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Meiosis
Gamete Formation
Prophase I
What’s happening in Prophase I?
• Nuclear envelope begins to dissolve
• Chromatin condenses into chromosomes
– Homologous chromosomes pair (“synapsis”)
– Already duplicated in S phase of Interphase  “tetrads”
• Centrosomes begin migrating to opposite poles
– Centrosome = amorphous material around centrioles
• Spindle fibers begin to form from centrosomes
– Spindle fibers made of microtubules & proteins
• Create “mitotic spindle”
– “Kinetochore fibers” attach to centromere
– “Polar fibers” link centrosomes but don’t attach to
chromosomes
Metaphase I
What’s happening in Metaphase I?
• Tetrads (4 sister chromatids/ 2 homologous
chromosomes) are pulled to middle of cell by
kinetochore fibers
• Chromosomes’ kinetochores are ½ way
between the 2 poles & kept in place there
• Homologues align so same genes are adjacent
• Crossing over (sister chromatids) occurs
• Karyotype from photomicrographs made here
Anaphase I
What’s happening in Anaphase I?
• Homologues (2 sister chromatids) are pulled
to opposite poles by kinetochores
• “Independent Assortment” = random
separation of homologous chromosomes
• Independent assortment  genetic variation
• Earlier crossing over  genetic recombination
• Genetic diversity improves species’ survival
Telophase I
What’s happening in Telophase I?
• Chromosomes reach opposite poles of cell
– 1 chromosome (½ of the homologue pair) to each
– Each chromosome still is paired sister chromatids
• Cytokinesis begins
– Splits cytoplasm between the cells
– Half of organelles will go to each cell
• Nuclear envelope reassembles in some cells
but not in others
– This is species-dependent
From Metaphase I to Cytokinesis
Meiosis II: Telophase II
Meiosis II in Lily Anthers
Summary of Meiosis
• Chromosomes replicated in Interphase
• Meiosis I separates homologues
• Meiosis II separates sister chromatids of each
homologue
• Four haploid cells form
– Each has only 1 copy of the chromosomes (1 n)
– Human ovum/ sperm has 23 chromosomes each
– Haploids join in fertilization  23 pairs in diploid