phase/stage between two mitotic divisions is inter mitotic/inter phase. No division occurs but in the nucleus and cytoplasm, active metabolic activities occur and also increase the volume of the cytoplasmic and nuclear substances takes place. The inter phase is the longest phase of the mitotic cycle. It takes 1 or 2 days for its completion. During inter-phase some events takes place. i. The nuclear envelope remains intact ii. The chromosomes occur long, coiled and indistinctly visible chromatin fibers iii. The DNA amount becomes double, due to accumulation of rrna & ribosomal protein. The nucleolus increases in size. iv. A daughter centriole is originated, so inter phase cell has two centrioles.

6 B. Mitotic phase: M phase includes four phases. i.e.; prophase, metaphase, anaphase & telophase. Prophase: It is the first phase of mitosis. During the prophase the events takes place in the cell. The cell becomes spheroid and viscous. Nuclear envelop disintegrates. Both chromatids remain connected with each other by the centramere. The chromatids become shortened and thickened nucleolus disappears. Each centriole separates and migrates towards to opposite poles of the cell. Each centriole duplicates, so that both poles of the cell contains paired centrioles. The centrosome forms delicate filamentous asters arise from the spindle.

7 Pro metaphase: Few changes occur in the cell. Nuclear membrane completely disintegrates. A clear zone equator appears between mid-line of the spindle & nucleus. Chromosomes move towards the equator. Metaphase: During this phase some events occur in the cell. Each chromosome reaches to the equator and arrange themselves radically at the periphery of the spindle. Spindle fiber attach with the centromere of each chromosome chromatin fibers. Inter zonal/inter chromosomal fibers present between the chromosomes.

8 Anaphase: changes occur in the cell during anaphase are: The centromere of each chromosome divides into two; the chromatids of each chromosome are separated and form two chromosomes. The chromosomes become shorter and thicker & migrate towards the opposite poles of the cells. The inter zonal fibers push the daughter chromosomes towards the opposite poles. Telophase: It is the final stage of mitosis. Events occur in telephase are: The chromosomes which reach at the opposite poles become elongate, thread like. The nucleolus reappears around the chromosomes new nuclear envelope and nucleolus forms. Mitotic spindle rearrange and disappear. After the telophase, two daughter nuclei are formed due to karyokinesis. It is followed by the cytokinesis.

9 Cytokenesis: It is the division of cytoplasm. In this process a constitute appears in the middle of the cell, it is gradually deepens and finally divides the cell into two daughter cells. Significance of mitosis: In mitotic division, the chromosome number in each daughter cell remains the same like parent cell. An equilibrium maintained in the amount of DNA & RNA contents. Mitosis provides opportunity for the growth and development of the body, old, decaying and dead cells are replaced by new cells.

10 MEIOSIS / MEIOTIC DIVISION The meiotic division includes two complete divisions of diploid cells, resulting into four haploid nuclei. The first meiotic division heterotypic division The second meiotic division homotypic division In heterotypic division, the haploid cells divides mitotically and results into four haploid cells. In homotypic division, pairing of chromosomes, exchange of genetic material and reduction of the chromosomes do not occur. Both meiotic divisions occur continuously and each includesprophase, metaphase, anaphase and telophase. The prophase of first meiotic division in very significant phase because the most cytological events such as synopsis, crossing over etc occur during this phase. The prophase is longest meiotic phase it is divided into sub-stages i.e. proleptotene, leptotene, zygotene, pachytene, diplotene and diakinesis.

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12 A. Heterotypic division/first meiotic division In the beginning; nucleus swell up by absorbing water from the cytoplasm and nuclear volume increases, it causes modification of nuclear components. After the changes, the cell passes to the first stage of first meiotic division i.e. prophase. STAGES OF FIRST MEIOSIS

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14 First prophase: It is the longest stage of meiotic division. During this stage, the amount of DNA becomes double & DNA synthesis occurs, it includes 6 sub stages. i. Proleptotene/Proleptonema: It is resemble like mitotic prophase. In this stage, chromosomes become long, thin, uncoiled slender thread like structures. ii. Leptotene/Leptonema: In this stage, chromosomes become more uncoiled as a long thread like shape inside the nucleus chromosomes take up a specific orientation. The centriole duplicates and each daughter centriole migrates towards the opposite pole of the cell.

