Chromosomes and the cell
cycle
for AS Biology
Chromosome structure
• The DNA of eukaryotic cells is organised into a
fixed number of linear chromosomes
• Each chromosome consists of a single DNA
molecule wound around a protein framework
• In non-dividing (interphase) cells this DNAprotein complex is too fine to be resolved using a
light microscope, and is barely resoluble in the
electron microscope
• During nuclear division this extended form of the
chromosome is multiply coiled and supercoiled to
form structures that can be seen in the light
microscope
Chromosome structure
The solenoid is
further coiled and
folded …
The chain of
nucleosomes is
coiled
and supercoiled
to form a solenoid
The DNA and
histones together
form the material
called chromatin,
of which the
nucleus is made
… to form the
visible chromosome
we see in dividing
cells
The DNA double
helix is wound
around clusters of
protein molecules
called histones
Each group of eight
histones with its two
turns of DNA is called
a nucleosome
This is the ‘naked’ DNA double
helix, with its sugar-phosphate
backbone and base pairs.
Chromosome structure
The shortest
human
chromosome
contains 4.6 x 107
bp of DNA: this is
equivalent to
14,000 µm of
extended DNA. In
its most
condensed state
during mitosis, the
chromosome is
about 2 µm long.
This gives a
packing ratio of
7000 (14,000/2).
Electron micrograph of
chain of nucleosomes
DNA
Histones
The cell cycle
• Cells in an actively dividing tissue go
through a repeated sequence of stages
called the cell cycle
Division occurs in
the M phase
In G2 the cell
synthesises
proteins and ATP
in preparation for
division
If the cell is going
to divide again it
enters S phase, in
which its DNA is
replicated
In G1 the cell
grows and carries
out its normal
metabolic
activities
A cell that is not
going to divide
again remains in
G1: proteins
called cyclins
determine
whether the cell
does this or
enters the next
stage
Have you been paying attention?
What is meant by packing ratio?
The ratio of the extended length of DNA in a chromosome to the minimum
length of that chromosome when condensed.
What is this?
A nucleosome.
DNA
?
Histones
?
What
A
mature
is the
liver
collective
cell does
name
not normally
for G1 + divide
S + G2?
again. What stage of the cell cycle is it in?
Interphase
G1.
Suggest why it is necessary for the cell to build
What
up
its happens
ATP and protein
in S phase?
reserves during G2
Because
DNA
replication
respiration and protein synthesis do
not occur during M phase.
What happens in G2 phase?
Synthesis of proteins and ATP
Mitosis: prophase
• At the beginning of mitosis chromosomes
begin to condense and become visible in
the light microscope: this is prophase
Animal cell in prophase
Plant cell in prophase
Mitosis: prophase
• At the beginning of mitosis chromosomes
begin to condense and become visible in
the light microscope: this is prophase
• As the chromosomes condense the
nuclear envelope breaks down
• In animal cells, the centrioles replicate
and move to opposite poles of the cell,
forming the spindle apparatus (an array
of microtubules) between them
Mitosis: prophase
Centrioles are cylinders made up of nine triplet
microtubules.
Animal cells contain a pair of centrioles, usually found
near the nucleus lying at right angles to each other.
In mitotic prophase they replicate and move to
opposite poles of the cell, forming the mitotic spindle
between them.
Plant cells do not contain centrioles, but still form a
spindle.
Centrioles
Mitosis: prophase
• By late prophase chromosomes have
condensed sufficiently to be visible as
pairs of sister chromatids joined at a
centromere.
Mitosis: metaphase
• In metaphase the chromosomes move to
the equatorial plane of the cells, and their
centromeres attach themselves to the
spindle fibres
Animal cell in metaphase
Plant cell in metaphase
Mitosis: metaphase
• Attachment is made by protein units called
kinetochores, to kinetochore microtubules in the spindle
Mitosis: anaphase
• At the end of metaphase the centromeres
divide and the sister chromatids – now
daughter chromosomes – are pulled
apart. This is anaphase
Animal cell in anaphase
Plant cell in anaphase
Mitosis: telophase
• In telophase the daughter chromosomes
reach opposite poles of the cell and start to
uncoil: a new nuclear envelope forms
around each set and the spindle disappears
Animal cell in telophase
Note that mitosis
Telophase
is usually,
refers
butonly
not
always,
to
nuclear
accompanied
division. by
cytokinesis (cytoplasmic
It is usually followed by
division).
cytokinesis, but not always.
In animal cells, this takes the
Muscle fibres are multiform of cytoplasmic
nucleate, as are some liver
constriction (‘pinching’),
cells. This is brought about by
leading to separation into two
mitosis occurring without
cells.
cytokinesis.
Mitosis: telophase
• In telophase the daughter chromosomes
reach opposite poles of the cell and start to
uncoil: a new nuclear envelope forms
around each set and the spindle disappears
Plant cell in telophase
Telophase is usually, but not
always, accompanied by
cytokinesis (cytoplasmic
division).
In plant cells, Golgi vesicles
lay down a cell plate across
the equator of the cell.
A new cell wall is laid down
either side of the cell plate.
Mitosis: overview
Mitosis animation
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Mitosis animation
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Mitosis animation
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Mitosis animation
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Mitosis: test yourself
What stage of mitosis is shown in this micrograph?
Metaphase
Name the structures indicated.
Spindle
?
Aster
?
Centrioles /
?
centrosome
Mitosis: test yourself
What stage of mitosis is shown in this micrograph?
Prophase
Name the structures indicated.
Chromosome
?
Nuclear
?
envelope
Mitosis: test yourself
What stage of mitosis is shown in this micrograph?
Metaphase
Name the structures indicated.
Chromatids
?
Mitosis: test yourself
What stage of mitosis is shown in this micrograph?
Telophase
Name the structure indicated.
Cell?plate
Mitosis: test yourself
What stage of mitosis is shown in this micrograph?
Anaphase
Name the structures indicated.
Daughter chromosomes
?
Spindle
?
Sequence and name these stages
Prophase
Name?
Metaphase
Name?
Name?
Anaphase
Telophase
Name?
Mitosis: significance
• In the cell cycle every chromosome is first
replicated (in the S phase) and then the
copies separated (in M phase / mitosis).
• Daughter cells of mitosis are therefore
genetically identical to each other and to
the parent cell.
• Mitosis is important in:
 growth and replacement;
 asexual reproduction: the offspring are
genetically identical clones of the parent.