Mit Flashcards
(29 cards)
Mammalian cells divide every
Other cells
Neurons are
24 hours
Do not divide once they are born
Terminally differentiated
M phase
Mitosis and nuclear division
Cytokinesis and cytoplasm division.
G1 phase
Can be very long or very short.
At the end a choice is made on whether to do DNA replication.
Checks DNA for damage.
S phase
Produces sister chromatids
DNA replication.
Prophase
The chromosomes condense
The mitotic spindle assembles
Picture shows fat chromosomes all in the nucleus.
Prometaphase
Starts with the breakdown of the nuclear envelope.
Chromosomes attach to the spindle microtubules by their kinetochores.
Metaphase
The chromosomes are lined up at the equator of the spindle.
Each of the sisters has an opposite microtubule attached.
Anaphase
The sister chromatids separate and are pulled to the poles of the cell.
The kinetochore microtubules get shorter.
Telophase
The two sets of chromatids arrive at the poles and decodense.
A new nuclear envelope assembled around each set forming two nuclei.
The contractile actin ring begins to contract.
Cytokinesis
The cytoplasm is divided by a contractile ring of actin and myosin which pinches the cell to create two daughter cells.
How is the cycle split up into time.
M phase takes one hour.
G phases and S phase takes 23 hours and is called interphase.
Yeast as a generic model
It is good because they have a rapid division rate of less than an hour.
The cell cycle control genes are highly conserved in humans and yeast.
Yeast can be grown as haploid or diploids and they are similar to us.
Types of yeast
Fission yeast- schizo pombe
In mitosis the cells divide but they are stuck together.
They have a short G1 and long G2.
Budding yeast- saccha cerevis
Creates two separate cells, one small and one big one.
Doesn’t have a G2 phase.
Because yeast can be maintained as diploids…
You can knock out one copy of the gene and then they can be turned into haploids to study the effects of knocking out the gene.
Temperature sensitive mutations
Some model organisms don’t maintain their body temperature so you can shift it up and down.
The temperature sensitive mutations usually work at low temperatures and high temperatures will inactivate the mutation.
At high temperatures all the cells would be stuck in a particular stage of the cycle and this will synchronise all the yeast cells.
Lowering the temperature again would let them continue the cycle and all begin at the same point.
CDC genes
Xenopus model
Easy to collect eggs
Rapid division rate 30 mins
Large eggs size for easy protein purification
Can be manipulated easily by injection.
The oocyte grows larger but does not divide.
Then after fertilisation the egg divided many times and does not grow.
Cell free mitosis
Purify the cytoplasm out of an oocyte by centrifugation.
Add a sperm nuclei and ATP to allow mitosis.
It will divide every 40-60 minutes and make more DNA.
Researchers can add antibodies for particular proteins and knockout those proteins and see what happens.
Take out cytoplasm as certain time points and see what changes over time.
Cell cycle checkpoints
From G1 to S-
Needs to decide if the cell is okay to replicate its DNA. is the environment favourable and is there enough energy.
From G2 to M-
Has all the DNA been replicated before the cell is allowed to divide. Is the environment favourable.
During M before anaphase- check if all the chromosomes are attached to the spindle.
Cyclins
Proteins expressed in different amounts at different stages in the cell cycle.
They bind to cyclin dependant kinases CDKs.
The CDKs are activated by cyclin binding and they phosphorylate proteins that are specific to certain stages of the cycle.
Which cyclins are where
G1/S cyclin is only active in G1 and binds to G1/S CDK.
S cyclin is active during S and G2 and some of M. It binds to S CDK.
M cyclin is active in M and G2 and binds to M CDK.
Other proteins that can modify CDKs.
Wee1 kinase phosphorylates CDKs in a different site to usual and inhibits them.
Cdc25 phosphatase removes the phosphate added by wee1 and reactivates the CDK.
P27 binds to the whole cyclin CDK complex and inhibits it’s activity.
Metaphase to anaphase checkpoint
Uniquitination causes M cyclin to drop.
Adding ubiquitin to M cyclin will cause it to be sent for degradation.
The degradation is done by ubiquitin ligases which add ubiquitin to proteins.
The APC (anaphase promoting complex) is the ubiquitin ligase used. APC will do this to M and S cyclin and a protein called securin which glues chromosomes together to prevent anaphase.
Difference between soma and germline cells.
The genes will be passed on in the germline and they will not be in the soma.
Meiosis then mitosis
Meiosis makes gametes
After fertilisation the gametes do mitosis to develop.
