Cell Division - Meiosis

Who Does Meiosis and Why? All Multicellular Eukaryotes Animals, Plants, Fungi Meiosis is needed for sexual reproduction Sexual Reproduction is beneficial because…. It allows genetic recombination New combinations of genes may .. 1. better suit environments 2. allow offspring to survive radical change in environment 3. allow offspring to survive a disease outbreak

Vocab Cells produced by meiosis = Gametes Sex Chromosomes = determine sex (in humans XX = female, Xy = male) Autosomes = any chromosome not used to determine sex of offspring

. Gametes = haploid cells made by meiosis Haploid gametes have one set of chromosomes (1 of each kind) Called 1n where n = number of different C types Humans have 23 different types 22 autosomes + 1 sex determining

. Diploid cells have 2 of each kind of chromosome Diploid cells are called 2n where n= The number of different Chromosome types Humans = 23 types Somatic cells have 2 of each type ->

. To determine 1n chromosome number multiply 1 * n Humans 1n C number is … 23 To determine 2n chromosome number multiply 2 * n Human’s 2n C number is… 2 * 23 = 46

Based on the Cat Karyotype below, calculate the 1n and 2n number for cats 18 different autosomes (C # 1-18) one pair of sex chromosomes So 19 different kinds 1n = 1(19) = 19 2n = 2(19) = 38

Haploid Cells produced by Meiosis Animal Meiosis only happens in gonads Gonads = ovary or testis Specialized cells called spermatocytes or oocytes are the only gonad cells that do meiosis Spermatocytes and oocytes are constantly replaced by specialized cells in gonads called Spermatogonia cells & oogonia cells

Spermatogenesis Spermatogonia do Mitosis to make 1Primary spermatocyte Which… Does meiosis to make 4 spermatids

Spermatids differentiate into sperm Acrosome = vesicle w/ penetrating enzymes Nucleus = 1n Midpiece makes ATP powers flagellum Tail = flagellum

Oogenesis Oogonia do Mitosis to make 1 primary Oocyte Which does… Meiosis I to make 1 secondary oocyte and one polar body Meiosis II makes 1 ovum &1 polar body

Meiosis I:separate homologous pairs (cells become 1n with X shape C) Prophase I : same as mitosis + Synapsis & crossing over Metaphase I : C line up in pairs

. Anaphase I : Homologs move to opposite poles (cohesin along arms breaks down) (cohesin at centromeres remains)

Telophase I : Cytokinesis sinultaneously X shaped C reach poles Many cells do not decondense C or reform nuclear E Spindle only partly broken down

Homologous Chromosomes Homologous Chromosomes are same C type same length code for same genes (but can be different versions) Unless they are X and y which are the only pair of Homologues that do not look alike

Synapsis Homologous Chromosomes pair up Line up gene to gene then physically connect along their arms Proteins form a synaptonemal complex that bind them together

. Crossing Over : Non-sister chromosomes trade corresponding segments of DNA Genes far from centromere cross over most often Location of a gene on a chromosome = the gene’s locus (loci) Distance from centromere measured in map units

Chiasmata X shaped region where crossing over has happened. Synaptonemal complex has broken down but … Cohesions still hold sister chromatids together

Meiosis in Fungi Fungal bodies formed of hyphae Filaments made of chains of 1n cells Hyphae interweave to form fungus body The body mass is called a mycelium

Phylum Ascomycota don’t make mushrooms Hypha can be + or – If one of each meet then… 2 cells (one + one-) fuse cytoplasm (plasmogamy) Then fuse their nuclei … now 2n (karyogamy) Then do meiosis…making 4 haploid nuclei

Each 1n nucleus does mitosis So now 8 nuclei in the cell Each nucleus grows a tough casing around it and becomes a Spore (ascospore) Spores disperse on wind Germinate to make new 1n hyphae .

The ascospores that form from one cell stay together in a capsule called an ascus

Homologous C both have gene for spore color…one tan color one black color No crossing over = 4:4 Any other pattern results from crossing over

Genetic Variation Increased by crossing over Increased by Independent Assortment.. Each pair of homologues lines up randomly and Independent of how other pairs line up # different gametes formed = 2n Humans = 223 or 8,399,608