Reduction-Division Genetic Recombination https://www.youtube.com/w atch?v=toWK0fIyFlY 1 cell division GAMETES, HALF CHROMOSOMES, The form of cell division by which GAMETES, with HALF the number of CHROMOSOMES, are produced. DIPLOID (2n) HAPLOID (n) MeiosisSEXUAL reproduction. Meiosis is SEXUAL reproduction. TWO divisionsMEIOSIS I MEIOSIS II TWO divisions (MEIOSIS I and MEIOSIS II). 2 GAMETES(sperm or egg) Sex cells divide to produce GAMETES (sperm or egg). HALF Gametes have HALF the # of chromosomes. GONADS (testes or ovaries). Occurs only in GONADS (testes or ovaries). Male: sperm Female: egg 3 mitosis Similar to mitosis interphase. CHROMOSOMES (DNA) S phase CHROMOSOMES (DNA) replicate in the S phase chromosometwo identical SISTER CHROMATIDS CENTROMERES Each duplicated chromosome consist of two identical SISTER CHROMATIDS attached at their CENTROMERES. CENTRIOLE CENTRIOLE pairs also replicate. 4 visible Nucleus and nucleolus visible. Nucleus nucleolus cell membrane chromatin 5 Cell division chromosome number one- half. Cell division that reduces the chromosome number by one- half. Four phases Four phases: a.Prophase I b.Metaphase I c.Anaphase I d.Telophase I 6 Prophase I Longest and most complex phase (90%). Chromosomes Chromosomes condense. SynapsisHomologous chromosomes tetrad Synapsis occurs - Homologous chromosomes come together to form a tetrad. Tetrad two chromosomes four chromatids Tetrad is two chromosomes or four chromatids (sister and non-sister chromatids). 7 8 Homologs contain DNA that codes for the same genes, but different versions of those genes Homologous chromosomes sister chromatids Tetrad 9 chromosomesmaternal paternalsimilar in shape and size. Pair of chromosomes (maternal and paternal) that are similar in shape and size. (tetrads) carry GENES controlling the SAME inherited traits Homologous pairs (tetrads) carry GENES controlling the SAME inherited traits. 10 PaternalMaternal eye color locus eye color locus hair color locus hair color locus 11 Crossing over chromatids chiasmata Crossing over may occur between non-sister chromatids at sites called chiasmata. Crossing over chromatids chromatid Crossing over: segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) exchange genes crossing over Chiasmata (chiasma) are where chromosomes touch each other and exchange genes (crossing over.) Genetic Recombination Causes Genetic Recombination 12 nonsister chromatids chiasmata: site of crossing over variation Tetrad 13 14 XX chromosome - female XY chromosome - male 15 Homologs separate 16 Meiosis I Nucleus & Nucleolus Nucleus & Nucleolus disappear Spindle Spindle forms coilSynapsis Chromosomes coil & Synapsis (pairing) occurs Tetrads Crossing over Tetrads form & Crossing over Occurs centrioles spindle fiber aster fibers 17 TETRAD Shortest phase Tetrads align on the equator. Independent assortment occurs chromosomes separate randomly causing GENETIC RECOMBINATION 18 Homologs line up at equator or metaphase plate OR 19 Homologous chromosomes Homologous chromosomes separate and move towards the poles. Sister chromatids centromeres Sister chromatids remain attached at their centromeres. 20 21 Homologs separate Homologs separate haploid chromosomes Each pole now has haploid (1n) set of chromosomes. Cytokinesis Cytokinesis occurs and two haploid daughter cells are formed. 22 23 cytokinesis Sister Chromatids Separate 24 Meiosis II No Interphase II or very short No DNA Replication Remember: Meiosis II is similar to mitosis 25 Same as Prophase in mitosis Nucleus & nucleolus disappear Chromosomes condense Spindle forms 26 Same as Metaphase in mitosis Chromosomes (not homologs) line up at equator 27 Anaphasemitosis Same as Anaphase in mitosis SISTER CHROMATIDS separate 28 Same as Telophase in mitosis. Nuclei and Nucleoli reform, spindle disappears CYTOKINESIS occurs. Remember: FOUR HAPLOID DAUGHTER cells are produced. Called GAMETES (eggs and sperm) 29 1n Sperm cell fertilizes 1n egg to form 2n zygote 30