Mitosis and Cytokinesis
Section 5.2
1
Objectives
• SWBAT describe the structure of a
chromosome.
• SWBAT follow chromosomes through the
processes of mitosis and cytokinesis.
2
Vocabulary
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Chromosome
Histone
Chromatin
Chromatid
Centromere
Telomere
Prophase
Metaphase
Anaphase
Telophase
3
Chromosomes
• A chromosome is one long, continuous,
thread of DNA.
– It consists of genes along with “regulatory (for
example start and stop instructions for gene
replication)” information.
– Humans have 46 chromosomes each (2 pairs of
23 chromosomes).
– The DNA you have in each cell, if stretched out,
would measure about 10 feet in length.
4
Human Karyotype
What does
diploid
mean?
What does
haploid
mean?
5
Salmon and Field Mouse Karyotypes
6
Chromosomes and Histones
• DNA wraps around proteins called histones
that help to organize and condense it.
– During interphase, DNA is loosely organized (looks
like spaghetti.
– During mitosis, however, the DNA is tightly
condensed.
7
Chromosomes and Histones
• The changes in DNA organization – from loose
and spaghetti-like to very condensed during
mitosis – are important for allowing cells to
carry out their functions.
– For example: during interphase, DNA needs to be
available so genes can be copied and transcribed
to make proteins and other molecules.
– During mitosis, DNA does not need to be
transcribed but it does need to be divided (hence
condensed is better than stringy to avoid “DNA
tangles”).
8
Chromosomes and Histones
DNA double
helix
DNA and
histones
Chromatin
Supercoiled
DNA
Each continuous double-stranded DNA molecule makes one chromosome. The
DNA wraps around the histones at regular intervals – looking like beads on a
string.
9
Chromosomes and Histones
• During interphase (when DNA is in its
“spaghetti phase”), the loose combination of
DNA and proteins is called chromatin.
• When interphase ends and prophase of
mitosis begins, the chromatin is condensed.
– It coils more tightly around the histones until it
forms “small thick rods” – like we saw in the
karyotypes.
10
Chromatids
• REMEMBER: each chromosome was
duplicated in S-Phase. So now, in
mitosis, we have identical
chromosomes that are condensed and
attached to one another at a region
called the centromere.
• One half of a duplicated chromosome
is called a chromatid. The two
identical chromatids are called sister
chromatids.
chromatid
telomere
centromere
telomere
11
Telomeres
• Telomeres are at the ends of DNA molecules.
They are made up of repeating nucleotides
(nitrogen bases) that do not form genes.
• The purposes of telomeres are:
1. to keep chromosomes from accidentally
attaching to one another;
2. and to help prevent the loss of genes.
• A short length of nucleotide is lost every time
a chromosome is copied. It is important that
the nucleotides are lost from telomeres and
not from the genes themselves.
12
13
1. Chromatin
condenses into
tightly coiled
chromosomes (two
sister chromatids).
Prophase
2. The nuclear
membrane breaks
down and the
nucleolus
disappears.
3. The centrioles and
centrosomes
migrate to
opposite sides of
the cell –
producing spindle
fibers
(microtubules).
14
Metaphase
1. Spindle fibers
attached to
protein structure
on the centromere
of each
chromosome.
2. The chromosomes
are aligned along
the equator of the
cell.
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Whitefish Blastula Metaphase
16
Anaphase
The sister
chromatids are
separated as the
spindle fibers
shorten - pulling
them apart.
17
Whitefish Blastula Anaphase
18
Telophase
A complete set of identical
chromosomes are now in the
developing daughter cells.
The nuclear envelope begins
to appear.
The chromosomes uncoil and
become spaghetti-like again.
The daughter cells complete
cytokinesis and separate
completely.
19
Whitefish Blastula Telophase
20