NATS 1675 Lecture Notes - Lecture 3: Spermatogenesis, Sister Chromatids, Mitochondrion
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6 Feb 2016
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NATS 1675
- MEIOSIS PHASES 2016-02-04 16:35
• Phases of Meiosis:
(1) Prophase I:
• Similar to mitosis with 2 major exceptions:
o 1. Synapsis
o 2. Crossover
• Synapsis: The pairing or coming together of homologous
chromosomes to form a TETRAD (See image below)
• Coming together of “like” chromosomes
• Crossover: Homologous recombination is the process by which two
chromosomes, paired up during prophase 1 of meiosis, exchange
some distal portion of their DNA.
• Can occur during synapsis
• Equal portions of non-sister chromatids can be exchanged
• Exchange of equal material
• The purpose of crossover is to further increase genetic
variability in gametes
• Crossing over can be replaced in the middle of the chromatids as
well and not only at the ends
Tetra
d'

NATS 1675
- MEIOSIS PHASES 2016-02-04 16:35
• In General: 23 2x2x2 = 8
• For Humans: 223 = > 8 million
o Females: +/- 8 million eggs
o Males: +/- 8 million sperm
(2) Metaphase I:
• Homologous pairs of chromosomes (those that have undergone
synapsis) line up on the equatorial plate
Chiasma'
Centromere''
Equatorial'Plate'
Centriole'
Spindle'Fibers'