Meiosis and Sexual Life Cycle PDF

Meiosis and Sexual Life Cycle Fig. 13-1 Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis A clone is a group of genetically identical individuals from the same parent In...

Meiosis and Sexual Life Cycle Fig. 13-1 Comparison of Asexual and Sexual Reproduction In asexual reproduction, one parent produces genetically identical offspring by mitosis A clone is a group of genetically identical individuals from the same parent In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents. Fertilization and meiosis alternate in sexual life cycles  The Gametes A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) For humans, the haploid number is 23 (n = 23) Each set of 23 consists of 22 autosomes and a single sex chromosome In an unfertilized egg (ovum), the sex chromosome is X In a sperm cell, the sex chromosome may be either X or Y Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-5 The alternation of Key Haploid gametes (n = 23) Haploid (n) meiosis and fertilization Egg (n) is common to all Diploid (2n) organisms that reproduce sexually …. alternation of generations Sperm (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)  Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid Like mitosis, meiosis is preceded by the replication of chromosomes Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis Each daughter cell has only half as many chromosomes as the parent cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  The Stages of Meiosis In the first cell division (meiosis I), homologous chromosomes separate Meiosis I results in two haploid daughter cells with replicated chromosomes; it is called the reductional division In the second cell division (meiosis II), sister chromatids separate Meiosis II results in four haploid daughter cells with unreplicated chromosomes; it is called the equational division Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Division in meiosis I occurs Division in meiosis II also occurs in four in four phases: phases: –Prophase I –Prophase II –Metaphase I –Metaphase II –Anaphase I –Anaphase II –Telophase I and cytokinesis –Telophase II and cytokinesis Meiosis II is very similar to mitosis Telophase I and Telophase II and Prophase I Metaphase I Anaphase I Prophase II Metaphase II Anaphase II Cytokinesis Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Centromere Sister Chiasmata (with kinetochore) chromatids Spindle Metaphase plate Sister chromatids separate Haploid daughter cells Homologous Cleavage forming Homologous chromosomes chromosomes furrow separate Fragments Microtubule of nuclear attached to envelope kinetochore Fig. 13-8a Telophase I and Prophase I Metaphase I Anaphase I Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Centromere Sister Chiasmata (with kinetochore) chromatids Spindle Metaphase plate Homologous Homologous Cleavage chromosomes chromosomes furrow separate Fragments Microtubule of nuclear attached to envelope kinetochore Meiosis I separates homologous chromosomes Prophase I Prophase I Prophase I typically occupies more than 90% of the time required for meiosis Chromosomes begin to condense In synapsis, homologous chromosomes loosely pair up, aligned gene by gene Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  In crossing over, nonsister chromatids exchange DNA segments Each pair of chromosomes forms a tetrad, a group of four chromatids Each tetrad usually has one or more chiasmata, X- shaped regions where crossing over occurred Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-8a Telophase I and Prophase I Metaphase I Anaphase I Cytokinesis Centrosome (with centriole pair) Sister chromatids remain attached Centromere Sister Chiasmata (with kinetochore) chromatids Spindle Metaphase plate Homologous Homologous Cleavage chromosomes chromosomes furrow separate Fragments Microtubule of nuclear attached to envelope kinetochore Meiosis I separates homologous chromosomes Fig. 13-8d Meiosis II separates sister chromatids Telophase II and Prophase II Metaphase II Anaphase II Cytokinesis Sister chromatids Haploid daughter cells separate forming Fig. 13-8e Prophase II Metaphase II Fig. 13-8f Telephase II and Anaphase II Cytokinesis Sister chromatids Haploid daughter cells separate forming A Comparison of Mitosis and Meiosis Mitosis conserves the number of chromosome sets, producing cells that are genetically identical to the parent cell Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-9a MITOSIS MEIOSIS Chiasma MEIOSIS I Parent cell Chromosome Chromosome Prophase replication replication Prophase I Homologous chromosome Replicated chromosome 2n = 6 pair Metaphase Metaphase I Anaphase Anaphase I Telophase Telophase I Haploid n=3 Daughter cells of meiosis I 2n 2n MEIOSIS II Daughter cells n n n n of mitosis Daughter cells of meiosis II Fig. 13-9b SUMMARY Property Mitosis Meiosis DNA Occurs during interphase before Occurs during interphase before meiosis I begins replication mitosis begins Number of One, including prophase, metaphase, Two, each including prophase, metaphase, anaphase, and divisions anaphase, and telophase telophase Synapsis of Does not occur Occurs during prophase I along with crossing over homologous between nonsister chromatids; resulting chiasmata chromosomes hold pairs together due to sister chromatid cohesion Number of Two, each diploid (2n) and genetically Four, each haploid (n), containing half as many chromosomes daughter cells identical to the parent cell as the parent cell; genetically different from the parent and genetic cell and from each other composition Role in the Enables multicellular adult to arise from Produces gametes; reduces number of chromosomes by half animal body zygote; produces cells for growth, repair, and introduces genetic variability among the gametes and, in some species, asexual reproduction  Genetic Variation in Population The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation Genetic variation produced in sexual life cycles contributes to evolution Three mechanisms contribute to genetic variation: – Independent assortment of chromosomes – Crossing over – Random fertilization Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Independent Assortment of Chromosomes Homologous pairs of chromosomes orient randomly at metaphase I of meiosis In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-11-3 Independent Assortment of Chromosomes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4  Crossing Over Crossing over produces recombinant chromosomes, which combine genes inherited from each parent Crossing over begins very early in prophase I, as homologous chromosomes pair up gene by gene In crossing over, homologous portions of two non-sister chromatids trade places Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Random Fertilization Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg) The fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Khatam