Cell cycle
- The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication.
- These events can be divided in two brief periods: interphase during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA and the mitotic (M) phase, during which the cell splits itself into two distinct cells, often called “daughter cells”.
- The cell-division cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed.
Phases of the cell cycle
- The cell cycle consists of four distinct phases: G1 phase, S phase, G2 phase (collectively known as interphase) and M phase.
- M phase is itself composed of two tightly coupled processes: mitosis, in which the cell’s chromosomes are divided between the two daughter cells, and cytokinesis, in which the cell’s cytoplasm divides forming distinct cells.
- Activation of each phase is dependent on the proper progression and completion of the previous one.
- Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase.
M phase
- The relatively brief M phase consists of nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).
- In plants and algae, cytokinesis is accompanied by the formation of a new cell wall.
- The M phase has been broken down into several distinct phases, sequentially known as prophase, Prometaphase, metaphase, anaphase and telophase leading to cytokinesis.
Interphase
- After M phase, the daughter cells each begin interphase of a new cycle.
- Although the various stages of interphase are not usually morphologically distinguishable, each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell division.
G1 phase
- The first phase within interphase, from the end of the previous M phase till the beginning of DNA synthesis is called G1 (G indicating gap or growth).
- During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate.
- This phase is marked by synthesis of various enzymes that are required in S phase, mainly those needed for DNA replication.
- Duration of G1 is highly variable, even among different cells of the same species.
S phase
- The S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids.
- Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same.
- Rates of RNA transcription and protein synthesis are very low during this phase.
- An exception to this is histone production, most of which occurs during the S phase.
- The duration of S phase is relatively constant among cells of the same species.
G2 phase
- The cell then enters the G2 phase, which lasts until the cell enters mitosis.
- Again, significant protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis.
- Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis.
G0 phase
- The term “post-mitotic” is sometimes used to refer to both quiescent and senescent cells.
- Non-proliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons).
- This is very common for cells that are fully differentiated.
- Cellular senescence is a state that occurs in response to DNA damage or degradation that would make a cell’s progeny nonviable; it is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis.
