Meiosis: Formation of Gametes PDF

MEIOSIS FORMATION OF GAMETES 1 FACTS ABOUT MEIOSIS Preceded by interphase which includes chromosome replication Two meiotic divisions --- Meiosis I and Meiosis II Called Reduction- division Original cell is diploid (2n)...

MEIOSIS FORMATION OF GAMETES 1 FACTS ABOUT MEIOSIS Preceded by interphase which includes chromosome replication Two meiotic divisions --- Meiosis I and Meiosis II Called Reduction- division Original cell is diploid (2n) Four daughter cells produced that are haploid (1n) Keep track of when this happens!!! 2  FACTS ABOUT MEIOSIS Daughter cells contain half the number of chromosomes as the original cell Produces gametes (eggs & sperm) Occurs in the testes in males (Spermatogenesis) Occurs in the ovaries in females (Oogenesis) 3  MORE MEIOSIS FACTS Start with 46 double stranded chromosomes (2n) 23 pairs of homologous chromosomes After 1 division – 23 double stranded chromosomes (n) After 2nd division - 23 single stranded chromosomes (n) 4 WHY DO WE NEED MEIOSIS? It is the fundamental basis of sexual reproduction Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote 5 FERTILIZATION – “PUTTING IT ALL TOGETHER” 2n = 6 1n =3 6  RECALL REPLICATION OF CHROMOSOMES Replication is the process of duplicating a chromosome Occurs in Occurs prior to division Interphase Replicated copies are called sister chromatids Held together at centromere 7  REPLICATED CHROMOSOMES IN THE CELL Gene X Sister Homologs Chromatids (same genes, (same genes, different alleles) same alleles) Homologs separate in meiosis I and therefore different alleles separate. 8 MEIOSIS FORMS HAPLOID GAMETES Meiosis must reduce the chromosome number by half from mom from dad child too much! meiosis reduces genetic content The right number! 9  MEIOSIS: TWO PART CELL DIVISION Sister Homologs chromatids separate separate Meiosis Meiosis I II Diploid Haploid Haploid 10  MEIOSIS I: REDUCTION DIVISION Nucleus Spindle fibers Nuclear Early Prophase I Late Metaphase I envelope (Chromosome Prophase I number doubled) Anaphase I Telophase I (diploid) 11  PROPHASE I Early prophase Late prophase Homologs pair. Chromosomes condense. Crossing over occurs. Spindle forms. Nuclear envelope fragments. 12  TETRADS FORM IN PROPHASE I Homologous chromosomes (each with sister chromatids) Join to form a TETRAD Called Synapsis 13  CROSSING-OVER Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring 14 HOMOLOGOUS CHROMOSOMES DURING CROSSING-OVER 15  METAPHASE I Homologous pairs of chromosomes align along the equator of the cell Centrosomes are at opposite sides of the cell. Microtubules attach to the Note: There can be a centromere/kinetochores mix of paternal & maternal chromosomes on either side which is called independent assortment and leads to genetic diversity 16 ANAPHASE I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres. Cell is on its way to becoming a haploid (n) 17 TELOPHASE I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. Cells are now haploid (n) 18  CYTOKINESIS I/ INTERKINESIS ▪ Chromosomes completely uncoil to become chromatin ▪ Nuclear membrane completely reforms  MEIOSIS II Gene X NOW! Only ONE homolog of each chromosome is present in the cell Sister chromatids carry identical genetic information. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. 20  MEIOSIS II: REDUCING CHROMOSOME NUMBER Prophase II Metaphase II Telophase II Anaphase II 4 Identical haploid cells 21 INTERPHASE II ▪ Cell prepares for division ▪ Chromatin begin to coil/condense into chromosomes ▪ DNA is NOT DUPLICATED so it remains a haploid (n) PROPHASE II Nuclear envelope fragments. Chromosomes condense Spindle forms. No crossing over 23 METAPHASE II Chromosomes line up at the center of the cell (no longer in pairs) Independent assortment occurs again (of the sister chromatids Microtubules attach to centromere/kinetochore 24 ANAPHASE II Pole Sister chromatids separate and move to opposite poles. Note: sister chromatids continue to be haploid! 25 TELOPHASE II Nuclear envelope assembles. Chromosomes uncoil Spindle disappears. Cytokinesis divides cell into two. 26  CYTOKINESIS II ▪ Cytokinesis occurs producing 4 haploid daughter cells RESULTS OF MEIOSIS Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome 28  COMPARISON OF DIVISIONS Mitosis Meiosis Number of 2 1 divisions Number of 2 4 daughter cells Genetically Yes No identical? Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Growth and Role Sexual reproduction repair 29