Cell Division and Cell Cycle MCQ Quiz in मल्याळम - Objective Question with Answer for Cell Division and Cell Cycle - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Key Points

• The cyclin-Cdk complex is responsible for the progression of the cell cycle from one phase to another.
• Cyclin is the regulatory subunit while the cdk is the catalytic subunit with kinase activity.
• Cdks are only functional when they are associated with their respective cyclins.
• Cyclin controls the ability of cdks to phosphorylate specific target proteins.
• Different cyclin-Cdk complexes perform different functions in the cell cycle.
• At different phases of the cell cycle, we see changes in the concentration of different cyclins, this is because cyclins undergo a cycle of synthesis and degradation.
• There are four main classes of cyclins -

1. G1 cyclin (Cyclin D) -
   • The concentration of G1 cyclin increases during the G1 phase where it targets proteins and makes the cell ready for DNA replication.
   • It interacts with Cdk4 and Cdk6 to perform their respective function.
2. G1/S cyclin-
   • Cyclin E is the G1/S cyclin.
   • The concentration of cyclin E increases in the late G1 phase, reaches its peak when the cell enters the S phase (not during the S phase) and declines during the S phase.
   • Cyclin E binds to the Cdk2 and  trigger
   • the transition of the cell from the G1 phase to the S phase.
3. S phase Cycline (cyclin A and cyclin E)-
   • The synthesis of this cyclin starts at the late G1 phase, Unlike cyclin E, the concentration of cyclin A remains high throughout the S phase.
   • This cyclin binds to the Cdk2.
   • Cyclin A /Cdk2 complex is responsible for the replication of DNA. Along wi
   • Cyclin A/ Cdk2 complex, the cyclin E/Cdk2 complex also helps in the replication, even though it is present only during the start of the S-phase.
4. M-cyclin( Cyclin A and cyclin B)-
   • Both bind with the Cdk1.
   • Their synthesis starts during the S-phase and the G2 phase and activity are under regulation until DNA replication is completed.

Explanation:

Statement A - CORRECT

• In cell cycle progression, cyclins undergo a cycle of synthesis and degradation in each cell cycle. 
• On the contrary, the concentration of CDKs remains constant.
• Cyclic changes in the concentration of cyclins results in cyclic assembly and activation of cyclin-CDKs complexes.

Statement B - INCORRECT

• The concentration of CDKs remain constant throughout the cycle, so it does not undergo synthesis and degradation in each cycle.

Statement C - CORRECT

• Activating phosphorylation of CDKs helps in their kinase activity and cell cycle progression.
• On the other hand, inhibitory phosphorylation of CDKs plays a role in the S-phase checkpoint in response to DNA damage during replication.

Statement D - INCORRECT

• Retinoblastoma is inhibitor of G1 to S transition. Rb is tumour suppressor factor and its binds to the E2F transcription factor and regulate E2F activity. 
• E2F is transcription factor that induces the transcription of genes required for cell to progress in S phase. 
• Rb bond to the E2F and inhibit it thereby stopping transition from G1 to S phase. 
• When Rb is phosphorylated, its affinity for E2F decreases and free E2F now can activate transcription of genes required for S-phase transition. 

Statement E - CORRECT

• Sic1 prevents the cell cycle from moving forward from G1 to S phase (DNA synthesis).
• At the end of the G1 phase, Sic1 is rapidly degraded, allowing the cell to start DNA replication.

Hence, the correct answer is option 3.

The correct answer is: (A), (C), (D), (B)

Explanation:

• Option A: G1 cyclin CDK Complex

  • This complex activates at the G1 phase of the cell cycle. It is responsible for pushing the cell into the S phase by phosphorylating target proteins that allow the initiation of DNA replication.

• Option C: S Phase cyclin CDK Complex

  • Once the G1 cyclin CDK complex has initiated the transition into the S phase, the S phase cyclin-CDK complex takes over. It ensures that DNA replication occurs by activating proteins involved in the DNA replication machinery.

• Option D: APC - cdc20

  • The APC (anaphase-promoting complex) associated with cdc20 is involved in promoting the progression into anaphase by targeting specific proteins (such as securin) for degradation. This ensures that the cell progresses properly from metaphase to anaphase during mitosis.

