19Meiosis separates chromosome pairs to form haploid gametes. Meiosis I: Homologous pairs are separated (as sister chromatids) into 2 cellsMeiosis II: Similar to mitosis: sister chromatids are separated = 4 haploid cells.
20MEIOSIS IInterphase IInterphase is very similar to interphase in mitosis
21MEIOSIS IProphase IDouble stranded chromosomes and spindle fibers appear;Nuclear membrane and nucleolus fade (synapsis aligns homologous chromosomes)
22MEIOSIS I Metaphase I Chromosome pairs (chromatids) line up Spindle fibers attach to centromeres and centrioles
23MEIOSIS IAnaphase IChromatids separate from matching pair (independent assortment occurs here)
24MEIOSIS I Telophase I Cytoplasm divides and 2 cells form Each cell still has a diploid (2n) number
25Telophase/cytokinesis II MEIOSIS IIProphase IIMetaphase IIAnaphase IITelophase/cytokinesis II
26SUMMARY OF MEIOSIS II Meiosis II follows the same steps as mitosis Key differences:In interphase II, there is no replication of genetic materialWhen the sister chromatids separate during anaphase II, they are now haploid (1n)After telophase II, you have 4 haploid (1n) cells with genetic variation (they are no longer identical)SUMMARY OF MEIOSIS II
27Genetic VariationIndependent Assortment: Homologous pairs can separate in different parental combinations.2n possible gametes created
28Possible Gametes In humans: 223 = 8 million possible gametes This is without crossing over, which increases the number of distinct chromatids (exact number of chiasma determines how many different chromatids will exist)Possible Gametes
29Genetic VariationCrossing Over: During synapsis (Prophase I) portions of non-sister chromatids are exchanged.
3064 trillion possible zygotes Genetic VariationRandom Fertilization: Each sperm will fertilize and egg randomly8 million possible gametes for each parent:8,000,0002 =64 trillion possible zygotesWithout Crossing over. With it, there are even more!
32These are a pair of homologous chromosomes with genes A - E. CROSSING OVERThese are a pair of homologous chromosomes with genes A - E.
33These are called TETRADS CROSSING OVERThese homologous chromosomes are made of 2 sister chromatids each.These are called TETRADS
34The tetrads line up side by side (synapsis) in Prophase I Synaptonemal Complex!CROSSING OVERThe tetrads line up side by side (synapsis) in Prophase ISegments cross over (form chiasma) and the pieces are traded.
35The sister chromatids end up with different copies of the genes. CROSSING OVERThe sister chromatids end up with different copies of the genes.
41With all of the variation produced in meiosis, how can parents have IDENTICAL TWINS? How does the production of sperm differ from production of egg?What do you think?
42Errors in meiosis are passed down to offspring (because they occur in sperm or eggs) These errors can create sperm and eggs that have the wrong number or types of chromosomesErrors in Meiosis
43Types of errors Deletion: part of a chromosome is deleted (removed) Duplication: part of a chromosome is repeatedTranslocation: part of a chromosome is moved to another chromosomeInversion: part of a chromosome is removed, then flipped around and reattachedTypes of errors
45Non-disjunction Failure of chromosomes to separate properly in meiosis Meiosis I: homologues don’t separateMeiosis II: Sister chromatids don’t separate
46Monosomy: one parent does not give a copy of one chromosome There is only 1 total in the child45 instead of 46 total chromosomesThe only monosomy that we see in humans (that is nonfatal) is Turner syndromeTypes of errors
47Turner syndromeFemales inherit only one X chromosome (XO), instead of XX1/2500 girls98% of fetuses with Turner Syndrome miscarryShort stature, broad chest, webbed neck
49Trisomies When one parent gives two copies of a chromosome The child has 3 total of that chromosome (47 instead of 46 total chromosomes)
50Trisomy 21 Down’s syndrome 3 copies of chromosome 21 Impairment of cognitive ability (avg. IQ 50, vs. 100 in rest of population)Almond-shaped eyesHealth problems: heart failure, GERD, ear infections, sleep apnea1/733 birthsMore common with older mothers, but 80% are born to women under 35Trisomy 21
52Trisomy 18 Edwards syndrome 95% miscarry May be a partial trisomy (less severe)Edwards syndrome2nd most common trisomy95% miscarryOf those born, 50% die within 2 months; 5-10% survive first yearApnea, heart and kidney defects1/3000 live births
53Trisomy 13 Patau syndrome May be a partial trisomy (less severe) Mental impairmentPolydactyly (extra fingers or toes)Microcephaly (small brain)85% do not survive 1 year
54Trisomies Trisomy 16 Trisomy 9 Most common trisomy, but results in miscarriageTrisomy 9Mental retardation, deformities of skull, nervous systemTrisomies may be partial (due to translocation) or mosaicTrisomies
55Mosaicism: During very early development, non-disjunction occurs in mitosis and as a result some percentage of an individual’s cells (but not all) have the trisomy/monosomy4647Mosaicism
56Trisomies in sex chromosomes XXX: No difference between this and a normal woman (XX)(X inactivation -> Barr bodies)XXY: KlinefelterSmall testicles, reduced fertilityMost common sex trisomy (1/500 males)XYY: Not named because normally asymptomatic (normal phenotype)Trisomies in sex chromosomes
57Trisomies in sex chromosomes Many other possibilities, many don’t have any symptomsTrisomies in sex chromosomes