Meiosis IB Biology

Meiosis – cell division for the production of gametes (sperm or egg or spores) Called “reduction division” because it divides the number of chromosomes in half Diploid (2n) = 4Haploid (n) = 2 OR cells NOT identical B D D b d d B d b D B d B D D b b d D b Cell of ovary or testes In humans... 2n = 46 n = 23 Gametes or spores

Karyotype for pea plants… The diploid number for this organism would be 14 (The haploid number is 7) “Locus” – a gene’s specific position on a chromosome The locus for flower color might be at this point on the 5 th chromosome

2 stages (meiosis I and meiosis II) Meiosis I –Prophase I Synapsis – homologous chromosomes pairing up closely on the same spindle fiber A A a a B b r r R R d D CHIASMATA –point where chromatids touch, to allow for crossing over Homologous pairs are called “bivalents” when together

Tetrad – 4 chromatids (paired) Crossing over – homologous chromosomes can exchange pieces at the chiasmata… this is a type of “recombination” A A a a B b r r R R d D d D * The lining up here of homologous pairs (ex: which homologue will be on the right and which will be on the left?) is completely random… this will eventually add to the variation of the gametes  Mendel’s Law of Independent Assortment

Metaphase I A A a a B b r r R R d D d D Chromosomes are at the equator (Homologous chromosomes still on the same spindle fiber) ****Note the different arrangement than in mitosis****

Anaphase I – chromosomes move to poles A A a a B b r r R R D d D d Spindle fibers pull chromosomes to poles (spindle fibers are shortening.) Homologous pairs are separated. Creates (nearly) infinite variety Mendel’s Law of Segregation: Two alleles of each gene separate into different gametes (A’s to one side, a’s to the other) Centro- meres do NOT split!... The bivalents separate.

Telophase I Chromosomes at the poles Spindle fibers disintegrate Nuclear membrane begins to reform – each new nucleus is haploid! r r A A d D B b R a a d D B b

Cytokinesis I Down to 2 chromosomes n (DNA is not replicated again after meiosis I) Down to 2 chromosomes n (DNA is not replicated again after meiosis I) After cytokinesis I, maybe a brief interphase. Otherwise, Meiosis II !!.

Meiosis II -Prophase II Chromosomes evident Nuclear envelopes break down A A r r B b d D R R a a d D B b

Metaphase II Spindle fibers attach Chromosomes line up at equator ****Note similar arrangement to mitosis**** A A B b r d D a a B b R R d D

Anaphase II Spindle fibers pull sister chromatids apart Chromosomes move to poles Centromeres have split A A B b r d D a B b R d D centriole (pole)

Telophase II A A B b r d D a B b R d D Spindle fibers disintegrate Chromosomes at poles! Nuclear envelopes reform! Still Haploid!!

Result after cytokinesis... 4 gametes: 4 spores, 4 sperms, or 1 egg and 2 polar bodies Potential for lots of variety!! Recombination – the re- assortment of genes into different combinations from those of the parents A A a a B b B b r r R R d D d D