1.10 Chromosomes, mitosis and the cell cycle
7 exam questions worth 16 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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How many chromosomes are there in a human body cell?
Choose one answer.
Correct Not quite: the answer is 46
Mark scheme
B (46)
Model answer
B: 46. Body cells have 23 pairs of chromosomes, which is 46.
Describe what happens to the DNA before a cell divides by mitosis.
Mark scheme
- the DNA is copied / replicated (1)
- so there are two copies of each chromosome (1)
Model answer
The DNA is copied, so that there are two copies of each chromosome.
State two reasons why mitosis is important in a multicellular organism.
Mark scheme
Any two from:
- growth / development (1)
- repair of damaged tissue (1)
- replacement of worn out cells (1)
- asexual reproduction (1)
Model answer
Mitosis is needed for growth and for repairing damaged tissue.
Describe the three stages of the cell cycle.
Mark scheme
- the cell grows and the DNA is copied / more sub-cellular structures are made (1)
- mitosis: one set of chromosomes is pulled to each end of the cell and the nucleus divides (1)
- the cytoplasm and cell membrane divide to make two identical cells (1)
Model answer
First the cell grows, makes more sub-cellular structures and copies its DNA. In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides. Finally the cytoplasm and cell membrane divide, forming two identical cells.
A cell divides by mitosis five times.
Calculate the number of cells there are at the end. Start with one cell.
Mark scheme
- 25 (1)
- = 32 (1)
- allow 32 for 2 marks with no working
Model answer
Each division doubles the number of cells, so after five divisions there are 25 = 32 cells.
Explain why the two daughter cells formed by mitosis are genetically identical to the parent cell.
Mark scheme
- the DNA is copied exactly / replicated before the cell divides (1)
- each daughter cell receives one full copy / the same set of chromosomes (1)
Model answer
The DNA is copied exactly before the cell divides, and each daughter cell receives one complete set of chromosomes, so they have the same genes as the parent cell.
A human skin cell has 46 chromosomes. It divides by mitosis.
State the number of chromosomes in each daughter cell, and explain how this number is produced.
Mark scheme
- 46 (1)
- the DNA is copied before mitosis (1)
- so there are two copies of each chromosome (1)
- one set of chromosomes is pulled to each end of the cell / each daughter cell gets one complete set (1)
Model answer
Each daughter cell has 46 chromosomes. Before mitosis the DNA is copied, so there are two copies of each chromosome. During mitosis one set of chromosomes is pulled to each end of the cell, so each daughter cell receives a full set of 46.
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