Lecture 2: Mitosis and meiosis
1.
2.
3.
4.
5.
6.
Chromosomes
Diploid life cycle
Cell cycle
Mitosis
Meiosis
Parallel behavior of genes
and chromosomes
Basic morphology of chromosomes
telomere
short arm (p)
centromere
long arm (q)
telomere
End of the 19th century: cytology – studies of cells at the light microscopy level
Discovery of ‘chromosomes’ – stained bodies (in Greek)
n=3
2n = 6
B
A
b
A
c
C
d
d
Each chromosomes contains a long (up to 2”) DNA
molecule and many proteins
Chromosome number
is a constant feature
within a species
(normally)
Different species often
can be distinguished
by their chromosome
numbers (e.g. human
and chimp)
A full set of human
male chromosomes
as seen in metaphase of mitosis,
after staining with a certain dye
46 chromosomes
(23 pairs of homologs):
male = 44 + XY
female = 44 + XX
One half of the set (23
chromosomes) come
from father, and the
other half from mother
Diploid life cycle
Zygote formation
Development
It takes 250 mitoses to make
an adult out of a zygote
Two types of cell
division in the
diploid life cycle
Mitosis:
Meiosis:
- in many types of cells
- produces identical
cells
- in haploid and diploid
cells
- one cell division
- in germ line cells to
produce gametes
- reduces ploidy:
2n -> n
- only in diploid cells
- two cell divisions
Cell cycle = division (mitosis) + interphase
Interphase = G1 + S + G2
Imagine a cell with just one pair of homologs (2n = 2)
In G1 there is only one DNA molecule (one
chromatid) per chromosome, then DNA replicates
during S, and in G2 there are already two
chromatids per each chromosome
Mitosis
Interphase
Late prophase
Metaphase
Early anaphase
Telophase
Mitosis is a continuous process with
stage boundaries somewhat blurred
Snapshots of mitosis
in a cell with 2n = 2
Prophase
G2 (interphase)
Metaphase
Two chromosomes but four
chromatids per cell
S (interphase)
DNA replicates
G1 (interphase)
centromere
Two
chromosomes
and
two chromatids
per cell
Telophase
Anaphase
A metaphase
chromosome
The genetic consequence of mitosis is simple:
it generates two identical copies of the parental cell
Stages of Prophase I
Meiosis is a bit more
complex …
From meiocyte
ÎÎÎ
Meiosis is not a cycle,
it is a linear process with
no turning back
to gametes
Snapshots of mitosis in a cell with two chromosomes (2n = 2)
Interphase
Prophase I
Duplication of
the
chromatids in
S phase
Pairing
(synapsis) of
homologous
chromosomes
In human females, oocytes
remain in Pro I since the
time when the fetus is just 7
months old, and they
remain paired until puberty.
Notice in passing: cross-overs happen in Prophase I
Snapshots of mitosis in a cell with two chromosomes (2n = 2)
Telophase I
Interphase
Prophase I
Metaphase I
Anaphase I
Duplication of
chromatids in
S phase
Pairing
(synapsis) of
homologous
chromosomes
Lining up of the
paired homologs
in the equatorial
plane
Separation
(disjoining) of
the homologs
Completion of
Meiosis I
Anaphase II
Telophase II
Separation
(disjoining) of
the sister
chromatids
Completion of
Meiosis II
Prophase II
Peparation for
Meiosis II
Metaphase II
Individual
homologs line up
in the equator
The genetic outcome of meiosis is …
Interphase
Telophase I
Prophase I
Metaphase I
Anaphase I
Pairing
(synapsis) of
homologous
chromosomes
Lining up of the
paired homologs
in the equatorial
plane
Separation
(disjoining) of
the homologs
Completion of
Meiosis I
Anaphase II
Telophase II
2n
Duplication of
chromatids in
S phase
Prophase II
Metaphase II
n n
n n
Peparation for
Meiosis II
Individual
homologs line up
in the equator
Separation
(disjoining) of
the sister
chromatids
Completion of
Meiosis II
…production of four haploid gamets (4 x n) out of one diploid (2n) meiocyte
Reduction of chromosome number from 2n to n
occurs during the first division of meiosis (Meiosis I)
A
a
Using meiosis to explain Mendel’s laws
Consider the cross
P:
F1:
A/A x a/a
A/a
Law I: equal segregation
How can we explain formation of
two gametic types with equal
frequency (½ A, ½ a) in such F1
heterozygote?
A/a
Using meiosis to explain Mendel’s laws
Consider the cross
P:
F1:
A/A x a/a
A/a
How can we explain formation
of two gametic types with
equal frequency (½ A, ½ a) in
such F1 heterozygote?
A
1/2
a
1/2
Law I: equal segregation of alleles is due to
orderly segregation of homologs in Anaphase I
A/a
Using meiosis to explain Mendel’s laws
Consider the cross
P:
F1:
A/A; B/B x a/a; b/b
A/a; B/b
Law II: independent assortment
How can we explain formation of
four gametic types
(¼ AB, ¼ ab, ¼ Ab, ¼ aB) in
such F1 heterozygote?
A/a; B/b
Using meiosis to explain Mendel’s laws
Consider the cross
P:
F1:
A/A; B/B x a/a; b/b
A/a; B/b
How can we explain formation of
four gametic types
(¼ AB, ¼ ab, ¼ Ab, ¼ aB) in
such F1 heterozygote?
½ AB
and
½ ab
A/a; B/b
?
Using meiosis to explain Mendel’s laws
Consider the cross
P:
F1:
b
B
Alternative
metaphase
alignment of the
second pair of
homologs
A/a; B/b
A/A; B/B x a/a; b/b
A/a; B/b
How can we explain formation of
four gametic types
(¼ AB, ¼ ab, ¼ Ab, ¼ aB) in F1
heterozygote?
Using meiosis to explain Mendel’s laws
Consider the cross
P:
F1:
b
B
Alternative
metaphase
alignment of the
second pair of
homologs
A/a; B/b
A/A; B/B x a/a; b/b
A/a; B/b
How can we explain formation of
four gametic types
(¼ AB, ¼ ab, ¼ Ab, ¼ aB) in F1
heterozygote?
Law II: independent
assortment of two pairs
of alleles is due to two
equally likely metaphase
alignments of different
homologs in Metaphase I