• Option B: APC - cdh1

  • APC-cdh1 complex is active after mitosis, in the late stages of the cell cycle. It targets cyclins and CDKs for degradation, helping to maintain the exit from mitosis and ensuring the cell does not re-enter the cell cycle prematurely.

 Key Points

• The role of cyclin-CDK complexes is critical for regulating the progression of the cell cycle by phosphorylating proteins that control each checkpoint.
• APC-cdh1 and APC-cdc20 are key components that regulate mitosis by controlling the degradation of cyclins and other regulatory proteins.
• The precise timing of cyclin-CDK activity and APC complexes ensures proper cell cycle progression and prevents abnormal cell division.

The correct option is: 3

Explanation:

• Paclitaxel: Paclitaxel, also known as Taxol, is a chemotherapeutic agent that binds to the β-tubulin subunit of microtubules. It stabilizes microtubules by preventing their depolymerization, which disrupts normal microtubule dynamics. This leads to a halt in cell division (mitosis) and eventually cell death, which is why paclitaxel is used in cancer treatment.
• Griseofulvin: Griseofulvin binds to tubulin but acts by disrupting the assembly of microtubules, rather than preventing their depolymerization. It is primarily used as an antifungal agent.
• Colchicine: Colchicine also binds to tubulin, but its action involves preventing microtubule polymerization rather than stabilizing microtubules. It inhibits the formation of microtubules, preventing proper cell division.
• Podophyllotoxin: Podophyllotoxin inhibits microtubule formation by binding to tubulin, but like colchicine, it disrupts the assembly of microtubules rather than stabilizing them.

Key Points

• Microtubule dynamics: Microtubules undergo dynamic instability, where they constantly alternate between polymerization and depolymerization. This process is essential for functions like mitosis and intracellular transport. Paclitaxel's role in stabilizing microtubules makes it particularly effective against rapidly dividing cells, such as cancer cells.
• Chemotherapeutic agents targeting microtubules: Other drugs, like vincristine and vinblastine, target microtubules by binding to tubulin and inhibiting polymerization, but paclitaxel is unique in its ability to prevent depolymerization, offering a distinct mechanism of action.

The correct option is 3.

Explanation:

• During the G2 phase, the cell performs a critical check for DNA damage before proceeding to mitosis. After DNA replication in the S phase, the cell enters G2, where it verifies whether the DNA has been properly replicated and is free of errors or damage. If any issues are detected, the cell activates repair mechanisms to fix the damage before proceeding to mitosis. This checkpoint ensures that the cell does not enter mitosis with faulty DNA, thereby maintaining genomic integrity.
• The G2 checkpoint is a key control mechanism that prevents cells with damaged or incomplete DNA from dividing, which could lead to mutations or cancer.
• G1 phase: The cell checks for favorable conditions to enter the S phase and begins to grow. DNA damage may be checked here as well, but the G2 phase is more critical for post-replication DNA integrity checks.
• S phase: DNA replication occurs, but the cell doesn't perform a full DNA damage check at this stage. Rather, it focuses on duplicating the genome.
• M checkpoint: This checkpoint occurs during mitosis, ensuring that chromosomes are properly aligned and attached to the spindle fibers before they are separated into daughter cells. It does not check for DNA damage before mitosis begins.

The correct option is 2

Explanation:

• Cyclins are regulatory proteins that bind to and activate cyclin-dependent kinases (CDKs). These activated CDKs then phosphorylate target proteins, promoting progression through the various checkpoints of the cell cycle. This binding and activation of CDKs by cyclins are essential for the proper regulation and timing of the cell cycle, ensuring that each phase transitions smoothly to the next (e.g., from G1 to S phase, or from G2 to mitosis).
• The levels of cyclins fluctuate throughout the cell cycle, rising and falling at specific points to coordinate the progression through phases like G1, S, G2, and mitosis.
• Apoptosis is generally triggered by other factors such as p53 (a tumor suppressor protein) in response to DNA damage, not directly by cyclins.
• Cyclins do not directly inhibit CDK activity to prevent the G1 to S transition; rather, they activate CDKs to enable progression. In contrast, CDK inhibitors (such as p21) can inhibit CDK activity to halt progression at certain checkpoints.
• Cyclins do not replicate DNA themselves. DNA replication is carried out by specific enzymes like DNA polymerase during the S phase, although cyclins and CDKs regulate the entry into the S phase.

