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Stage relationships between the
cell cycle and DNA packaging:
Loop (Step #4) – Interphase chromatin
Rosette (Step #5) – Leptotene of Prophase I
Chromatid/Chromosome (Step #7) –
Pachytene of Prophase I
M-phase
“Meiosis”
Meiosis I
Prophase I
L
e
p
t
o
t
e
n
e
Meiosis II
Metaphase I
a
Z
y
g
o
t
e
n
e
Anaphase I
y
P
a
c
h
y
t
e
n
e
a
Telophase I
D
i
p
l
o
t
e
n
e
“Lazy Paddy”
D
i
a
k
e
n
e
s
i
s
y
Diplotene
centromere
chiasma
chiasma
Diakenesis
Metaphase I
centromere
centromere
centromere
centromere
centromere
More than one chiasma = chiasmata
Terminalization of Chiasmata (“slipping off of the ends”)
N = # of complete sets of chromosomes; or gene (allele) complement #
C = Amount (mass) of DNA in the nucleus
Mitosis
Equational division (No pairing of homologs)
1 X (2N, 4C; G2)
2 X (2N, 2C; G1)
Meiosis
Meiosis I
- Reductional division (Pairing of homologs)
Meiosis II
- Equational division (Like Mitosis in that sister chromatids are separated,
however, division of 1N, 2C cells instead of 2N, 4C cells – no replication
prior to Meiosis II)
Meiosis I
1 X (2N, 4C; G2)
2 X (1N, 2C; no S)
1 X (1N, 2C)
2 X (1N, 1C)
Meiosis II
Final Product
4 X (1N, 1C)
Mitosis
In somatic cells
One cell division,
resulting in two
daughter cells
2n
Chromosome number
per nucleus maintained
(e.g., for a diploid cell)
One premeiotic
S phase per cell
division (e.g., for
a diploid cell)
Normally, no
pairing of
homologs
Daughter
cells
Parental cell
2n
2n
4
3
2
1
G1 S
G2
M
G1
Normally, no
pairing of
homologs
Normally, no
crossovers
Centromeres
divide at
anaphase
Conservative process:
daughter cells’ genotypes identical to parental cell’s genotype
Cell undergoing mitosis can be diploid or haploid
Meiosis
In cells in the sexual cycle
Two cell division,
resulting in four
products of meiosis
Chromosome number
halved in the products
of meiosis
One premeiotic S
phase for both
cell divisions
Products
of meiosis
Meiocyte
2n
n
n
n
MI
MII
4
3
2
1
S
Full synapsis of
homologs at prophase I
n
Full synapsis of
homologs at prophase I
At least one crossover
per homologous
pair
Centromeres do not
divide at anaphase I
but do at anaphase II
Promotes variation among the products of meiosis
Cell undergoing meiosis is diploid
Chromosome Segregation
• Two models to explain chromosome separation by
spindle fibers:
• Traction-fiber--- Separation force is produced
along the fibers (sliding model). Best explains
separation in meiosis.
• Pac-man---Separation forces by disassembly of
microtubules at kinetochore (centromere) region.
Best explains separation in mitosis.
• One or both models are probably responsible for
separation in both mitosis and meiosis.
Traction-Fiber Chromosome
Segregation Model
Pac-man Chromosome
Segregation Model