Cell division – meiosis


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– Male and female gametes fuse together during fertilization to form a zygote. The chromosome number is halved during the formation of gametes by the process of meiosis. This maintains the chromosome number generations after generations. Meiosis leads to genetic diversity which is very essential for evolution.

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Cell division – meiosis

  1. 1. Cell Division – Meiosis Sexual reproduction in eukaryotes takes place by the fusion of gametes and they maintain their ploidy. How is this possible?
  2. 2. Asexual and Sexual Reproduction - Differences <ul><li>Asexual or vegetative reproduction </li></ul><ul><ul><li>Takes place only through mitosis which involves duplication of cells. </li></ul></ul><ul><ul><li>Produces a genetically identical offspring for example a new plant originating from a shoot or root </li></ul></ul><ul><ul><li>This method is rapid and allows the spread of the organisms to a large extent. </li></ul></ul>
  3. 3. Asexual and Sexual Reproduction - Differences <ul><li>Sexual reproduction </li></ul><ul><ul><li>Takes place only in eukaryotes. </li></ul></ul><ul><ul><li>A new individual is formed by the fusion of two gametes which are haploid cells. </li></ul></ul><ul><ul><li>The new individual so formed is a single cell – zygote and is diploid having two sets of chromosomes. </li></ul></ul><ul><ul><li>Gametes usually come from two different parents </li></ul></ul><ul><ul><ul><li>Female – egg </li></ul></ul></ul><ul><ul><ul><li>Male –sperm </li></ul></ul></ul><ul><ul><li>Meiosis increases the genetic diversity as it changes the genetic information. </li></ul></ul>
  4. 4. Chromosomal Characters <ul><li>Ploidy – Number of sets of chromosomes in a single cell </li></ul><ul><li>Depending on the number of sets of chromosomes the cells are known as </li></ul><ul><ul><li>Haploid (n) – Those which have a single set of chromosome </li></ul></ul><ul><ul><li>Diploid (2n) - Those which have two sets of chromosomes </li></ul></ul><ul><ul><li>Triploid (3n) - Those which have three sets of chromosomes </li></ul></ul><ul><li>Many plants and higher organisms are diploid (2n) however their gametes are haploid (n). </li></ul><ul><li>In case of human beings – </li></ul><ul><li>Diploid (2n) – 23 pairs or 46 chromosomes </li></ul><ul><li>22 pairs of homologus chromosomes are known as Autosomes </li></ul><ul><li>One pair of sex chromosomes </li></ul><ul><ul><li>Female-sex chromosomes are homologous (XX) </li></ul></ul><ul><ul><li>Male-sex chromosomes are non-homologous (XY) </li></ul></ul>
  5. 5. An Outline of Meiosis <ul><li>The term meiosis was coined by J.B Farmer and J.B Moore in 1905. </li></ul><ul><li>It is alternatively known as reduction division. </li></ul><ul><li>Nuclear division takes place in two different phases which are: </li></ul><ul><ul><li>Meiosis 1 (reduction) – which results into two haploid cells </li></ul></ul><ul><ul><li>Meiosis 2 (Division) – Each haploid cell divides into two producing 4 haploid cells. </li></ul></ul><ul><ul><ul><ul><ul><li>Image reference - http://www.daviddarling.info/encyclopedia/M/meiosis.html </li></ul></ul></ul></ul></ul>
  6. 6. Interphase <ul><li>The preparatory stage of meiosis and mitosis is </li></ul><ul><li>named as interphase and is similar in both. </li></ul><ul><li>It is divided into three phases: </li></ul><ul><li>Growth 1 (G1) phase: </li></ul><ul><ul><li>An active period marked by the synthesis of </li></ul></ul><ul><ul><li>various proteins and enzymes needed for cellular </li></ul></ul><ul><ul><li>growth. </li></ul></ul><ul><ul><li>DNA appears like a single long coiled molecule. </li></ul></ul>
  7. 7. Interphase <ul><li>Synthesis (S) phase: </li></ul><ul><ul><li>Replication of the genetic material takes place. </li></ul></ul><ul><ul><li>Each chromosome divides into two sister chromatids. </li></ul></ul><ul><ul><li>RNA transcription is low. </li></ul></ul><ul><ul><li>Protein synthesis is low. </li></ul></ul><ul><li>Growth 2 (G2) phase: </li></ul><ul><ul><li>This stage is absent in meiosis </li></ul></ul>
  8. 8. Meiosis I- Prophase I <ul><li>Four stages of prophase I are as follows: </li></ul><ul><li>Leptotene ( leptonema) </li></ul><ul><ul><li>This stage is of a short duration during which condensation and coiling of chromosomes takes place. </li></ul></ul><ul><ul><li>Chromosomes become visible as long slender threads bearing numerous bead-like nucleosomes (chromomeres). </li></ul></ul><ul><ul><li>Chromosomes are arranged in a linear fashion along their lengths. </li></ul></ul><ul><ul><li>The nuclear envelope and the nucleolus are prominently visible. </li></ul></ul><ul><ul><li>The thin chromosomes are scattered in the nucleus. </li></ul></ul>
