1. Lecture 6
The Cell Cycle

2. G0
RESTRICTION POINT
The cell cycle can be divided into interphase and mitosis/cytokinesis. Interphase consists of the first gap phase – G1, the DNA synthesis phase – S, and the second gap phase – G2. The gap phases – G1 and G2 – are also called the growth phases of the cell cycle. G1 has what is known as the restriction point. This is the point at which the cell must decide if it will proceed with the remaining phases of the cell cycle, or go into the G0 phase – this is a non-dividing state of the cell. Many cells can re-enter the cell cycle from the G0 phase, but other do not.
If the cell receives the OK to continue with the cell cycle – the cell proceeds to S, G2, and eventually mitosis and cytokinesis. These two daughter cells enter the G1 phase and reach the restriction point again.
But what make the cell decide it is going to continue on to S phase, or any other phase of the cell cycle?

3. G0 G1 G1 S R R Rb represses gene expression P P Rb Rb
relieves repression P Rb R
growth factor G1 S
In many cases, the decision to continue on to S phase depends on the presence of a growth factor signal.
Last class – growth factors bind to receptor tyrosine kinases – see next slide….
and in this case, one response is to activate expression of the cyclin D gene
cyclin D protein is made, binds to cdk4 protein already present (cycllin dependent kinase 4) and results in activation of its kinase activity
cyclin D/cdk4 phosphorylates Rb protein – cells pass the restriction point
this a protein that regulates expression of a variety of genes necessary for DNA synthesis
one gene it activates is cyclin E, which activates its cdk partner, and that complex of cyclin E/cdk2 phosphorylates Rb at another amino acid, further changing it ability to regulate gene expression
ultimate result is that cells enter S phase
note: cyclin D and cyclin E proteins are targeted for destruction once cells enter S phase P R Rb P
activate expression of cyclin D
activate cdk2 P
activate cdk4 Rb
activate expression of cyclin E

4. growth factor binds to RTK
activation of the receptor kinase activity
transduction of the signal
(go back)
connective tissue
digest the ECM with enzymes
grow with PDGF or without
with PDGF – cells divided
signal transduction

5. S G2 M cyclin B cyclin A cdk1 cdk2
cyclin A/cdk2 activity necessary to progress through S
need cyclin B/cdk1 to enter mitosis – part of MPF
MPF will stimulate formation of the mitotic spindle
checkpoints at G2 and M to make sure all is proceeding properly
DNA damage checkpoints:
at start of S
at the end of S
at the start of M
Replicated DNA checkpoint at start of M
Spindle assembly checkpoint during the beginning of M
Segregated chromosomes checkpoint during late M

6. Cancer uncontrolled cell division genetic damage not corectable
specific damage to genes that regulate cell division GF GF P P P P
cancer cells – do not respond to normal mechanisms that control cell division
density-dependent inhibition of growth
anchorage dependence of growth
they are also able to secrete signals that cause blood vessels to grow – gives them a nutrient source
they can lose or destroy their attachment to other neighboring cells and breakdown their local area of the extracellular matrix
result: the cells can move; can enter blood or lymphatic vessels
in culture – cancer cells can grow without growth factors
either make their own growth factors
or there is an abnormality in the signaling pathway
activation of cyclin D
expression
cell division

7. Mitosis and Cytokinesis
mitosis: division of the nucleus and its chromosomes
cytokinesis: division of the cytoplasm
RESULT: one cell divides into 2 daughter cells

8. Phases of Mitosis
Prophase
Prometaphase
Metaphase
Anaphase
Telophase

9. During S phase
Going back to S phase – all DNA is replicated so that each chromosome becomes 2 sister chromatids connected by a centromere
each of the two sister chromatids has a kinetochore – a protein-based structure that associates with specific portions of the centromere
the two kinetochores face in oppsoite directions

10. Just outside the nucleus – two centromeres
coordinating center for formation of the microtubules for the mitotic spindles
when the cell knows that it will continue on with cell division, the centrosome is replicated
the microtubules that make up the cytoskeleton partially break down and the assembly of the mitotic spindle begins
an aster forms – a radial array of short microtubules
the centrosomes push apart and will end up on opposite sides of the cell
show video

11. Prophase – chromatin more tightly coiled
each chromosome appears as two identical sister chromatids
mitotic spindle begins to form
centrosomes move away from each other
Prometaphase – nuclear envelope begins to break down
chromosomes form kinetochores
chromosomes begin to interact with the microtubules
begin the tug of war between the opposite poles
asters begin to make associations with the plasma membrane
the mitotic spindle continues to grow – microtubules that will not bind kinetochores reach out to each other
and interact

12. Metaphase:
centrosomes are at opposite sides of the cell
chromosomes are attached to the spindle – such that the kinetochore of each sister chromatid is attached to a
separate microtubule – one to a microtubule from one pole and the other to a microtubule from the other pole - will allow separation of the sister chromatids into the two separate cells later
lined up at the metaphase plate
Anaphase:
the proteins holding the two sister chromatids together are inactivated
two sister chromatids pull apart and move to opposite poles of the cell
accomplish this through the kinetochore and motor proteins – uses ATP
the microtubules not associated with chromosomes lengthen while the ones associated with chromosomes
shorten (depolymerization of tubulin)
Telophase:
two new nuclear envelopes begin to form
chromosomes become less condensed
microtubules not bound to chromosomes continue to grow to push apart the two poles
Cytokinesis:
division of the cytoplasm

13. microtubles shorten as the chromosomes move towards their respective poles

14. In animal cells:
cleavage furrow forms
result of interactions between actin and myosin microfilaments – like muscle contraction
tightening of a drawstring
furrow deepens until the cell is pinched into 2 cells
Plant cells:
have a cell wall
vessicles derived from the Golgi apparatus line up along the center of the cell using microtubules
come together to form the cell plate
vessicles carry material necessary to form a cell wall
cell plate grows until it fuses with the existing plasma membrane

15. go over again
rewatch movie?