15 iii. Zygotene/Zygonema: In this stage, the pairing of homologous chromosomes takes place. The chromosome which comes from by other (mother) and by sperm (father) are attracted towards each other and their pairing takes place. The pairing of homologous chromosomes is called synopsis. Pairing of chromosomes is very exact and specific. iv. Pachytene/pachynema: In this stage, pair of chromosomes becomes twisted spirally around each other. Each homologous chromosome splits lengthwise to form two chromatids. Actually it is the doubling of DNA. The pachytene chromosome consists of four chromatids closely joined together in one complex unit bivalent. During pachynema stage, crossing over takes place. The crossing over involves reshuffling, redistribution and mutual exchange of hereditary material of two parents between two homologous chromosomes.

16 v. Diplotene/Diplonema: In this stage, the homologous chromosomes repel each other, they are not separated completely, because both remain united at the point of inter change or chaismata. vi. Diakinesis: In this stage, the bivalent chromosomes become more condensed and evenly distributed in the nucleus. The nucleolus detaches and disappears. During diakineses, the chaisma moves from the centramere towards the ends of chromosomes. This type of movement of chaismata is teminalisation. The chromatids still remain connected by the terminal chaisma, exist up to the metaphase.

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18 Metaphase I: In this stage, spindle fibers are attached with the centromer of the chromosome of each tetrad. The centromere of each chromosome is directed towards the opposite poles. The repulsive forces between the homologous chromosomes increase greatly and the chromosomes become ready to separate. Anaphase I: In this stage, due to the contraction of chromosomal fibers, each chromosomes with its two chromatids moves towards the opposite poles of the cell. The actual reduction occurs at this stage, during the chaisma formation out of two chromatids of chromosomes, one has changes its counterpart. So that, the two chromatids of a chromosomes do not resemble with each other in the genetical terms. Telophase I: In this stage, nuclear envelop forms around the chromosomes and the chromosomes uncoil. The nucleolus reappears and two daughter chromosomes are formed. After the karyokinesis, cytokinesis occurs and two haploid cells are formed.

19 B. Homotypic division of meiosis Second meiotic division/homotypic division: It is actually the mitotic division which divides each haploid meiotic cell into two haploid cells. The second meiotic division includes four stages.

20 Prophase II: In this stage, each centriole divides into two each centriole migrate to the opposite pole of the cell. The fibers get arranged in the form of spindle at the right angle of the spindle of first meiosis. The nuclear membrane and nucleolus disappears. The chromosome with two chromatids becomes short and thick. Metaphase II: During this stage, the chromosomes get arranged on the equator of the spindle. The centramere divides into two and each chromosome produces two daughter chromosomes/monads. The spindle fibers attached with the centramere of the chromosomes. Anaphase II: In this stage, the daughter chromosomes move towards the opposite poles due to the contraction of chromosomal fibers and stretching of inter-zonal fibers of the spindle.

21 Telophase II: In this stage, chromatids migrate to the opposite poles and now known as chromosomes. The ER forms nuclear membrane around the chromosomes and the nucleolus reappear due to synthesis of rrna and rdna. After the karyokinesis, in each haploid meiotic cell cytokinesis occurs then four haploid cells are formed. These cells have different types of chromosomes due to the crossing over in the prophase I. Significance of meiosis: The meiosis division has the greatest significance for the biological world. Meiosis maintains a definite and constant number of chromosomes in the organisms. By crossing over, meiosis provides an opportunity for the exchange of the genes and causes the genetical variation among the species. The meiosis has a peculiar taxonomic, genetical and evolutionary importance for the sexually reproducing organisms.

22 THANK YOU