Meiosis Part 2
SBI 3U
November 4th, 2012
Recall
What is the function of meiosis?
 How many cell divisions are there in the
process of meiosis?

Random Assortment of
Chromosomes
Random assortment of chromosomes
during Anaphase I
 1 of the ways by which meiosis creates
genetic variability
 2n=46  how many variations can we have?

Crossing Over




Genetic recombination
During prophase of Meiosis I
Breakage and reunion of chromatids
Large genetic diversity
Sperm & Egg Cells
Spermatids: in male animals. 4 functional cells 
later become sperm cells
-Have a head (containing nucleus)
-Have a tail (flagellum)
-Middle piece (containing mitochondria)

Spermatogenesis
Sperm & Egg Cells



In meiosis I  division of the cytoplasm is unequal = one large cell (a
secondary oocyte) and one small cell (the first polar body)
Polar body contains only the nucleus
Oogenesis
Sperm & Egg Cells
Meiosis II  secondary oocyte divides
unequally again = one large ovum/egg
and a small polar body
 End of meiosis for females: 1 egg

Mistakes in Meiosis
Mistakes may be in separation of the
chromosomes during division or an
incorrect exchange of genetic information
 Mistake in separation of chromosomes
during division: Can result in an abnormal
# of chromosomes in an egg or sperm
cell

Mistakes in Meiosis
Resulting zygote will have too few or too many chromosomes:
aneuploidy
 Down syndrome
 Mistake in exchange of information during crossing over: correct
# of chromosomes but altered genetic information

Mistakes in Meiosis

Altered chromosomes may contain
duplications or deletions
Abnormal Chromosome Number
Nondisjunction  occurs when the
homologous chromosomes do not split
properly during meiosis I or the sister
chromatids do not separate during
meiosis II
 What will happen to the resulting
gametes?

Abnormal Chromosome Number
Monosomy: when a child only has 1 copy
of a chromosome
 Polysomy: when a child has 3 copies of a
particular chromosome
 Polyploidy: 3 sets of chromosomes!
Very rare.

Abnormal Chromosomal Structure
Occurs during crossing over
 Deletions and duplications
 If a segment reattaches to a complete
homolog, there will be a duplication
 If a segment reattaches to the correct
homolog  inversion

Abnormal Chromosomal Structure

Translocation: segment attaches to a
non-homologous chromosome