Objectives for today:
• What is the cell cycle?
• When is DNA replicated in the cell cycle?
• What are the main stages of mitosis?
Syllabus: 3.2 cells
Now:
Interphase:
G1 - making new organelles & proteins
Original cell
Daughter cell
Daughter cell
Interphase:
G1 - making new organelles & proteins
Original cell
Daughter cell
Daughter cell
Interphase:
G1 - making new organelles & proteins
S - replicating DNA
Original cell
46
chromosomes
2x23 or 2n
Daughter cell
Daughter cell
46
46
chromosomes
chromosomes
Organism
How many chromosomes in a
somatic (body) cell?
Barley
14
Carp
104
Cat
38
Chimpanzee
48
Earthworm
36
Hedgehog
88
Horse
64
Kangaroo
12
Mouse
40
Yeast
32
Interphase:
G1 - making new organelles & proteins
S - replicating DNA
Original cell
46  92
chromosomes
2n  4n
Daughter cell
Daughter cell
46
46
chromosomes
chromosomes
2n
2n
Somatic (body) cells are diploid:
Interphase:
G1 - making new organelles & proteins 2 copies of every chromosome.
Think “double.”
S - replicating DNA
Original cell
sperm
46
23
chromosomes
chromosomes
n
2x23 or 2n
Ovum (egg)
23
Daughter cell
Daughter cell
chromosomes
n
46
46
chromosomes
chromosomes
Gametes are haploid: one
copy of each chromosome.
Think “half.”
Interphase:
G1 - making new organelles & proteins
S - replicating DNA
G2 - growth
Original cell
Daughter
cell
Daughter
cell
Daughter
cell
Daughter
cell
Daughter
cell
Daughter
cell
Interphase:
G1 - making new organelles & proteins
S - replicating DNA
G2 - growth
Original cell
Daughter cell
Daughter cell
The cell cycle: a series of processes that cells have to undergo before
cell division (mitosis or meiosis).
G1: growth, synthesis of
proteins and new
Mitosis
organelles. Synthesis of
new centrosomes starts.
G2: rapid
growth of cell
& checking for
DNA damage.
S: DNA synthesis, DNA
repair & centrosomes
finish being synthesized.
G0: resting state. The
cell leaves the cell
cycle permanently
or temporarily.
Checkpoints regulate the cell cycle: if mutations or other problems are
found then the cell enters G0 then may undergo apoptosis.
G0 or apoptosis
(programmed cell death)
G2 checkpoint.
Check for:
• Cell size
• DNA
replication
• DNA damage
Spindle assembly checkpoint
Check for chromosome
attachment to spindle
G1 checkpoint.
Check for:
• Cell size
• Nutrients &
growth factors
• DNA damage
G0 or apoptosis
(programmed
cell death)
apoptosis
(programmed cell death)
G0 resting state
cytokinesis
The cell cycle
interphase
S phase
interphase
centrosomes
G1
Mitosis: a type of cell division.
• For growth & repair of
tissues.
• For asexual reproduction,
e.g. in yeast. It’s called binary
fission in bacteria but is very
similar.
• Each daughter cell is
genetically identical to the
original cell; they are clones.
• The original cell must
replicate (copy) its DNA
before cell division so that
each daughter cell gets one
copy of each chromosome.
A karyotype: a set of chromosomes
Humans have 23 pairs of chromosomes called
homologous chromosomes. 1 chromosome
from each homologous pair is from your mother
and the other is from your father.
DNA is replicated during S phase in interphase.
The DNA has to replicate (copy itself)
DNA is replicated during S phase in interphase.
The DNA has to replicate (copy itself)
The DNA then supercoils to prevent it from breaking during cell
division. This happens in the first stage of mitosis (called prophase)
and makes the DNA visible as chromosomes
Chromosomes are visible
Chromatin is diffuse
The sister chromatids are held together by a centromere.
The sister chromatids
are held together by a
centromere.
Chromosomes during interphase and mitosis.
• Each chromosome makes a pair of sister chromatids: identical
copies of one original chromosome.
• So each homologous pair is now made of 2 x 2 chromatids called a
bivalent.
Homologous
chromosomes
Sister
chromatids
cytokinesis
Q: When do the chromosomes become visible?
Chromosomes during interphase and mitosis.
• Each chromosome makes a pair of sister chromatids: identical
copies of one original chromosome.
• So each homologous pair is now made of 2 x 2 chromatids called a
bivalent.
