Prophase Metaphase Anaphase Telephase Cellular Control Meiosis 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? Starter 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 2n 1n 1n haploid haploid

only found in sexual reproduction 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)

Chromosomes reach poles 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/anisamples/biology/biology For animation of meiosis

Role of chiasmata Hold homologous chromosomes together as a bivalent Separate alleles on same chromosome  increased genetic variation in gametes

Genetic Recombination Cellular Control Meiosis 2n 1 cell 2n 2 cell replication Prophase I Prophase II 4n 1 cell Genetic Recombination Metaphase I Metaphase II Anaphase I Anaphase II Telephase I Telephase II 2n 2 cell 1n 4 cell

Identify one difference between mitosis and Meiosis Cellular Control Cellular Control Apoptosis Identify one difference between mitosis and Meiosis

2 Without genetic recombination: 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

gene for a characteristic gene for a characteristic Cellular Control Meiosis gene for a characteristic (protein) 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

Evolution via natural selection requires variation within a population Cellular Control Meiosis 2 Evolution via natural selection requires variation within a population

Cellular Control Meiosis 4 Evolution via natural selection requires variation within a population Sexual Reproduction Combining gametes from 2 organisms doubles genetic variation

Cellular Control Meiosis 529 Evolution via natural selection requires variation within a population Sexual Reproduction Combining gametes from 2 organisms doubles genetic variation Independent assortment Maternal and paternal chromatids distributed to gametes independently from each other (randomly)

Cellular Control Meiosis 8 Evolution via natural selection requires variation within a population Sexual Reproduction Combining gametes from 2 organisms doubles genetic variation Independent assortment Maternal and paternal chromatids distributed to gametes independently from each other (randomly) Crossing Over Alleles on Chromatids are randomly distributed between sister chromatids

Cellular Control Cellular Control Apoptosis Homologous pairs Chromatids Loci Alleles Chiasmata Crossing over

Errors in Meiosis Cellular Control Cellular Control Apoptosis Multiple copies of genes leads to variability in gene expression and adaptability Mutations Polypoidy Mutation rate of DNA is higher during meiosis and those mutations are passed on to offspring Increases genetic variability in plants Genetic Abnormalities in most animals Multiple copies of genes leads to inbalance of gene products and abnormalities

Kleinfelter’s Syndrome Cellular Control Cellular Control Apoptosis Errors in Meiosis Kleinfelter’s Syndrome XXY, XXXY, XXXY Sterile or low fertility Hormonal imbalances Learning difficulties Increase risk of cancers Polypoidy Increases genetic variability in plants 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? Plenary Match up

Cellular Control Meiosis Chiasmata Cytokinesis loci Homologous pairs Crossing over Centrioles Gametes Chromatid Chromatin Spindle fibres Centromeres