Cell Cycle and Apoptosis – MCQs
1. Which of the following correctly represents the sequence of phases in the cell cycle?
A. G1 → G2 → S → M
B. G0 → G1 → M → S
C. G1 → S → G2 → M
D. M → G1 → G2 → S
Answer: C. G1 → S → G2 → M
2. What occurs during the S phase of the cell cycle?
A. Cell undergoes mitosis
B. DNA replication
C. Chromosomes line up at the equator
D. Microtubule formation
Answer: B. DNA replication
3. During which phase are microtubules synthesized for use in mitosis?
A. G1 phase
B. S phase
C. G2 phase
D. M phase
Answer: C. G2 phase
4. What is the name of the state when a cell temporarily stops dividing?
A. S phase
B. Apoptosis
C. G0 phase
D. Telophase
Answer: C. G0 phase
5. In which phase does cytokinesis occur?
A. Interphase
B. Prophase
C. S phase
D. M phase
Answer: D. M phase
6. Which of the following is NOT true about the G1 phase?
A. It follows the M phase
B. It involves high biosynthetic activity
C. It includes chromosome replication
D. Enzymes for DNA synthesis are produced
Answer: C. It includes chromosome replication
7. Which of the following phases are collectively known as interphase?
A. G1, G2, and M
B. G1, S, and G2
C. S, M, and G0
D. G0, S, and M
Answer: B. G1, S, and G2
8. Which process is inhibited if protein synthesis is blocked during the G2 phase?
A. DNA replication
B. Cell wall formation
C. Mitosis
D. Chromatin condensation
Answer: C. Mitosis
9. In plant cells, cytokinesis is usually accompanied by:
A. Breakdown of the nuclear envelope
B. Cell wall formation
C. Apoptosis
D. Protein degradation
Answer: B. Cell wall formation
10. What is apoptosis?
A. Uncontrolled cell division
B. Natural cell death
C. DNA replication error
D. Cell growth phase
Answer: B. Natural cell death
11. Which of the following best describes the ploidy of a cell after S phase?
A. Haploid, DNA content doubled
B. Diploid, DNA content doubled
C. Haploid, DNA content unchanged
D. Diploid, DNA content halved
Answer: B. Diploid, DNA content doubled
12. During which phase of mitosis do chromosomes align at the equator of the cell?
A. Prophase
B. Anaphase
C. Metaphase
D. Telophase
Answer: C. Metaphase
13. The production of which proteins is particularly high during S phase?
A. Tubulin
B. Histones
C. Cyclins
D. Actin
Answer: B. Histones
14. The process of karyokinesis occurs during which part of the cell cycle?
A. G2 phase
B. M phase
C. S phase
D. G1 phase
Answer: B. M phase
15. What is the main characteristic of a senescent cell?
A. Undergoes rapid division
B. Prepares for apoptosis
C. Permanently exits the cell cycle due to DNA damage
D. Continues RNA transcription
Answer: C. Permanently exits the cell cycle due to DNA damage
16. Which of the following phases shows the lowest rate of RNA transcription and protein synthesis?
A. G1 phase
B. G2 phase
C. S phase
D. M phase
Answer: C. S phase
17. What happens to the chromosome number of a cell after the M phase?
A. It becomes haploid
B. It doubles
C. It remains the same
D. It becomes zero
Answer: C. It remains the same
18. Which checkpoint ensures DNA replication is complete before the cell enters mitosis?
A. G1 checkpoint
B. S phase checkpoint
C. G2 checkpoint
D. M checkpoint
Answer: C. G2 checkpoint
19. What is the fate of most fully differentiated cells in multicellular organisms?
A. Apoptosis
B. Repeated mitosis
C. Entry into G0 phase
D. Entry into M phase
Answer: C. Entry into G0 phase
20. Which event marks the end of mitosis?
A. Disappearance of the nuclear membrane
B. Separation of chromatids
C. Reformation of nuclear envelopes around chromosomes
D. Alignment of chromosomes at equator
Answer: C. Reformation of nuclear envelopes around chromosomes
21. The transition from G1 phase to S phase is tightly regulated by:
A. Cyclin D and CDK4
B. Cyclin A and CDK2
C. Cyclin B and CDK1
D. Cyclin E and CDK1
Answer: A. Cyclin D and CDK4
22. Apoptosis is most likely to occur in a cell when:
A. It enters G0 phase
B. It fails the cell cycle checkpoint due to DNA damage
C. It finishes mitosis
D. Protein synthesis is high
Answer: B. It fails the cell cycle checkpoint due to DNA damage
23. Which of the following is not a characteristic feature of apoptosis?
A. Cell shrinkage
B. Inflammatory response
C. DNA fragmentation
D. Membrane blebbing
Answer: B. Inflammatory response
24. Caspases involved in apoptosis are primarily:
A. Lipases
B. Kinases
C. Proteases
D. Ligases
Answer: C. Proteases
25. Which of the following is the correct order of mitotic phases?
A. Prophase → Telophase → Metaphase → Anaphase
B. Prophase → Prometaphase → Metaphase → Anaphase → Telophase
C. Prometaphase → Prophase → Anaphase → Telophase
D. Telophase → Anaphase → Metaphase → Prometaphase → Prophase
Answer: B. Prophase → Prometaphase → Metaphase → Anaphase → Telophase
26. What prevents a cell from entering M phase if DNA is damaged?
A. Spindle assembly checkpoint
B. G2/M checkpoint
C. G1/S checkpoint
D. S phase checkpoint
Answer: B. G2/M checkpoint
27. Which of the following proteins are involved in regulating apoptosis?
A. Tubulin and Actin
B. Bcl-2 and Bax
C. DNA Polymerase and Ligase
D. Cyclin D and Cyclin E
Answer: B. Bcl-2 and Bax
28. A neuron is considered a post-mitotic cell because:
A. It rapidly divides
B. It stays permanently in G0 phase
C. It completes the cell cycle in 12 hours
D. It undergoes frequent apoptosis
Answer: B. It stays permanently in G0 phase
29. If a cell is stuck in metaphase and cannot proceed to anaphase, which checkpoint might be active?
A. DNA damage checkpoint
B. G0 checkpoint
C. Spindle assembly checkpoint
D. G1 checkpoint
Answer: C. Spindle assembly checkpoint
30. Which term refers to programmed cell death that avoids inflammation and protects surrounding cells?
A. Necrosis
B. Apoptosis
C. Lysis
D. Senescence
Answer: B. Apoptosis
31. Which checkpoint is responsible for ensuring that all chromosomes are properly attached to the spindle before anaphase begins?