  9. 9. Meiosis I- Prophase I <ul><li>Zygotene (z ygonema ) </li></ul><ul><ul><li>Pairing of homologus chromosomes known as synapsis takes place. </li></ul></ul><ul><ul><li>Pairing or synapsis takes place in a zipper like fashion and may start at the centromere, chromosomal ends or at any other position. </li></ul></ul><ul><ul><li>Thus pairing is highly specific and exact. </li></ul></ul><ul><ul><li>The paired chromosomes are called Bivalent or tetrad chromosome. </li></ul></ul><ul><ul><li>The chromosomes at this stage are shorter thicker and more distinct. </li></ul></ul>
  10. 10. Meiosis I- Prophase I <ul><li>Pachytene (Pachynema) </li></ul><ul><ul><li>Within a tetrad consisting of homologous pair of chromosomes, the paternal and maternal chromatids are non-sister to one another. </li></ul></ul><ul><ul><li>The non- sister chromatids take part in crossing over. </li></ul></ul><ul><ul><li>Non-sister chromatids randomly exchange segments over regions of homology. </li></ul></ul><ul><ul><li>The exchange of information between the non-sister chromatids results in a recombination of genes </li></ul></ul>
  11. 11. Crossing Over <ul><li>Crossing over consists of a mutual exchange of equal quantity (segments) of chromosomal material between two non-sister chromatids. </li></ul><ul><li>It involves the following events: </li></ul><ul><ul><li>The non-sister chromatids, taking part in crossing over, break simultaneously at the identical points (i.e. at homologous points). </li></ul></ul><ul><ul><li>The broken segments are of equal lengths. </li></ul></ul><ul><ul><li>The broken segments may rejoin or there may be an exchange of the segments between the non-sister chromatids. </li></ul></ul>
  12. 12. Crossing Over <ul><li>During crossing over, the genes located on the segments are exchanged between the two chromatids. </li></ul><ul><li>It results in the recombination of genes (characters). </li></ul><ul><li>Crossing over does not take place between sister chromatids. </li></ul><ul><li>The point where the non sister chromatids exchange is known as chaisma. </li></ul>
  13. 13. Crossing Over <ul><ul><li>Image reference - http://www.accessexcellence.org/RC/VL/GG/comeiosis.php </li></ul></ul>
  14. 14. Significance of Crossing Over <ul><li>The gametes produced through meiosis receive a new combination of characters (genes). </li></ul><ul><li>Therefore individuals with new combination of characters are produced in each generation. </li></ul><ul><li>This forms the genetic basis for variations and plays important role in evolution. </li></ul>
  15. 15. Meiosis I- Prophase I <ul><li>Diplotene (Diplonema) </li></ul><ul><li>Two major events involved in this stage are: </li></ul><ul><li>Repulsion of homologous chromosomes </li></ul><ul><ul><li>Homologous chromosomes of a pair start repelling each other. </li></ul></ul><ul><ul><li>Homologous chromosomes begin to separate and uncoil. </li></ul></ul><ul><ul><li>The non-sister chromatids which are involved in the cross-overs are still held together at the points of crossing over. </li></ul></ul><ul><li>Terminalization </li></ul><ul><ul><li>The separation and uncoiling of the homologues begins at the centromeres and proceeds towards the ends. </li></ul></ul><ul><ul><li>This causes progressive shifting of the chiasmata towards the ends of the chromatids. This is called terminalization of chiasma. </li></ul></ul>
  16. 16. Meiosis I- Diakinesis <ul><li>This is the last phase of Prophase-I. </li></ul><ul><li>Chromosomes are still in the pairs and in contact with each other by terminal chiasma. The chromosomes become shorter, thicker and more prominent. </li></ul><ul><li>The nucleolus and nuclear envelope disappear completely. </li></ul><ul><li>The pairs of chromosomes are seen distributed in the nucleoplasm. </li></ul>
  17. 17. Meiosis I- Prophase I Image reference – http://www.pinkmonkey.com/studyguides/subjects/biology- edited/chap6/b0606302.asp
  18. 18. Meiosis I- Metaphase I <ul><li>Spindle apparatus showing two types of spindle fibers is formed </li></ul><ul><ul><li>Continuous fibers extending from pole to pole and </li></ul></ul><ul><ul><li>Chromosomal fibers extending from pole to the equator of the spindle body. </li></ul></ul><ul><li>The homologous chromosomes, still in pairs, are arranged at the equator of the spindle. </li></ul><ul><li>Maternal homologues of all the pairs face toward one pole while the paternal homologues face the opposite pole. </li></ul><ul><li>The chromosomal fibers connect the centromeres of all the homologues to the pole on their respective side. </li></ul>
  19. 19. Meiosis I- Anaphase I <ul><li>Shortening of the chromosomal fibers </li></ul><ul><li>The sets of maternal and paternal chromosomes separate (segregate) and start moving towards opposite poles. </li></ul><ul><li>The cell starts elongating in preparation for division into two. </li></ul>Image reference –http://www2.estrellamountain.edu/faculty/farabee/biobk/biobookmeiosis.html
