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Cellular Control Meiosis Prophase Spindle fibers attach to the two sister chromatids of each chromosome contract and separate chromosomes which move to opposite poles of the cell. Metaphase Spindle apparatus attaches to sister chromatids of each chromosome. All the chromosomes are line up at the equator of the spindle Anaphase DNA and proteins start to condense. The two centrioles move toward the opposite end of the cell to form a spindle. The nuclear envelope and nucleolus also start to break up. Telephase the chromosomes become less condensed and reappear as chromatin. New membrane forms nuclear envelopes and the nucleolus is reformed. Can you think of 3 reasons why mitosis is important? Meiosis Aims: Why is meiosis necessary? What happens during meiosis? How does meiosis create genetic variation? 1) Match up Meiosis Produces 4 daughter cells each with half the amount of DNA as the parent cell. In sexual reproduction 2 gametes fuse to give rise to new offspring. This requires 2 haploid cells to join together to make 1 diploid cell. In order to maintain a constant number of chromosomes in the adults of species, the number of chromosomes needs to be halved during meiosis. SNA 14/12/04 Meiosis Every diploid cell of an organism has 2 sets of chromosomes, one from each parent. During meiosis, the homologous pairs of chromosomes separate, so that only one chromosome from each parent enters each gamete. These are know as haploid cells. SNA 14/12/04 Cellular Control Meiosis 2n 2n diploid 2n 2n 4n Cellular Control Meiosis 2n 2n diploid 1n haploid 1n 2n haploid Meiosis • only found in sexual reproduction • produces haploid gametes (egg/sperm, pollen/ovule) • takes place in the sex organs (ovary/testes, stamens/carpel) • Introduces genetic variation Meiosis - Involves 2 nuclear divisions During interphase each chromosome replicates each chromosome is made up of 2 chromatids Meiosis I • Homologous chromosome form bivalents • Chiasmata form & crossing over occurs • Homologous chromosomes separate Meiosis II • Chromatids separate Significance of meiosis • Halves the diploid chromosome number haploid gametes • Diploid cells are formed on fertilisation • Increases variation Independent segregation of maternal & paternal chromosomes crossing over so that genes on the same chromosome are not always inherited together Meiosis I • Homologous chromosomes separate Interphase • Thread-like chromosomes • Chromosomes replicate (DNA synthesis) • Centrioles replicate Prophase I • Chromatin condenses and coils chromosomes become visible • Nuclear membrane breaks down & nucleolus disappears • Spindle starts to form • Homologous pairs of chromosomes form bivalents • Chiasmata form crossing over • Homologous chromosomes repel but remain attached at chiasmata Metaphase I • Bivalents line up along the equator attached to the spindle by their centromeres Anaphase I • Homologous chromosomes separate and are pulled by the spindle fibres towards opposite poles of the cell • Independent segregation (random separation of maternal & paternal chromosomes) Telophase I • Chromosomes reach poles Often the cell goes straight into meiosis II • Spindle disappears • Nuclear membrane starts to reform • Cell divides by cytokinesis Meiosis II chromatids separate Prophase II • Spindle forms (at right angles to original spindle) • (nuclear membrane disappears if it had reformed) Metaphase II • Chromosomes line up at equator attached to spindle by their centromere Anaphase II • Centomeres divide & chromatids separate Telophase II • Chromosomes reach poles & uncoil • Spindle disappears • Nuclear membrane reforms • Cytoplasm divides (cytokinesis) • Tetrad of 4 haploid cells see http://www.sumanasinc.com/webcontent/anis amples/biology/biology.html For animation of meiosis Role of chiasmata • Hold homologous chromosomes together as a bivalent • Separate alleles on same chromosome increased genetic variation in gametes Cellular Control Metaphase I 2n 1 cell 4n 1 cell Genetic Recombination replication Prophase I Meiosis Prophase II Anaphase II Telephase I Telephase II 2 cell 2 cell 1n 4 cell Metaphase II Anaphase I 2n 2n Cellular Control Apoptosis Identify one difference between mitosis and Meiosis Cellular Control Meiosis Without genetic recombination: How many genetically different gametes could a human make? • 23 Chromosomes • 2 genetically distinct chromatids • Maternal stays with maternal • Paternal stay with paternal 2 Cellular Control gene for a characteristic (protein) Meiosis gene for a characteristic (protein) Inherited from male Inherited from female Paternal Chromatid Maternal Chromatid Genes have a position on a chromosome (in a genome) that is conserved among species LOCUS (loci) Cellular Control Meiosis Different versions of a gene, that code for different versions of a characteristic, are called alleles Cellular Control 2 Meiosis Evolution via natural selection requires variation within a population Cellular Control 4 Meiosis Evolution via natural selection requires variation within a population Sexual Reproduction Combining gametes from 2 organisms doubles genetic variation Cellular Control 529 Meiosis Evolution via natural selection requires variation within a population Sexual Reproduction Independent assortment Combining gametes from 2 organisms doubles genetic variation Maternal and paternal chromatids distributed to gametes independently from each other (randomly) 8 Cellular Control Meiosis Evolution via natural selection requires variation within a population Sexual Reproduction Independent assortment Crossing Over Combining gametes from 2 organisms doubles genetic variation Maternal and paternal chromatids distributed to gametes independently from each other (randomly) Alleles on Chromatids are randomly distributed between sister chromatids Cellular Control Apoptosis Homologous pairs Chromatids Loci Alleles Chiasmata Crossing over Cellular Control Apoptosis Errors in Meiosis Mutations Mutation rate of DNA is higher during meiosis and those mutations are passed on to offspring Polypoidy Multiple copies of genes leads to variability in gene expression and adaptability Increases genetic variability in plants Genetic Abnormalities in most animals Multiple copies of genes leads to inbalance of gene products and abnormalities Cellular Control Apoptosis Errors in Meiosis Kleinfelter’s Syndrome • XXY, XXXY, XXXY • Sterile or low fertility Polypoidy Increases genetic variability in plants • Hormonal imbalances • Learning difficulties • Increase risk of cancers Genetic Abnormalities in most animals Multiple copies of genes leads to inbalance of gene products and abnormalities Meiosis Aims: Why is meiosis necessary? What happens during meiosis? How does meiosis create genetic variation? 1) Match up Cellular Control loci Homologous pairs Gametes Centromeres Meiosis Chiasmata Crossing over Chromatin Cytokinesis Centrioles Chromatid Spindle fibres