Cell cycle control

Question 1

What are the main phases of the cell cycle?

Answer 1

G1 phase (cellular growth and preparation for DNA replication), S phase (DNA replication), G2 phase (further cell growth and preparation for mitosis), and M phase (mitosis, where cell division occurs into two daughter cells).

Question 2

What is MPF and what is its role in the cell cycle?

Answer 2

MPF (Mitosis Promoting Factor) is a key regulatory molecule that drives cells from the G2 phase into the M phase (mitosis). MPF consists of cyclin and CDK; cyclin binds to and activates CDK, forming the active MPF complex, which triggers events leading to cell division.

Question 2

How is the transition between cell cycle phases controlled?

Answer 2

Transition between cell cycle phases is controlled by checkpoints and regulatory molecules, such as the restriction point (G1-S transition) influenced by growth factors, the G2-M transition ensuring accurate DNA replication, and the metaphase-anaphase transition ensuring proper chromosome alignment and attachment to the spindle apparatus.

Question 3

How is cyclin concentration controlled during the cell cycle?

Answer 3

Cyclin concentration is controlled through regulated synthesis and degradation. Cyclin genes are transcribed and translated in response to cell cycle signals, and cyclin degradation is mediated by the ubiquitin-proteasome system, where cyclins are tagged with ubiquitin and broken down by the proteasome.

Question 4

What are feedback regulatory circuits in the cell cycle?

Answer 4

Feedback regulatory circuits ensure the oscillations of cyclins, involving positive feedback (activation of cyclin-CDK complexes promoting further cyclin synthesis) and negative feedback (activation of pathways leading to cyclin degradation), coordinating the progression of the cell cycle.

Question 4

Explain the role of growth factors in cell cycle progression.

Answer 4

Growth factors are extracellular signaling proteins that regulate cell proliferation by activating receptors with tyrosine kinase activity. This activation triggers the Ras pathway, leading to MAPK activation, which enters the nucleus and activates transcription factors to promote the synthesis of cyclins and CDKs, facilitating the transition from G1 to S phase.

Question 5

What are CdK inhibitors and their function?

Answer 5

CdK inhibitors are proteins that regulate the activity of cyclin-dependent kinases (Cdks). The Cip/Kip family (e.g., p21, p27, p57) inhibits Cdk4/Cyclin D, Cdk6/Cyclin D, and Cdk2/Cyclin E complexes, while the INK4 family (e.g., p15, p16, p18, p19) inhibits Cdk4/Cyclin D or Cdk6/Cyclin D complexes, preventing premature cell cycle progression.

Question 6

What is the role of the Rb tumor suppressor in the cell cycle?

Answer 6

The Rb (retinoblastoma) protein regulates the cell cycle by acting as a gatekeeper for the G1-S transition. It binds to E2F transcription factors, preventing them from activating DNA replication genes. When phosphorylated by CDKs, Rb releases E2F, allowing the transcription of S-phase genes. Mutations in the Rb gene can lead to uncontrolled cell proliferation.

Question 7

Describe the function of the p53 tumor suppressor.

Answer 7

The p53 protein maintains genomic stability by preventing the propagation of damaged DNA. It can induce cell cycle arrest to allow for DNA repair or trigger apoptosis if the damage is irreparable. p53 activates p21 to halt the cell cycle and Bax to promote apoptosis. Mutations in p53 are common in cancers, leading to the survival and proliferation of damaged cells.

Question 8

What are some common assays used to detect apoptosis?

Answer 8

Common apoptosis assays include the DNA-ladder assay (detecting DNA fragmentation), TUNEL assay (labeling fragmented DNA), caspase assay (measuring caspase activity), annexin V assay (detecting phosphatidylserine translocation), and propidium iodide assay (distinguishing between apoptotic and necrotic cells).

Question 9

What is the role of receptor tyrosine kinases (RTKs) in cellular signal transduction?

Answer 9

RTKs are membrane receptors that, upon ligand binding, dimerize and autophosphorylate, activating intracellular signaling cascades.

Question 10

How is the cell cycle controlled?

