5.03 Cell Cycle

Question 1

Define the cell cycle and outline its main phases. (5 marks)

Mark Scheme:

1. The cell cycle is the sequence of events from one cell division to the next. (1 mark)
2. Interphase: The phase where the cell grows, performs normal functions, and prepares for division. (1 mark)
3. Mitosis (M phase): Nuclear division, creating two genetically identical nuclei. (1 mark)
4. Cytokinesis: Division of the cytoplasm, forming two daughter cells. (1 mark)
5. The cycle ensures growth, repair, and genetic continuity in cells. (1 mark)

Question 2

Describe the events that occur during interphase of the cell cycle. (6 marks)

Mark Scheme:

1. G₁ Phase (Gap 1): Cell grows, produces RNA, enzymes, and proteins needed for growth. (1 mark)
2. Decision point: The cell decides whether to continue division or enter G₀. (1 mark)
3. S Phase (Synthesis): DNA replication occurs, producing two identical chromatids for each chromosome. (1 mark)
4. G₂ Phase (Gap 2): Additional growth occurs; DNA is checked for errors and repaired. (1 mark)
5. Tubulin production increases to form microtubules for the mitotic spindle. (1 mark)
6. Interphase prepares the cell for mitosis while performing normal cellular functions. (1 mark)

Question 3

What is the significance of the G₀ phase in the cell cycle? (5 marks)

Mark Scheme:

1. The G₀ phase is a resting or non-dividing phase where cells exit the cycle after G₁. (1 mark)
2. Cells in G₀ continue to perform their normal functions but do not prepare for division. (1 mark)
3. Examples: Neurons and muscle cells remain in G₀ permanently, while liver cells can re-enter the cycle if needed. (1 mark)
4. This phase prevents unnecessary division, conserves energy, and allows cells to specialize. (1 mark)
5. It acts as a checkpoint, ensuring cells only divide when conditions are favourable. (1 mark)

Question 4

Explain the role of checkpoints in the cell cycle. (6 marks)

Mark Scheme:

1. G₁ Checkpoint: Occurs at the end of G₁; checks cell size, nutrients, growth signals, and DNA integrity. (1 mark)
2. If conditions are unfavourable, the cell enters G₀. (1 mark)
3. G₂ Checkpoint: Occurs at the end of G₂; checks DNA replication accuracy and cell readiness for mitosis. (1 mark)
4. Repairs DNA or triggers apoptosis if damage is irreparable. (1 mark)
5. Metaphase (Spindle) Checkpoint: Ensures chromosomes are correctly attached to spindle fibers before anaphase. (1 mark)
6. Checkpoints maintain genomic stability and prevent cell cycle progression with errors. (1 mark)

Question 5

Describe the process of mitosis and its importance in the cell cycle. (6 marks)

Mark Scheme:

1. Mitosis is nuclear division that produces two genetically identical nuclei. (1 mark)
2. Ensures each daughter cell receives the same chromosome number and type as the parent cell. (1 mark)
3. Stages:
   • Prophase: Chromatin condenses into visible chromosomes; spindle fibers form. (1 mark)
   • Metaphase: Chromosomes align at the cell’s equator. (1 mark)
   • Anaphase: Sister chromatids separate and move to opposite poles. (1 mark)
   • Telophase: Nuclear envelopes reform around each set of chromosomes. (1 mark)

Question 6

How do plant and animal cells differ during cytokinesis? (4 marks)

Mark Scheme:

1. In animal cells, the cytoplasm constricts to split the cell, forming a cleavage furrow. (1 mark)
2. This furrow deepens until the cell divides into two daughter cells. (1 mark)
3. In plant cells, a new cell wall forms between the daughter nuclei. (1 mark)
4. Vesicles from the Golgi apparatus fuse to form the cell plate, which becomes the cell wall. (1 mark)

Question 7

What is the significance of DNA replication during the S phase of interphase? (5 marks)

Mark Scheme:

1. During the S phase, DNA is replicated to ensure each chromosome has two identical chromatids. (1 mark)
2. This guarantees that each daughter cell receives a complete set of genetic material during mitosis. (1 mark)
3. Accurate replication is crucial for maintaining genetic continuity and function. (1 mark)
4. Errors during replication can lead to mutations, genomic instability, or cell death. (1 mark)
5. The S phase prepares the cell for mitosis, ensuring it is ready for division. (1 mark)

Question 8

Explain the role of tubulin in the cell cycle. (4 marks)

Mark Scheme:

1. Tubulin is a protein produced during the G₂ phase of interphase. (1 mark)
2. It is used to form microtubules, which make up the mitotic spindle. (1 mark)
3. The spindle is essential for aligning chromosomes during metaphase and separating sister chromatids during anaphase. (1 mark)
4. Proper tubulin production ensures accurate chromosome segregation and successful mitosis. (1 mark)

Question 9

Why do some cells have a shorter cell cycle duration than others? (5 marks)

Mark Scheme:

1. The duration of the cell cycle depends on cell type and environmental conditions. (1 mark)
2. Cells with high turnover rates, such as intestinal epithelial cells, have shorter cycles (~10 hours). (1 mark)
3. Root tip cells in plants divide approximately every 20 hours to support growth. (1 mark)
4. Cells in the G₀ phase, such as neurons, have no division and therefore a very long or indefinite cycle duration. (1 mark)
5. The cell cycle length reflects the cell’s function, division need, and external factors like nutrient availability. (1 mark)

Question 10

Describe the appearance of a cell under the microscope during interphase and explain its significance. (5 marks)

Mark Scheme:

• The nuclear membrane appears intact, and the nucleoli are visible as dark-stained spots. (1 mark)
• Chromatin is spread out and appears as a light or finely granular network. (1 mark)
• No visible chromosomes, as DNA is uncondensed. (1 mark)
• Interphase accounts for ~90% of the cell cycle, during which the cell grows, performs normal functions, and prepares for division. (1 mark)
• This preparation ensures the cell is ready for DNA replication and mitosis. (1 mark)