Lecture 5:
Feb. 1, 2006
Chromosomes,
mitosis and meiosis
Chapter 4
Chromosomes
At right: A newt lung cell in culture at an
intermediate stage of mitotic spindle
formation, when most of the
chromosomes (blue) have already
connected to spindle microtubules
emanating from the centrosomes but have
not yet congressed to the spindle equator.
Immunofluorescence highlights:
alpha-tubulin (green)
gamma-tubulin (magenta)
keratin (red)
Image: A. Khodjakov
Chromosome Anatomy

Each duplicated chromosome

Has two sister chromatids, which separate during
cell division
0.5 µm
A eukaryotic cell has multiple
chromosomes, one of which is
represented here. Before
duplication, each chromosome
has a single DNA molecule.
Once duplicated, a chromosome
consists of two sister chromatids
connected at the centromere. Each
chromatid contains a copy of the
DNA molecule.
Mechanical processes separate
the sister chromatids into two
chromosomes and distribute
them to two daughter cells.
Figure 12.4
Chromosome
duplication
(including DNA
synthesis)
Centromere
Separation
of sister
chromatids
Centromeres
Sister
chromatids
Sister chromatids
A cell cycle showing the 4
stages
The M-stage is the shortest stage. In addition to the 4 stages,
two checkpoints and the 2 kinds of protein complexes (cyclin+kinase)
needed to drive through the checkpoints are also shown.
Steps in a mitotic cell division
The chromosome
number does
not change.
Two daughter cells are
produced in the end.
Steps in a meiotic cell division
Two successive cell
divisions occur
resulting in 4 daughter
cells. Pairing of
homologous
chromosomes occurs
and the chromosome
number is halved in the
first division. The
second division is
identical to mitosis.
G2 OF INTERPHASE
PROMETAPHASE
PROPHASE
Aster
Centrosomes
(with centriole pairs)
Chromatin
(duplicated)
Early mitotic
spindle
Centromere
Fragments
of nuclear
envelope
Kinetochore
Nonkinetochore
microtubules
Nucleolus
Nuclear
Plasma
Chromosome, consisting
Kinetochore
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at
one spindle pole
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
Details of the steps in meiosis prophase I
During this phase, exchange of parts of chromatids
(visible as X-shaped structures called chiasmata)
follows chromosome pairing.
There is a
mistake in this
diagram. Each
chromosome in
leptotene,
zygotene and
pachytene
should have 2
chromatids. The
paired
homologous
chromosomes
in zygotene and
early pachytene
should have 4
strands.
Subsequent steps in meiosis I
Steps in meiosis II
Pairing of homologous chromosomes
and chiasma formation [due to crossing
over (recombination) between sister
chromatids]
Alternative alignments of paired
chromosomes in metaphase I
Meiosis and fertilization
in females (XX) and males (XY)
The female
produces only
one kind of
gamete
while the male
produces 2
kinds of
gametes.
The chromosome composition of
male and female Drosophila
Inheritance of the X-linked eye
color gene in Drosophila
This complex
diagram shows
the results of 2
separate
crosses
(reciprocal
crosses) for the
X-linked eye
color gene
(w+ or w alleles)
Inheritance of the X-linked gene for the disease
hemophilia in the descendants of Queen Victoria
Consequences of nondisjunction (lack of segregation)
of the X-chromosomes during meiosis in Drosophila