Homologous
chromosomes
Sister
chromatids
cytokinesis
Q: When do the chromosomes become visible?
During the first stage of mitosis (called prophase). So even though we know
what they are doing during interphase, we can’t actually see them.
Now
Write down definitions for the words on p.4 of your
booklets.
Diploid – a cell or an organism containing two copies
of each chromosome. e.g., most body cells.
Haploid – a cell or an organism with a single set of
chromosomes, i.e., a single copy of each homologous
pair. Found in gametes (sperm and eggs).
Homologous chromosomes – two chromosomes that
carry the same genes (although the alleles may be
different) at the same loci (positions on the
chromosome).
Bivalent – two homologous chromosomes as two
pairs of sister chromatids.
Centromere – attaches a pair of sister chromatids to
each other; split apart by the spindle fibres
shortening.
Chromatids – identical copies of a chromosome. Held
together with its sister chromatid by a centromere.
Made by DNA replication.
Centrosome – two centrioles forming a T shape at the
poles of the cell where the spindle grows from.
Spindle – the microtubules that pull the
chromosomes towards opposite poles.
This somatic cell is in G0 resting phase.
1. Would we be able to see the
chromosomes? Why?
2. What is the chromosome number of
this species?
3. Draw the cell after S phase.
4. Draw a gamete made by this animal.
This somatic cell is in G0 resting phase.
1. Would we be able to see the
chromosomes? Why?
2. What is the chromosome number
of this species? 3
3. Draw the cell after S phase.
4. Draw a gamete made by this
animal.
What are the stages in the cell cycle?
1. Interphase I
2. Prophase
Paint
3. Metaphase My
4. Anaphase
Ankles
5. Telophase
Tangerine
6. Cytokinesis Colour
Remember that
interphase &
cytokinesis aren’t
part of mitosis!
Prophase (pro = before)
• Chromosomes become visible.
• Nuclear membrane disintegrates.
• The spindle begins to form.
Prophase
The spindle begins to form. The
centrosomes migrate to opposite poles.
The nuclear
membrane
disintegrates
Chromosomes
supercoil so are visible
Metaphase (meta = adjacent)
Chromatids are moved to the equator of the cell by the spindle
fibres.
Metaphase
The chromosomes line up on
the equator.
The spindle fibres
pull the
chromosomes to
the equator.
Why does this result
in genetically
identical daughter
cells?
Metaphase
The chromosomes line up on
the equator.
The spindle fibres
pull the
chromosomes to
the equator.
Why does this result in
genetically identical daughter
cells? Sister chromatids will
be pulled to opposite poles
from each other, so daughter
cells get one copy of each
chromosome.
Anaphase (Ana = away)
The sister chromatids are pulled apart to opposite poles by the
spindle fibres shortening.
Anaphase
The chromosomes are
pulled to opposite
poles by the spindle
fibres getting shorter.
Telophase (telo = end)
A new nuclear membrane forms around each set of chromosomes
(not called chromatids once they reach the poles).
Telophase
The nuclear
membrane reforms.
The chromosomes
uncoil to become
indistinct again.
Now
Fill in the table on p.5 of your handout.
Extension:
Match each dark card with a pale card.
Doubles
 Chromosomes are
during
unwound so invisible
interphase
(chromatin).
: 2n – 4n.
 Chromosomes become
visible as they condense:
the DNA coils up.
Double:
4n.
 Chromosomes appear as
identical chromatids joined
by centromere.
Double:
 Chromatids line up on
4n.
equator.
 Sister chromatids separate
to give daughter
chromosomes.
Double:
4n.
 Daughter chromosomes
pulled to opposite poles of
cell.
 DNA replication.
 Protein synthesis
(new cellular
components).
 Spindle develops.
 End of prophase
when nuclear
membrane
disintegrates.
 Daughter chromosomes
Double at
arrive at poles,
start (4n):
2n in each  Chromosomes begin to
disappear as they uncoil
nucleus.
(chromatin).
 Nuclear membrane
re-forms.
 Cell begins to
constrict across
middle… to form two
daughter cells.
4n at start
(2n in
 Daughter nuclei return to
each
interphase state.
nucleus).
 Chromosomes become
Ends up
invisible again (chromatin).
as 2n
(diploid).
 Cytoplasm divides
into two separate
cells each with
nucleus.
haploid
anaphase
interphase
G1
S phase
Budding in yeast
spindle
centromere
meiosis