Lecture 5 – Meiosis
I. A few important points about meiosis
A. cell division that reduces chromosome number by 1/2
2n
four 1n cells
B. crossing over used to increase genetic diversity of gametes produced
II. Overview of meiosis
A. homologues pair
B. 1st division: 2n cell divides to give two 1n cells
C. 2nd division: each 1n cell divides to give total of
four 1n cells
meiosis I
Two stages:
meiosis I and
meiosis II
2n
1n
1n
meiosis II
1n
III. Meiosis in detail
A. meiosis I
1. prophase I
a. leptotene
b. zygotene
synapsis =
c. pachytene
bivalent =
tetrad =
1
1n
1n
1n
d. diplotene
chiasmata =
crossing over =
e. diakinesis
2. metaphase I
3. anaphase I
4. telophase I
IV. Meiosis II
A. between meiosis I and II
1. no DNA replication occurs
2. nuclear envelope may reform transiently
3. chromosomes may decondense briefly
4. meiosis II resembles mitosis, but:
2
B. steps of meiosis II
1. prophase II
2. metaphase II
3. anaphase II
4. telophase II
V. Differences in gametogenesis
A. spermatogenesis
vs.
B. oogenesis
2n
2n
1n
1n
1n
1n
1n
1n
1n
1n
1n
3
1n
1n
1n
VI. Mitosis versus meiosis
A. side-by-side comparison
B. final thoughts on mitosis vs. meiosis
reduces ploidy
mitosis
meiosis
# cells produced
homologues pair
VII. Principle of segregation – Hereditary determinants separate in such a way that each
gamete is equally likely to inherit either member of the pair.
A A a a
A A
A
4
a a
A
a
a
VIII. Independent assortment – unlinked genes assort independently from one another.
A. Products depend on metaphase I alignment of chromosomes.
W Ww
W Ww
w
w
Wg
W
WG
W
g
G
G
Gg
g
g
w
wg
gG
wG
G
w
G
g
B. How metaphase I alignment determines gamete genotype.
w
w
w
W W
g
G
g
g
G
G
w
W W
G
w
G
G
WG
WG
g
g
wg
w
g
w
w
W
g
G
W W
g
G
W
w
W W
W
g
wg
Wg
5
G
g
W
g
w
G
w
w
g
Wg
G
wG
G
wG