III. Signal transduction and cell cycle | 49. Regulation of the cell cycle in the G1 phase, transition to S phase Flashcards
I. Cell cycle
1. What are the main features of cell cycle?
- 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.
I. Cell cycle
1. What are the main features of cell cycle?
- 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.
I. Cell cycle
3. What happen in S phase?
DNA synthesis – doubling of the 23 pairs of chromosomes
I. Cell cycle
4. What happen in G2 phase?
G2 phase: restricting mitosis until DNA is completely doubled and the cell is big enough
I. Cell cycle
5. What happen in M phase?
M phase: consists of two important events – (1) mitosis = cell nucleus divides (2) cytokinesis = cytoplasmic division, forming two daughter cells
I. Cell cycle
6. What happen in G0 phase?
- 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
II. Checkpoints
1. What is the role of cell cycle checkpoints?
- 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.
II. Checkpoints
2. What does cell cycle checkpoints consist of?
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
II. Checkpoints
3. What happen in In G1 phase: restriction point?
Cell decides whether to continue cell cycle or not. ‘’Is the mitogenic stimulus strong enough?’’
II. Checkpoints
4. What happen in In G1/S transition?
Cell checks if the environment is favorable and whether it has sufficient nutrients before starting DNA replication. ‘’Am I ready for replication?’’
II. Checkpoints
5. What happen in in G2/M transition?
Cells completed and if it is big enough to enter mitosis
II. Checkpoints
6. What happen in in M phase: mitotic spindle checkpoint?
- 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
III. Cell-cycle control system - Cyclins and Cdks
1. What is the role of Cell-cycle control system?
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
III. Cell-cycle control system - Cyclins and Cdks
2. How do Cyclins and Cdks work?
- 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 stagesdifferent Cdk-cyclin complexes (see table) will trigger different stages of the cell-cycle
III. Cell-cycle control system - Determinants of Cdk activity
3. How is Cdk activity activated?
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)
III. Cell-cycle control system - Determinants of Cdk activity
3. How is Cdk activity inhibited?
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
III. Cell-cycle control system
4. What are the most important inhibitors of cyclin-dependent kinases (CKIs)?
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?
- Growth factor (mitogen) binds to its receptor
- Signaling pathways are set into motion (MAP kinase cascade)
- G1 cyclins and Cdks get induced, the cell grows
- 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 Cdks deactivate an S phase CKI, thus SPF (S-phase promoting factor) is released from inhibition (the phosphorylated CKI gets ubiquitinated and degraded)
- SPF activates pre-replication complexes (already attached to DNA)
- DNA starts doubling
- MPF (M-phase promoting factor) remains inactive until the completion of replication and cell growth
IV. Main steps of the cell cycle and their regulation
2. What happen in step 1 “Growth factor (mitogen) binds to its receptor”
- 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)
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”?
- Cdks, cyclin D, E2F↑
- CKIs (Cdk inhibitor proteins )↓
IV. Main steps of the cell cycle and their regulation
3A. What happen in “step 4. G1 Cdk-cyclin complexes reach a critical point”?
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
IV. Main steps of the cell cycle and their regulation
3B. What do 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
IV. Main steps of the cell cycle and their regulation
4. What happen in step 5. G1 Cdks deactivate an S phase CKI?
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)