The correct answer is A, B and C

Explanation:

A. Cdc25 is activated by Cdk1-Cyclin B complex during the G2/M transition: This is correct, as Cdc25 is activated by phosphorylation events from Cdk1-Cyclin B to ensure the cell enters mitosis.

B. Cdc25 removes inhibitory phosphates from Cdk1: This is correct, as the role of Cdc25 is to activate Cdk1 by removing its inhibitory phosphates, promoting mitotic entry.

C. Cdc25 is inactivated by phosphorylation through Chk1/Chk2 in response to DNA damage, preventing premature mitotic entry: This is also correct. In the presence of DNA damage, Cdc25 gets phosphorylated by Chk1/Chk2, which prevents it from activating Cdk1, thereby halting the cell cycle to allow for DNA repair.

D. Cdc25 is degraded by proteolysis during the metaphase-to-anaphase transition to ensure proper cell cycle progression: This is incorrect. Cdc25 is not specifically targeted for degradation during this transition; instead, proteins like securin and cyclin are degraded to promote anaphase progression.

Key Points

• Cdc25 is activated by the Cdk1-Cyclin B complex during the G2/M transition through phosphorylation.
• Cdc25 removes the inhibitory phosphates from Cdk1, allowing for cell cycle progression into mitosis.
• Chk1/Chk2 mediate the inactivation of Cdc25 in response to DNA damage, preventing damaged DNA from being propagated.

The correct answer is: Cell assures complete replication of DNA

Explanation: The G1 checkpoint is primarily responsible for ensuring that the cell has a favorable extracellular environment, checking for DNA damage, and directing any damaged DNA to repair mechanisms. Ensuring complete replication of DNA, however, is a function of the G2/M checkpoint, which checks that the DNA has been fully and accurately replicated before the cell proceeds to mitosis:
Additional Information:

• The G1 checkpoint (also known as the restriction point in mammalian cells) is critical in determining whether a cell proceeds from the G1 phase to the S phase, where DNA replication occurs. It ensures the cell is ready to enter the cell cycle and duplicate its DNA. The G2/M checkpoint, on the other hand, ensures that cells do not enter mitosis (M phase) with any unreplicated or damaged DNA.

The correct answer is Activation of CDK1 requires post-translational modifications.

Concept:

Role of Cyclins and CDKs in Cell Cycle Regulation

• Cyclins are proteins whose levels fluctuate during the cell cycle. They bind to and activate CDKs, which are crucial for driving the cell through different phases of the cell cycle.
• CDK1 (also known as Cdc2 in some organisms) plays a critical role in the transition from the G2 phase to the M phase (mitosis).

Gradual Increase in Cyclin Levels vs. Sudden Activation of CDK1

1. Mitotic Cyclin Accumulation:

   • Mitotic cyclins (such as cyclin B) gradually accumulate during the G2 phase.
   • The accumulation of cyclins alone is not sufficient to activate CDK1 fully.

2. CDK1 Activation Mechanism:

   • Although cyclin binding is necessary for CDK1 activation, CDK1 activity is tightly regulated by a series of post-translational modifications.
   • CDK1 has to undergo specific phosphorylation and dephosphorylation steps to become fully active.

3. Post-Translational Modifications of CDK1:

   • Initially, CDK1 is phosphorylated at inhibitory sites (Thr14 and Tyr15) by enzymes such as Wee1 kinase.
   • For CDK1 to become active, these inhibitory phosphates must be removed by the enzyme Cdc25 phosphatase.
   • Once dephosphorylation occurs, CDK1 is fully activated, leading to a sudden increase in its activity at the onset of the M phase.