  20. 20. Meiosis I- Telophase I <ul><li>The two sets of chromosomes reach the opposite poles. </li></ul><ul><li>The chromosomes, each with two chromatids and one centromere, become thin and long. </li></ul><ul><li>A nucleolus reappears. </li></ul><ul><li>Each nucleus is haploid as it has received only one set of chromosomes. </li></ul><ul><li>Hence M-I is called reduction division. </li></ul><ul><li>Cytokinesis leads to creation of two daughter cells. </li></ul><ul><li>Microtubules disappear and a new nuclear membrane surrounds each haploid set. </li></ul>Image reference - http://www2.estrellamountain.edu/faculty/farabee/biobk/biobookmeiosis.html
  21. 21. Meiosis-II <ul><li>Interkinesis : The time interval between M-I and M-II is called interkinesis. </li></ul><ul><li>The stages of Meiosis – II are similar to mitosis. </li></ul><ul><li>The two nuclei formed after M-I divide during M-II and produce four haploid nuclei. </li></ul>
  22. 22. Meiosis-II - Prophase-II <ul><li>The chromosomes become shorter, thicker and distinct. </li></ul><ul><li>Each chromosome has two sister chromatids which are joined by a centromere. </li></ul><ul><li>The nuclear envelope and the nucleolus disappear by the end of Prophase-II. </li></ul>Image reference –http://www2.estrellamountain.edu/faculty/farabee/biobk/biobookmeiosis.html
  23. 23. Meiosis-II <ul><li>Metaphase-II </li></ul><ul><ul><li>The bipolar spindle body is formed. </li></ul></ul><ul><ul><li>Chromosomes are arranged along the equators in such a way that their chromatids are facing the opposite poles. </li></ul></ul><ul><ul><li>The centromere of each chromosome is connected with both the poles by chromosomal fibers. </li></ul></ul><ul><li>Anaphase-II : </li></ul><ul><ul><li>During early anaphase-II, the centromere of each chromosome divides longitudinally into two. </li></ul></ul><ul><ul><li>Therefore each chromosome is divided into two halves (chromatids) or daughter chromosomes. </li></ul></ul><ul><ul><li>During late anaphase-II, the two sets of daughter chromosomes are pulled away from each other and move towards the opposite poles. </li></ul></ul>
  24. 24. Image reference –http://www2.estrellamountain.edu/faculty/farabee/biobk/biobookmeiosis.html
  25. 25. Meiosis-II – Telophase -II <ul><li>The sets of chromosomes reach the opposite poles and a new nucleus is organized at each pole. </li></ul><ul><li>In all, four daughter nuclei are formed. </li></ul><ul><li>Each nucleus has half the number of chromosomes as compared to the original mother nucleus. </li></ul><ul><li>These nuclei also differ from each other in the structure and characters of chromosomes. This is because of the crossing over during prophase-I. </li></ul>
  26. 26. <ul><li>Cytokinesis : </li></ul><ul><li>This is the division of the cell cytoplasm. It follows the nuclear division and may be successive or simultaneous. </li></ul>Image reference –http://www2.estrellamountain.edu/faculty/farabee/biobk/biobookmeiosis.html
  27. 27. Significance of Meiosis <ul><li>Facilitates stable sexual reproduction </li></ul><ul><li>Recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the offspring. </li></ul>
  28. 28. Comparison between Mitosis and Meiosis Four daughter cells are formed. Two daughter cells are formed. Dividing cells are diploid. Dividing cells can be haploid or diploid. May take place only once (simultaneous type) or twice (successive type). Cytokinesis takes place only once. Consists of two nuclear divisions M-I and M-II. Consists of only one nuclear division. Occurs in reproductive cells. Occurs in somatic cells. Meiosis Mitosis
  29. 29. Comparison between Mitosis and Meiosis Meiosis Mitosis Chromosomal characters are altered due to &quot;crossing over&quot; causing recombination of genes. Original characters of the chromosomes are maintained in the daughter cells. It is a reduction division. It is an equational division. Pairing of homologous chromosomes and crossing over occur during Prophase-I. Does not involve either pairing of homologous chromosomes or crossing over.
  30. 30. Comparison between Mitosis and Meiosis Meiosis Mitosis Helps in the sexual reproduction and regulation of chromosome number in the life cycle of sexually reproducing organism. Helps in growth and body repairs. Daughter cells differ from each other as well as from the original mother cell. Daughter cells are similar to each other and also to the original mother cell.
  31. 31. Summary of the class <ul><li>Meiosis is a reductional division which helps in sexual reproduction. </li></ul><ul><li>During he first part of meiosis the chromosome number is reduced to half. </li></ul><ul><li>The second part of meiosis is similar to mitosis during which the haploid cell duplicates. </li></ul><ul><li>The end result of meiosis is 4 haploid cells. </li></ul><ul><li>Meiosis leads to genetic diversity. </li></ul>
  32. 32. <ul><li>Thank you </li></ul>