The Cell Cycle

Question 1

What phase does a cell spend the majority of its time in?

Answer 1

Interphase

Question 2

What does the cell do in G1 phase?

Answer 2

This is most of the cell’s life, where it makes extra organelles and proteins.

Question 3

What does the cell do in S phase?

Answer 3

DNA replication.

Question 4

What does the cell do in G2 phase?

Answer 4

Prepare for mitosis.

Question 5

What does the cell do in the mitotic phase?

Answer 5

Mitosis

Question 6

What does the cell-cycle control system do?

Answer 6

This system ensures that the events of the cell cycle is completed in a set sequence, and each is completed before moving on to the next.

Question 7

What are housekeeping genes?

Answer 7

Housekeeping genes are genes that are always active anytime.

The products of these genes are absolutely needed by all cells in all organisms at any time, therefore they are genes required for the cell to live and hence are always expressed.

Question 8

What are condition type specific genes?

Answer 8

They replicate at specific moments depending on whether they are on or off.

Question 9

What are cyclically dependent protein kinases (Cdk)?

Answer 9

Protein kinases that carries out phosphorylation reactions to regulate protein activity involved in the cell cycle.

Question 10

Why are there different types of Cyclin-Cdk complexes?

Answer 10

Because different Cyclin-Cdk complexes trigger different steps in the cell cycle.

For example, the cyclin that trigger entry into M phase is called M-cyclin, and forms a complex with Cdk calles M-Cdk.

Question 11

What are cyclin concentrations regulated by?

Answer 11

The rise in cyclin concentrations can be determined by the rate at which its transcribed.

They can also be degraded by an anaphase promoting complex (APC), which tags these cyclins with a chain of ubiquitin, resulting in their degradation.

Question 12

How can Cyclin-Cdk complexes be switched on abruptly?

Answer 12

They contain inhibitory phosphates and needs to be dephosphorylated by a specific protein phosphatase.

Question 13

How can other proteins also regulate the activity of Cyclin-Cdks?

Answer 13

Cdk inhibitor proteins can bind to block the assembly or activity of certain cyclin-cdk complexes.

Question 14

Why does the cell cycle control system transiently pause the cycle at various points?

Answer 14

It monitors the cell’s internal state and conditions, to ensure the important events occur in the right order.

Question 15

What mechanisms does the cell use at various points to pause the cycle?

Answer 15

G1-S stage: Cdk inhibitors are used to block entry into S phase.

G2-M phase: Suppresses activation of M-Cdk by inhibiting the phosphotase required to activate.

Exit from M: Inhibition of APC delays exit from mitosis.

Question 16

How do cells inactivate S-Cdks and M-Cdks towards the end of M phase?

Answer 16

By blocking the synthesis of new ones and using Cdk inhibitor proteins.

Question 17

How do mitogens work to stimulate mitosis?

Answer 17

They switch on cell signalling pathways that stimulate the synthesis of G1 cyclins, G1/S cyclins and other proteins involved in DNA synthesis and chromosome duplication.

Question 18

What happens if no mitogens are present?

Answer 18

The cell cycle arrests in G1.

If deprived for long enough time, the cell can withdraw from the cell cycle and enter a non-proliferative state.

Question 19

What happens if there’s DNA damage?

Answer 19

This causes an increase in concentration of p53, which is a transcription regulator that activates transcription of p21.

P21 binds to G1/S-Cdk and S-Cdk to prevent then from taking the cell into S phase.

This gives time for the cell to heal itself of the damage, though if this is too great, p53 can induce the cell to undergo apoptosis.

Question 20

How can missing or defective p53 lead to cancer?

Answer 20

The unrestrained replication of damaged DNA leads to a high chance for mutation and giving rise to cancerous cells.

Question 21

What happens in cells that completely withdraw from the cell cycle?

Answer 21

In some terminally differentiates cells, like muscle cells or nerve cells, the cell cycle control system is completely dismantled.

This means that the genes encoding for relevant cyclind and cdks are shut down.

Question 22

How does the cell delay entry into M phase?

Answer 22

M-Cdk is inhibited by inhibitory phosphotases which needs to be removed by an activating protein phosphatase called Cdc25.

When DNA is damaged, Cdc25 is inhibited. Thus M-Cdk is inactive.

Question 23

How does S-Cdk initiate and block DNA replication?

Answer 23

ORC (origin recognition complex) recruits Cdc6. These proteins load DNA helicase to form the pre-replicative complex.

S-Cdk activates DNA helicase and promotes the assembly of the rest of the proteins to form the replicative fork.

S-Cdk phosphorylates Cdc6, marking it for degradation.

Question 24

What is cell necrosis?

Answer 24

Cells that die as a result of acute injury, they typically swell and burst, splling their contents over neighbour cells.

Question 25

What happens in cell apoptosis?

Answer 25

The cell may develop irregular bulges on its surfaces (blebs), which shrinks and condenses.

The cytoskeleton collapses, nuclear envelope disassembles and the nuclear DNA breaks up into fragments.

Question 26

Describe the process of apoptosis through the intracellular proteolytic cascade.

Answer 26

Ths involves a family of proteases called caspases. They are initially precursors called procaspases that are activated in reponse to signals inducing apoptosis.

There are 2 types of caspases, initiator casapases cleave amd activate downstrea, executioner caspases.

Some of executioner caspases activate additional executioners, starting a proteolytic cascade. While other dismember other key proteins in the cell.

Question 27

How can apoptosis be regulated?

Answer 27

By the Bcl2 family.

Bax and Bak induce the release of e- transport protein cytochrome C from the mitochondria to the cytosol. This activates initiator procasapases by promoting the assembly of apoptosomes.

Other proteins of this family inhibits apoptosis by preventing Bax and Bak from releasing cytochrome C.

Question 28

How can extracellular signals induce apoptosis?

Answer 28

Killer lymphocytes have fas ligands present on their surface, which they can use to activate fas, a death receptor.

This induces the assembly of a death-inducing signalling complex, which can launch a caspase cascade leading to apoptosis.

Question 29

Describe the 3 major categories of cell extracellular signal.

Answer 29

Survival factors - promote cell survival, largely by apoptosis.

Mitogens - stimulate cell division, primarily by overcoming the intracellular breaking mechanisms that tend to block progression through the cell cycle.

Growth factors - promote cell growth by promoting the synthesis and inhibiting degradation of proteins and other macromolecules.

Question 30

Why do cells need survival factors from other cells?

Answer 30

If deprived of survival factors, the cell activates a caspase-dependent cellular suicide programme. This helps ensure that cells only survive when needed.

E.g. nerve cells are produced in excess in the developing nervous system and compete for limited amounts of survival factors secreted by target cells they contact.