Answer 10

By checkpoints and regulatory molecules. Eg: G1-S transition-restriction point influenced by growth factors. Checkpoints prevent unregulated growth and ensure genomic stability.
MPF is a key regulatory molecule involved in the control of the
cell cycle. It plays a critical role in driving cells from the G2 phase
into the M phase (mitosis) of the cell cycle.

Question 10

Describe the function of each checkpoint that regulates the transition between cell cylce phases:

Answer 10

G1-S Transition: Cells commit to DNA replication if conditions are favorable.
G2-M Transition: Ensures that DNA has been accurately replicated and the cell is ready for division.
Metaphase-Anaphase Transition: Ensures proper chromosome alignment and attachment to the spindle apparatus before segregation. CDKs, Cyclins, CKIs, Checkpoint proteins, Ubiquitin proteasome system.

Question 11

How do the MPF levels change during embryonic cell division?

Answer 11

Before fertilization, the oocyte is typically arrested in the G2 phase with low MPF levels. After fertilization, MPF levels increase to promote the progression of meiosis I.
In meiosis II MPF levels may again play a role in regulating progression. In metaphase arrest if conditions are not favorable for further division MPF levels may decrease. During embryonic mitosis MPF levels rise again to drive progression through mitosis, promoting cell division and embryonic growth. Progesterone can influence MPF activation, and upon fertilization, MPF activity increases to facilitate the resumption (επαναληψη) of the cell cycle.

Question 12

How are oscillations of cyclin generated?

Answer 12

Feedback regulatory circuits generate oscillations of cyclins. These circuits ensure that cyclin levels rise and fall in a controlled manner, coordinating the progression of the cell cycle. The regulation involves:
- Positive Feedback: Activation of cyclin-CDK complexes that promote further cyclin synthesis.
- Negative Feedback: Activation of pathways that lead to cyclin degradation.

Question 13

How do CdK/Cyclin complexes cause breakdown of nuclear envelope necessary for chromosome segregation?

Answer 13

CDK1-Cyclin B, induce the breakdown of the nuclear envelope during mitosis by phosphorylating proteins associated with the envelope such as the nuclear pore complex (NPC) proteins and lamins. This phosphorylation weakens interactions within the nuclear envelope, leading to disassembly of nuclear pore complexes and depolymerization of nuclear lamins. Additionally, CDK-Cyclin complexes activate factors that facilitate structural changes, ultimately allowing for the mixing of nuclear and cytoplasmic contents necessary for chromosome segregation.

Question 14

How is the transition from G1 to S triggered by CdK/Cyclin complexes?

Answer 14

By the activation of CDK4/6-Cyclin D and CDK2-Cyclin E complexes. These complexes phosphorylate the retinoblastoma protein (Rb), leading to the release of E2F transcription factors. E2F then promotes the transcription of genes necessary for DNA replication, driving the cell from G1 into S phase. If conditions are not favorable, cells may enter a dormant state (G0).

Question 14

What do CdK-Cyclins do during M-phase?

Answer 14

During the M-phase (mitosis) of the cell cycle, CDK-Cyclin complexes promote chromosome condensation, ensuring proper organization of genetic material. CDK-Cyclins also orchestrate spindle assembly, facilitating the segregation (separation) of chromosomes to daughter cells. By regulating the mitotic checkpoint, they ensure accurate chromosome segregation. Additionally, CDK-Cyclins contribute to cytokinesis, the division of the cytoplasm, by aiding in the assembly and contraction of the contractile ring. Overall, CDK-Cyclin complexes drive the precise and coordinated events of mitosis, ensuring the faithful transmission of genetic material to daughter cells.

Question 15

How do CdK/Cyclin complexes cause breakdown of nuclear envelope necessary for chromosome segregation?

Answer 15

CDK1-Cyclin B, induce the breakdown of the nuclear envelope during mitosis by phosphorylating proteins associated with the envelope such as the nuclear pore complex (NPC) proteins and lamins. This phosphorylation weakens interactions within the nuclear envelope, leading to disassembly of nuclear pore complexes and depolymerization of nuclear lamins. Additionally, CDK-Cyclin complexes activate factors that facilitate structural changes, ultimately allowing for the mixing of nuclear and cytoplasmic contents necessary for chromosome segregation.

