III. Signal transduction and cell cycle | 49. Regulation of the cell cycle in the G1 phase, transition to S phase

Question 1

I. Cell cycle
1. What are the main features of cell cycle?

Answer 1

• The eukaryotic cell cycle consists of 4 distinct phases: G1, S, G2 (collectively known as interphase) and M phase (mitosis and cytokinesis).
• The duration of the cycle is about 24 hours, out of which mitosis takes about 1 hour.

Question 2

I. Cell cycle
1. What are the main features of cell cycle?

Answer 2

• The eukaryotic cell cycle consists of 4 distinct phases: G1, S, G2 (collectively known as interphase) and M phase (mitosis and cytokinesis).
• The duration of the cycle is about 24 hours, out of which mitosis takes about 1 hour.

Question 3

I. Cell cycle
3. What happen in S phase?

Answer 3

DNA synthesis – doubling of the 23 pairs of chromosomes

Question 4

I. Cell cycle
4. What happen in G2 phase?

Answer 4

G2 phase: restricting mitosis until DNA is completely doubled and the cell is big enough

Question 5

I. Cell cycle
5. What happen in M phase?

Answer 5

M phase: consists of two important events – (1) mitosis = cell nucleus divides (2) cytokinesis = cytoplasmic division, forming two daughter cells

Question 6

I. Cell cycle
6. What happen in G0 phase?

Answer 6

• G0 phase: resting phase – no cell division occurs here.
• From this phase, the cell can enter the cycle again or it can withdraw from it

Question 7

II. Checkpoints
1. What is the role of cell cycle checkpoints?

Answer 7

• Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle.
• Checkpoints prevent the replication of damaged DNA and premature entry (or exit) from mitosis, and allow time for DNA repair if DNA damage occurs.

Question 8

II. Checkpoints
2. What does cell cycle checkpoints consist of?

Answer 8

Checkpoints typically consist of a network of regulatory proteins that monitor and dictate the progression of the cell through the different stages.
1) In G1 phase: restriction point
2) G1/S transition
3) G2/M transition
4) In M phase: mitotic spindle checkpoint

Question 9

II. Checkpoints
3. What happen in In G1 phase: restriction point?

Answer 9

Cell decides whether to continue cell cycle or not. ‘’Is the mitogenic stimulus strong enough?’’

Question 10

II. Checkpoints
4. What happen in In G1/S transition?

Answer 10

Cell checks if the environment is favorable and whether it has sufficient nutrients before starting DNA replication. ‘’Am I ready for replication?’’

Question 11

II. Checkpoints
5. What happen in in G2/M transition?

Answer 11

Cells completed and if it is big enough to enter mitosis

Question 12

II. Checkpoints
6. What happen in in M phase: mitotic spindle checkpoint?

Answer 12

• Occurs between metaphase and anaphase.
• Cell checks if the mitotic spindle has formed (microtubules attached to kinetochores) and all chromosomes are properly attached to it before it segregates them into 2 daughter cells in anaphase

Question 13

III. Cell-cycle control system - Cyclins and Cdks
1. What is the role of Cell-cycle control system?

Answer 13

The cell-cycle control system regulates the cell-cycle by activating and inactivating
proteins that initiate/regulate DNA replication, mitosis and cytokinesis – carried out
by phosphorylation and dephosphorylation of these proteins

Question 14

III. Cell-cycle control system - Cyclins and Cdks
2. How do Cyclins and Cdks work?

Answer 14

• Different protein kinases of the cell cycle are activated at different times of the cell-
  cycle by proteins called cyclins -> therefore, these kinases are known as cyclin dependent protein kinases (Cdks)
• Cyclins bind to Cdks and activate them only when they are present during their stage of the cell-cycle, and are absent in other cell-cycle stagesdifferent Cdk-cyclin complexes (see table) will trigger different stages of the cell-cycle

Question 15

III. Cell-cycle control system - Determinants of Cdk activity
3. How is Cdk activity activated?

Answer 15

Activation:
- When cyclin binds to Cdk, it acts as an activating cofactor which also influences the specificity of the enzyme
- The phosphorylation of a threonine residue on Cdk is also required for Cdk activity. This phosphorylation is mediated by CAK (Cdk- activating kinase)

