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Meiosis
Chapter 13
What you need to know!
• The role of meiosis and fertilization in sexually reproducing organisms
• The importance of homologous chromosomes to meiosis
• How the chromosome number is reduced from diploid to haploid through the stages of meiosis
• Three important differences between mitosis and meiosis
• The importance of crossing over, independent assortment, and random fertilization to increasing genetic variability
Meiosis
• A process by which an organism produces haploid (n) sex cells from diploid (2n) germ cells
• Meiosis is divided into meiosis 1 and meiosis 2
• Meiosis 1 = separation of homologous chromosomes
• Meiosis 2 = separation of sister chromatids
Interphase
• G1, S, G2• S Phase:• Duplication of DNA 2n 4n• Assembly of centrioles
Prophase I
• 90% of meiotic time• Condensation of chromosomes• Nuclear envolope disappears• Spindles form• Synapsis:
– Homologous chromosomes pair up as tetrads– The chiasmata are where crossing-over takes
place
Crossing Over
• Leads to the creation of novel chromosomes• Genes from homologous maternal and paternal
chromosomes mixa) Synapsis: homologous chromosomes form tetradsb) Chiasmata: homologous chromosomes overlap on multiple
pointsc) Crossing Over: identical parts break off and reattach
Crossing Over Frequency
• The recombination frequency between two genes on one chromosome is mathematically related to the distance between them
• The further apart 2 genes are the higher the probability of a crossing over event separating them
• Recombination frequencies vary between 0% and 50%
• This method is useful for establishing gene maps (location of genes on a chromosome
• Example: Recombination frequencies between genes A and B is 45% while A and C is only 20%. Map the genes.
Metaphase 1
• Homologous chromosomes line up at the equator side by side
• One spindle attaches to each chromosome• 4n = 92 chromosomes in humans
Anaphase 1
• Homologous chromosomes separate (sister chromatids remain together)
Telophase 1, Cytokinesis, and Interkinesis
Telophase 1/cytokinesis:• Two cells separate• Each contains one homologous chromosome
(haploid) with two sister chromatids• Often no nuclei form and the DNA remains
condensed 2 x (n) = 46 (no pairs)
Interkinesis:• Short interphase with little growth• Sometimes partial uncoiling of the chromosomes• Duplication of centrioles (no S phase)
Prophase 2
• Spindle fibers form• 2 kinetochore spindle fibers attach to each
chromosome
Metaphase II
• Chromosomes line up at the equator of the cell
• Spindle fibers attach to each Centromere of the sister chromatids
Anaphase 2
• Sister chromatids are pulled apart• Sister chromatids are not identical
(crossing over)
Telophase 2 and Cytokinesis
• Four new cells form• All are genetically different• All are haploid with 1n chromosomes
– Humans 23
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