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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
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G1, S, G2
S Phase:
Duplication of DNA 2n  4n
Assembly of centrioles
Prophase I
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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
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Leads to the creation of novel chromosomes
Genes from homologous maternal and paternal
chromosomes mix
a) Synapsis: homologous chromosomes form tetrads
b) Chiasmata: homologous chromosomes overlap on multiple
points
c) 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