Explanation

• Option 1 (Active CDK1 subunit is synthesized in M phase): Incorrect. CDK1 protein is already present before the M phase, but it is not active due to inhibitory phosphorylation.
• Option 2 (Mitotic cyclin is sequestered in the cytosol): Incorrect. Cyclin levels increase gradually and bind to CDK1 during the G2 phase, but cyclin localization is not the primary reason for the sudden activation of CDK1.
• Option 3 (Activation of CDK1 requires post-translational modifications): Correct. The sudden activation of CDK1 at the onset of the M phase is due to the removal of inhibitory phosphates by Cdc25 phosphatase, which is a post-translational modification.
• Option 4 (The inhibitor of CDK1 is degraded in the M phase): Incorrect. While the regulation of CDK1 can involve degradation of inhibitors in some contexts, the primary mechanism of its sudden activation involves dephosphorylation by Cdc25 phosphatase.

Conclusion: The correct answer is Activation of CDK1 requires post-translational modifications. The sudden increase in CDK1 activity at the onset of the M phase is due to the dephosphorylation of inhibitory sites on CDK1, which is a key post-translational modification. This precise regulation ensures that the cell transitions into mitosis at the appropriate time.

The correct answer is Cell assures complete replication of DNA

Concept:

The eukaryotic cell cycle is a highly regulated series of events that cells go through to grow and divide. It consists of:

• G1 phase (Gap 1): The cell grows and prepares the necessary machinery for DNA replication.
• S phase (Synthesis): DNA replication occurs, resulting in two copies of each chromosome.
• G2 phase (Gap 2): The cell prepares for mitosis, making sure DNA replication has been completed successfully.
• M phase (Mitosis): The cell undergoes mitosis, resulting in two daughter cells.

Checkpoints are control mechanisms in the eukaryotic cell cycle which ensure proper division of the cell. The main checkpoints are:

G1 Checkpoint (also known as the Restriction Point in mammals):

• This is the primary decision point for a cell, determining whether it should divide, delay division, or enter a resting state (G0 phase).
• The cell ensures that conditions are favorable for division, such as the availability of nutrients and growth factors (statement 1).
• DNA Damage Check: The cell examines its DNA for any damage. If damage is present, the cell must repair the DNA before proceeding to the S phase (statement 2).
• Repair Mechanisms: If damage is detected, the cell will try to repair it. If the damage is irreparable, the cell may undergo apoptosis to prevent the propagation of damaged DNA (statement 3).

G2 Checkpoint: Ensures that DNA replication in the S phase has been completed correctly and that there is no DNA damage before proceeding to mitosis.
M Checkpoint (Metaphase Checkpoint): Ensures all chromosomes are properly aligned and attached to the mitotic spindle before proceeding with cell division

Explanation:

Statement 4 : "Cell assures complete replication of DNA." This statement is incorrect to the G1 checkpoint for the following reasons:-

• The G1 checkpoint occurs before the S phase, where DNA replication takes place. Therefore, the complete replication of DNA cannot be assured at this point since it hasn't started yet.

Correct Phase and Checkpoint:

• DNA replication is the process that happens during the S phase.
• Ensuring complete and correct replication of DNA is the primary function of the G2 checkpoint, which occurs after the S phase, not the G1 checkpoint.

The correct answer is Option 2 i.e. Only kinases require ATP as the source of phosphate.

Explanation:

• Phosphorylases and kinases are both enzymes that catalyze the addition of phosphate groups to substrates, but they do so using different sources of phosphate.

Phosphorylases are a class of enzymes that add a phosphate group from inorganic phosphate (Pi) to their substrate. This reaction does not require ATP (adenosine triphosphate) as the phosphate source.

• Instead, phosphorylases directly use inorganic phosphate (Pi) to phosphorylate specific substrates. An example of a phosphorylase is glycogen phosphorylase, which acts on glycogen and breaks it down to produce glucose-1-phosphate by incorporating an inorganic phosphate.

Kinases, on the other hand, are enzymes that transfer a phosphate group from ATP to their substrate. In this reaction, ATP serves as the donor of the phosphate group.

• The kinase-mediated phosphorylation process is critical for many cellular processes, including signal transduction, regulation of enzyme activity, and protein function. Because kinases use ATP as the source of the phosphate group, they directly mediate the transfer of energy within cells.
•  For example, cyclin-dependent kinases (CDKs) regulate the cell cycle by phosphorylating key proteins.

Conclusion:

Therefore, the key difference between the two types of enzymes in the context of the provided options is that only kinases require ATP as the source of phosphate for their activity. Phosphorylases do not use ATP; they use inorganic phosphate (Pi) directly.