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Agenda. Homework out Review homework A few cancer presentations Meiosis No HW. 1. Replicated Chromosome. 2 chromatids. 2. . Mitosis creates identical cells Growth, repair etc. 3. . 4-5 . Someone draw each one on the board?. Meiosis. Sexual Reproduction. Purpose.
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Agenda • Homework out • Review homework • A few cancer presentations • Meiosis • No HW
1. Replicated Chromosome 2 chromatids
2. • Mitosis creates identical cells • Growth, repair etc.
4-5 • Someone draw each one on the board?
Meiosis Sexual Reproduction
Purpose • To allow individuals to pass on half of their chromosomes to offspring • In vertebrate animals it produces the gametes (sperm and egg cells)
Homologous Chromosomes • You have 2 versions of each chromosome • They contain slightly different versions of the same genes, called alleles • You inherit one from each parent
Meiosis • Produces cells that contain only 1 copy of each homologous chromosome • Also pass on one of the sex chromosomes • So each gamete contains 22 autosomal chromosomes and an X or Y chromosome
Steps of Meiosis • Very similar to mitosis, except they happen twice
Meiosis I • Instead of the replicated chromosomes being separated – the homologous chromosomes get pulled apart
Meiosis II • Similar to mitosis, except the cell begins with only half the chromosomes
Why Sexual Reproduction? • Because diversity is good! • Increases chances that our species will survive • Meiosis produces an incredible amount of diversity
How? • You can get either 1 of your mother’s pair of chromosomes for all 23 • So you can get 1A2A3A…23A, 1B2A3A..23A, 1A2B3A…23A- basically a whole lot of combinations • You get genes from your mother and your father- new combinations can be beneficial
In Humans • The ovaries/testes contain cells called germ cells which undergo meiosis • Produce sperm and egg (23 chromosomes) • Sperm and egg fuse in fertilization forming a zygote
Bacteria • Simply divide asexually • No mixing of genes- offspring are identical
Single Celled Eukaryotes • Often divide simply by mitosis • All offspring are identical
Haplodiploidy • In bees, wasps and ants unfertilized eggs develop into males • Only half one set of chromosomes • Fertilized eggs develop into females • Have 2 sets of chromosomes
Hermaphroditic Eukaryotes • No male/female division – individuals can make both sperm and egg • Most plants • Many snails and other solitary creatures • Still require other individuals to reproduce with (usually) http://www.youtube.com/watch?v=FhVi4Z6CjZk
Sequential Hermaphroditism • In a group of clownfish only one individual is the female and one is the male • When the female dies, the male grows and turns into the female • Another fish can then assume the role of the male If Nemo is lucky he might one day become a man… then a woman.