cell division

Question 1

1. what are the two essential features of cell division?
2. what happens within each phase:
   - M phase, G1 phase, S phase, G2 phase,

Answer 1

1. Essential features of cell division:
   - Faithfully replicate genetic material (DNA synthesis) - it takes 8 hours to replicate the human genome
   - Accurately segregate into daughter cells - it takes 2 hours to segregate the replicated genome
2. S phase - DNA is replicated

M phase (mitosis + cytokinesis) - where the replicated DNA is separated

^ these two phases are separated by gap phases (G1 and G2)

• in G1, the DNA which is inherited is not damaged and there is no nutrient deficit
• in G2, the cell is checking that it has replicated the DNA accurately and nothing needs repair

Question 2

what is the cell cycle critically controlled by?

what word is used to describe the activity of the regulators?

what regulates the activity of the regulators?

Answer 2

1. the cell cycle is critically controlled by cyclin-dependent kinase (CDKs).
   - it is active when it is bound to cyclin (mitosis occurs during binding). there are several different cyclin CDK complexes which have been identified working at different stages of the cell (S cylin and S-CDK during S phase etc)
2. the activity of cyclin-dependent kinases is cyclical as the cyclin concentration comes in waves
3. CDKs are regulated by phosphorylation and dephosphorylation of the cyclin dependent kinase

Question 3

what is the process of ubiquitination?

what is ubiquitin?

Answer 3

1. ubiquitination is the process of deactivating the enzymes using a protein called ubiquitin
2. ubiquitin is a 76 amino acid protein that is added onto cyclin or any other proteins which needs to be destroyed through a little signal which then takes them to the proteosome where the cyclin is then degraded and this controls the activity of CDKs

Question 4

what are the 5 steps of ubiquitination?

Answer 4

1. enzyme 1 (Ubiquitin activating enzyme) becomes coupled to ubiquitin (only 2 enzyme 1s) which is an energy dependent reaction (require energy)
2. the ubiquitin is transferred from the E1 enzyme onto an E2 enzyme (ubiquitin conjugating enzyme) (approx. 35 different enzyme 2s**)
3. this ubiquitin moiety can then be transferred onto the substrate with the presence of a E3 enzyme (ubiquitin ligase) (approx hundreds of enzyme 3s - more specificity)
4. multiple ubiquitins are added to a substrate which adds specificity
5. once ubiquitin is attached to the substrate, it is recognised by the proteosome (the dustbin lid recognies the ubiiquitin) which causes the protein to unfold and move through the proteosome parts where is is chopped up into different constituent amino acid which is then released from the proteosome. - this process is not only used in the cell cycle but also as a post-translational modification

Question 5

what is enzyme activity also tightly regulated by?

what are the 4 checkpoints?

what are these checkpoint pauses created by?
when can transcription of inhibition be induced?

Answer 5

1. enzyme activity is also tightly regulated through checkpoints in which the cell cycle cannot proceed until the cell has checked everything is in order
2. checkpoints in each phase:
   G1: damaged DNA and any unfavourable extracellular environment factors

S1: damaged or incompletely replicates DNA

G2: damaged or incompletely replicated DNA

M: chromosome improperly attached to mitotic spindle

3. these checkpoint pauses are created by a protein called cyclin dependent kinase inhibitors:
   - transcription of inhibition can be induced (producing the cyclin dependent kinase inhibitors to be synthesised) if conditions are not right for cell division (active CDK into inactivated CDK)

Question 6

looking at how the cell moves between phases:
1. what is the transition between G1 phase and S phase also known as?

2. what does the retinoblastoma gene (Rb) do?
3. how does the cell cycle move onto S phase?

Answer 6

1. growth or quiescence (R-point aka G1/S transition) - can be disrupted in cancer
2. retinoblastoma gene product (Rb) which sits on transcription factors and prevents the expression of genes required for the S phase
3. when the conditions are right and cell gives a thumbs up to cell division, the CDK becomes active and phosphorylates the retinoblastoma gene product which inactivates it and allows the stimulation of the expression of genes requires for the S phase

Question 7

1. what initiates origin replication:
2. what are the 4 steps of DNA replication?