Question 16

III. Cell-cycle control system - Determinants of Cdk activity
3. How is Cdk activity inhibited?

Answer 16

Inhibition:
- Binding of CKIs (Cdk inhibitor proteins) will inactive the Cdk-cyclin complexes
- There will also be inhibitory phosphorylations of a
tyrosine and a threonine residue on Cdk.
- The Wee1 kinase mediates this phosphorylation, while Cdc25 will be the phosphatase that dephosphorylates Cdk -> removing the inhibitory phosphates
=> INHIBITION IS ALWAYS DOMINANT

Question 17

III. Cell-cycle control system
4. What are the most important inhibitors of cyclin-dependent kinases (CKIs)?

Answer 17

Question 18

IV. Main steps of the cell cycle and their regulation
1. What are the 8 steps of the cell cycle in the G1 phase, transition to S phase?

Answer 18

1. Growth factor (mitogen) binds to its receptor
2. Signaling pathways are set into motion (MAP kinase cascade)
3. G1 cyclins and Cdks get induced, the cell grows
4. G1 Cdk-cyclin complexes reach a critical point, where they overcome CKIs (restriction point). They lead to induction of proteins needed for replication and of S cyclins
5. G1 Cdks deactivate an S phase CKI, thus SPF (S-phase promoting factor) is released from inhibition (the phosphorylated CKI gets ubiquitinated and degraded)
6. SPF activates pre-replication complexes (already attached to DNA)
7. DNA starts doubling
8. MPF (M-phase promoting factor) remains inactive until the completion of replication and cell growth

Question 19

IV. Main steps of the cell cycle and their regulation
2. What happen in step 1 “Growth factor (mitogen) binds to its receptor”

Answer 19

• Mitogens are EC signals stimulating growth and multiplication of cells
• If it is not present, the cell will arrest in G1 and eventually enter the G0 phase (can
  stay there for a lifetime)

Question 20

IV. Main steps of the cell cycle and their regulation
2. What happen in “step 3. G1 cyclins and Cdks get induced, the cell grows”?

Answer 20

• Cdks, cyclin D, E2F↑
• CKIs (Cdk inhibitor proteins )↓

Question 21

IV. Main steps of the cell cycle and their regulation
3A. What happen in “step 4. G1 Cdk-cyclin complexes reach a critical point”?

Answer 21

G1 Cdk-cyclin complexes reach a critical point, where they overcome CKIs (restriction
point). They lead to induction of proteins needed for replication and of S cyclins
- G1 Cdk (4,6)-cyclin D complex activates the E2F transcription factors
- During G1 phase, E2F is held inactive through binding of the tumor suppressor gene pRb (retinoblastoma protein)
- Once G1 Cdk (4,6) phosphorylates pRb, it will release E2F and DP (TFs), allowing them to induce S cyclins for the S phase
- Genes transcribed by E2F encode for:
+) Enzymes of nucleotide and DNA synthesis, proteins of replication: ORC, MCM 2-7, Cdc6, Cdt1, DNA polymerase α, PCNA, thymidine kinase, thymidylate synthase
+) Proteins of further progress of the cycle: Cdk1, Cyclin A + E, Cdc25
+) Transcription factors: B-Myb, c-myc, E2F

Question 22

IV. Main steps of the cell cycle and their regulation
3B. What do Genes transcribed by E2F encode for?

Answer 22

1. Enzymes of nucleotide and DNA synthesis, proteins of replication: ORC, MCM 2-7, Cdc6, Cdt1, DNA polymerase α, PCNA, thymidine kinase, thymidylate synthase
2. Proteins of further progress of the cycle: Cdk1, Cyclin A + E, Cdc25
3. Transcription factors: B-Myb, c-myc, E2F

Question 23

IV. Main steps of the cell cycle and their regulation
4. What happen in step 5. G1 Cdks deactivate an S phase CKI?

Answer 23

5/ G1 Cdks deactivate an S phase CKI, thus SPF (S-phase promoting factor) is released from inhibition (the phosphorylated CKI gets ubiquitinated and degraded)
- the G1/S Cdk (2)-cyclin E complex can phosphorylate the CKI (p27) of the S Cdk (2)- cyclin A complex, causing p27 to be ubiquitinated and eventually degraded
- End up with the active S Cdk (2)-cyclin A complex = S-phase promoting factor (SPF)