Meiosis Cell Division Meiosis A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis. Meiosis does two things - 1) Meiosis - diploid cell => haploid sex cells In meiosis, one diploid cells produces four haploid cells. Why do we need meiosis? Meiosis is necessary to add diversity and give the organism a better chance to adapt to changes. Meiosis is necessary to add diversity and give the organism a better chance to adapt to changes. Why halve the chromosomes in gametes? At fertilization the male and female sex cells will provide of the chromosomes each so the offspring has genes from both parents At fertilization the male and female sex cells will provide of the chromosomes each so the offspring has genes from both parents 2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. Genetic diversity is important for the evolution of populations and species. Meiosis Parent cell chromosome pair Chromosomes copied 1 st division - pairs split 2 nd division produces 4 gamete cells with the original no. of chromosomes Meiosis Reduction division Parent cell 4 gametes 1 st division 2 nd division Metaphase I Telophase I & Cytokinesis I Anaphase I Prophase IIMetaphase II Prophase I Anaphase II Telophase II & Cytokinesis Meiosis I : Separates Homologous Chromosomes Prophase I During this phase each pair of chromatids match up with their homologous pair and form a tetrad. During this phase each pair of chromatids match up with their homologous pair and form a tetrad. Extremely IMPORTANT!!! It is during this phase that crossing over occur. Extremely IMPORTANT!!! It is during this phase that crossing over occur. Crossing Over is the exchange of segments during synapsis. Crossing Over is the exchange of segments during synapsis. Spot the difference in the chromosomes Metaphase I The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles. The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles. Still in homologous pairs Still in homologous pairs Anaphase I The spindle guides the movement of the chromosomes toward the poles The spindle guides the movement of the chromosomes toward the poles Sister chromatids remain attached Sister chromatids remain attached Move as a unit towards the same pole Move as a unit towards the same pole The homologous chromosome moves toward the opposite pole The homologous chromosome moves toward the opposite pole Contrasts mitosis chromosomes appear as individuals instead of pairs (meiosis) Contrasts mitosis chromosomes appear as individuals instead of pairs (meiosis) Anaphase vs Anaphase I Telophase I This is the end of the first meiotic cell division. This is the end of the first meiotic cell division. The cytoplasm divides, forming two new daughter cells. The cytoplasm divides, forming two new daughter cells. Each of the newly formed cells has half the number of the parent cells chromosomes, but each chromosome is already replicated ready for the second meiotic cell division Each of the newly formed cells has half the number of the parent cells chromosomes, but each chromosome is already replicated ready for the second meiotic cell division Cytokinesis Occurs simultaneously with telophase I Occurs simultaneously with telophase I Forms 2 daughter cells Forms 2 daughter cells Plant cells cell plate Plant cells cell plate Animal cells cleavage furrows Animal cells cleavage furrows NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS Figure 13.7 The stages of meiotic cell division: Meiosis I Meiosis II : Separates sister chromatids Proceeds similar to mitosis Proceeds similar to mitosis THERE IS NO INTERPHASE II ! THERE IS NO INTERPHASE II ! Prophase II Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator Metaphase II The chromosomes are positioned on the median or equator in a mitosis-like fashion The chromosomes are positioned on the median or equator in a mitosis-like fashion Anaphase II The centromeres of sister chromatids finally separate The centromeres of sister chromatids finally separate The sister chromatids of each pair move toward opposite poles The sister chromatids of each pair move toward opposite poles Now individual chromosomes Now individual chromosomes Telophase II and Cytokinesis Nuclei form at opposite poles of the cell and cytokinesis occurs Nuclei form at opposite poles of the cell and cytokinesis occurs After completion of cytokinesis there are four daughter cells After completion of cytokinesis there are four daughter cells All are haploid (n) All are haploid (n)