Question 16

How do growth factors control the cell cycle?

Answer 16

How do growth factors control the cell cycle? Growth factors activate specific receptors on the cell surface, which possess tyrosine kinase activity. This activation initiates a cascade of biochemical events within the cell, leading to cell division. Growth factors such as PDGF and EGF target receptors with tyrosine kinase activity. Activation of these receptors triggers the Ras pathway which leads to the activation of MAP kinases. MAPKs enter the nucleus and activate transcription factors like Jun and Ets. This results in the synthesis of cyclins and Cdks. Cdk-cyclin complexes phosphorylate the Rb protein, releasing E2F transcription factors and promoting the transition from G1 to S phase.

Question 17

How is an accidental transition over the restriction point prevented?

Answer 17

Dependency on growth factorr signaling, controlled activation of the MAPK pathway, regulated expression and activity of cyclins and CDKs, maintenance of Rb’s inhibitory role, checkpoint proteins, and CDK inhibitors, work together to ensure that cells only transition over the restriction point under favorable growth conditions. This tightly regulated system prevents accidental or inappropriate cell cycle progression, which is crucial for maintaining cellular and organismal homeostasis.

Question 18

What is the double safeguard mechanism?

Answer 18

The double safeguard mechanism ensures the precise control of CdK activity through consecutive phosphorylation-dephosphorylation events. - CdKs are activated by phosphorylation at specific sites. - Dephosphorylation at other sites ensures that CdKs are only active when appropriate. - This dual control mechanism prevents accidental activation and ensures proper cell cycle regulation.

Question 19

How to make sure that DNA is replicated only once per cell cycle?

Answer 19

Licencing factor model. By licensing replication origins in early G1 and then preventing this licensing from happening again through phosphorylation and degradation of CDC6 and Cdt1, the cell ensures that each replication origin is used only once per cell cycle. This precise control mechanism prevents the DNA from being replicated more than once, which is crucial for maintaining the stability of the genome.

Question 20

How can tumour suppressors regulate the cell cycle (Rb)?

Answer 20

The retinoblastoma (Rb) protein is a crucial tumor suppressor that regulates the cell cycle. It acts as a gatekeeper for the transition from the G1 phase to the S phase. The Rb protein binds to E2F transcription factors, preventing them from activating genes required for DNA replication. When the cell is ready to progress to the S phase, cyclin-dependent kinases (CDKs) phosphorylate Rb, causing it to release E2F, which then activates the transcription of S-phase genes. Mutations in the Rb gene can lead to uncontrolled cell proliferation, as the cell cycle checkpoint is bypassed. This is a common feature in many cancers, highlighting the importance of Rb in maintaining cellular integrity.

Question 21

How can tumour suppressors regulate the cell cycle (p53)?

Answer 21

Upon detecting DNA damage, p53 can induce cell cycle arrest, allowing time for DNA repair, or trigger apoptosis if the damage is irreparable. The p53 protein (guardian of the genome) is another critical tumor suppressor that plays a key role in maintaining genomic stability.

Question 22

How is apoptosis regulated by p53?

Answer 22

The tumour suppressor protein p53 plays a crucial role in regulating apoptosis, particularly in response to DNA damage and other cellular stress signals. While p53 is well-known for its role in transcription-dependent apoptosis, it can also induce apoptosis through transcription-independent mechanisms. In transcription-independent apoptosis, p53 translocates to the mitochondria in response to stress signals. p53 interacts with pro-apoptotic proteins (e.g., Bax, Bak) and anti-apoptotic proteins (e.g., Bcl-2, Bcl-XL) at the mitochondria. This interaction promotes the release of cytochrome c from the mitochondria into the cytosol. Cytochrome c binds to Apaf-1 and procaspase-9, forming the apoptosome. The apoptosome activates caspase-9, which in turn activates executioner caspases (e.g., caspase-3), leading to apoptosis.