Answer 7

1. target of a cyclin dependent kinase is protein called cdc6:
2. protein cdc6 sits on the DNA at origin sites of replication, it primes this site before replication
   - when this replication is ready to begin, the cyclin dependent kinase can start phosphorylating cdc6
   - this causes cdc6 to detach from the complex and is targeted for degradation through the ubiquitin machinery
   - this allows the completion of the complex and the conversion from the pre-replicative state to the replication fork and then the completion of DNA replication

Question 8

what are the 2 responses used in the cell to check whether the cell is damaged?

Answer 8

-Response depends on stage of cell cycle:
- p53- directs transcription of cdk-inhibitors (repress the activity of CDKs)
- CHK2 is activated- inhibits cdc25

Question 9

what are the 6 stages of mitosis?

what occurs at each stage?

Answer 9

Question 10

what occurs during DNA condensation

Answer 10

protein called condensins, create ‘handcuffs’ which encourage DNA to adopt a very highly condensed form which then stack on top of each them to form a very condensed chromosome:

Question 11

what occurs during the forming of the spindle?

Answer 11

• the spindle initially comes from the centrosome, which is replicated very early in the cell cycle and comprises of two centriols that are 90 degrees to each other and forms a microtubule organising centre
• this begins to occur before the nucleus is broken down

Question 12

what occurs during the nuclear envelope breakdown

Answer 12

• before mitosis can proceed, the nuclear envelope must be broken down
• this is done with the help of one of the targets for the cyclin dependent kinases comes in (targeting the M phase cyclins), one of the targets is the nuclear lamin which is a component of the nuclear envelope
• then cyclin dependent lamins phosphorylate the nuclear lamins which will encourage the nuclear envelope to break apart, in prometaphase
• the lamins and nuclear pore proteins are then transported to the daughter cells accompanying the chromosomes and allow the nuclear envelope to be regenerated in the daughter cell

Question 13

how do the chromosomes attach after the nuclear envelope is broken down?

Answer 13

• chromosomes attach to the spindle through the specialised region on the chromosome called the centromere
• the centromere attracts a multiple protein complex called the kinetochore
• the kinetochore assembles on the centromere and attaches to the spindle

Question 14

metaphase:
1. what is the spindle checkpoint?

2. when can mitosis proceed?
3. what binds sister chromatids?

Answer 14

1. the spindle checkpoint is when the cell ensures that chromosomes are attached onto spindles, which have to be attached to opposite poles to bring about tension
2. mitosis cannot proceed until chromosomes are properly attached and under tension
3. there are cohesion complexes** between the different chromatids
   • defects in this cohesion can cause disease called robert syndrome in which there is a very severe phenotype, which is very rare

Question 15

anaphase:
1. what happens during anaphase?

2. what do separase enzymes do?
3. what is separase activity regulated by?
4. what can a defect at the spindle checkpoint cause?

Answer 15

1. • the separation of the chromosomes
2. separase enzyme separates the chromatids as it chops up the cohesion units
3. the separase activity is regulated and controlled by securin which inhibits separase until needed and checkpoints have been satisfied,
4. defects at the spindle checkpoint can cause aneuploidy (many cancer cells are aneuploid) - Aneuploidy is the presence of an abnormal number of chromosomes in a cell

Question 16

1. what are anti-mitotic drugs used for?
2. what is an example of an anti-mitotic drug and how does it work?

Answer 16

1. anti-mitotic drugs which are used to prevent cancerous growth
2. and example is taxol
   which stabilises microtubules, and in a normal cell would cause cell arrest and not produce the cell, the cancerous cells do not check the cell integrity and continue to produce the cell, this causes a formation of abnormal cells which eventually are killed

Question 17

how does the cell exit mitosis?

Answer 17

through the degradation of cyclins and securin

Question 18

what occurs during cytokinesis, the final step of mitosis?

Answer 18

the separation of the cell into two daughter cells is done through the contractile ring of actin and the interactions with the microtubules