A. G1/S checkpoint
B. S phase checkpoint
C. Spindle assembly checkpoint
D. G2/M checkpoint
Answer: C. Spindle assembly checkpoint
32. Which of the following organelles plays a key role in initiating intrinsic apoptosis?
A. Nucleus
B. Mitochondria
C. Ribosome
D. Golgi apparatus
Answer: B. Mitochondria
33. Which event occurs last in mitosis before cytokinesis begins?
A. Alignment of chromosomes
B. Breakdown of nuclear envelope
C. Chromatid separation
D. Reformation of nuclear envelopes
Answer: D. Reformation of nuclear envelopes
34. The enzyme responsible for DNA replication in S phase is:
A. DNA ligase
B. DNA polymerase
C. RNA polymerase
D. Reverse transcriptase
Answer: B. DNA polymerase
35. In apoptosis, chromatin undergoes:
A. Condensation and fragmentation
B. Duplication
C. Expansion
D. Histone degradation
Answer: A. Condensation and fragmentation
36. Which of the following is true regarding cytokinesis in plant cells?
A. It involves cleavage furrow formation
B. It results in multinucleated cells
C. It involves formation of a cell plate
D. It does not occur in mitosis
Answer: C. It involves formation of a cell plate
37. A key feature distinguishing apoptosis from necrosis is:
A. DNA damage
B. Energy consumption
C. Inflammation
D. Cell swelling
Answer: B. Energy consumption
(Apoptosis is energy-dependent; necrosis is passive and often causes inflammation)
38. The G0 phase is most commonly observed in:
A. Rapidly dividing cells
B. Stem cells
C. Fully differentiated cells
D. Cancer cells
Answer: C. Fully differentiated cells
39. What happens if a cell fails to repair DNA damage and cannot proceed through the cell cycle?
A. It undergoes mitosis anyway
B. It exits to G0
C. It undergoes apoptosis
D. It fuses with another cell
Answer: C. It undergoes apoptosis
40. Which of the following is an executioner caspase in apoptosis?
A. Caspase-2
B. Caspase-3
C. Caspase-8
D. Caspase-9
Answer: B. Caspase-3
41. Which phase of the cell cycle is most variable in duration among different cells?
A. G1 phase
B. S phase
C. G2 phase
D. M phase
Answer: A. G1 phase
42. Which protein family regulates the progression through various phases of the cell cycle?
A. Histones
B. Cyclins
C. Ribosomal proteins
D. Chaperones
Answer: B. Cyclins
43. Which of the following triggers the onset of anaphase during mitosis?
A. Cohesin formation
B. Chromatin condensation
C. Cyclin B degradation
D. Spindle fiber breakdown
Answer: C. Cyclin B degradation
44. Which of the following statements about S phase is incorrect?
A. DNA replication occurs
B. Histone synthesis is high
C. RNA transcription is high
D. Sister chromatids are formed
Answer: C. RNA transcription is high
45. Which process is associated with membrane blebbing, DNA fragmentation, and cell shrinkage?
A. Mitosis
B. Meiosis
C. Apoptosis
D. Necrosis
Answer: C. Apoptosis
46. The cell cycle is arrested at G1/S checkpoint primarily due to:
A. Incomplete DNA replication
B. Damaged DNA
C. Improper spindle formation
D. Lack of histone synthesis
Answer: B. Damaged DNA
47. Which of the following checkpoints ensures that mitosis does not proceed until chromosomes are properly aligned?
A. G1 checkpoint
B. G2 checkpoint
C. S checkpoint
D. Metaphase checkpoint
Answer: D. Metaphase checkpoint
(also called Spindle Assembly Checkpoint)
48. Apoptotic bodies are formed during:
A. Necrosis
B. Mitosis
C. Apoptosis
D. Cytokinesis
Answer: C. Apoptosis
49. What is the main role of the p53 protein in the cell cycle?
A. Promotes S phase entry
B. Inhibits mitosis
C. DNA replication
D. Halts the cycle for DNA repair
Answer: D. Halts the cycle for DNA repair
50. Which of the following correctly pairs the cyclin-CDK complex with its phase?
A. Cyclin A/CDK1 – G1
B. Cyclin E/CDK2 – G2
C. Cyclin D/CDK4 – G1
D. Cyclin B/CDK2 – S
Answer: C. Cyclin D/CDK4 – G1