The Mitotic Cell Cycle: Biology 9700 (Cambridge International AS & A Level)
Syllabus 5.1, 5.2 · Strand 2 Genetics, inheritance and evolution
- Questions
- 10
- Total marks
- 48
- Tier mix
- 10 Core
0 of 10 questions completed
Syllabus coverage
- 5.1 10 questions completed
- 5.2 5 questions completed
When a cell grows and divides, it must pass on a complete and accurate copy of its genetic information, and this topic (syllabus 5.1 and 5.2) explains how that is achieved. It begins with the structure of a chromosome, built from DNA wound around histone proteins, and made of two identical sister chromatids joined at a centromere, with protective telomeres at each end. The mitotic cell cycle is then described as a repeating sequence: a long interphase, during which the cell grows and copies its DNA in the S phase, followed by mitosis, the division of the nucleus, and finally cytokinesis, the division of the cytoplasm into two daughter cells.
Because mitosis produces genetically identical cells, it underlies the growth of multicellular organisms, the replacement of damaged or dead cells, tissue repair and asexual reproduction. You should be able to describe the behaviour of chromosomes through prophase, metaphase, anaphase and telophase, and identify these stages in photomicrographs and slides. The topic also introduces telomeres as structures that prevent the loss of genes during repeated replication, stem cells as a source of new cells for repair, and the way that a loss of control over cell division can lead to a tumour.
The exam-style questions below are original, written to match these objectives, each with a full worked solution so you can check your reasoning step by step.
Question 1
A cell in an onion root tip meristem completes one full mitotic cell cycle, passing through interphase, mitosis and cytokinesis.
Which statement correctly describes what happens during the S phase of interphase?
Question 2
By the time a cell enters mitosis, each of its chromosomes has already been copied during the S phase of the preceding interphase.
(a) State what is meant by the term sister chromatids, and name the structure that joins the two sister chromatids of a chromosome together. [2]
(b) Explain why DNA replication during S phase is essential if mitosis is to produce two daughter cells that are genetically identical to the parent cell. [3]
(c) Telomeres are regions of repetitive, non-coding DNA found at each end of a chromosome. Explain the role of telomeres in protecting a cell's genes during repeated rounds of DNA replication. [3]
Question 3
The broad bean, Vicia faba, has a diploid chromosome number of 12. A root tip cell from a broad bean seedling completes interphase and then enters mitosis, passing through prophase, metaphase, anaphase and telophase before the cytoplasm divides.
(a) Describe the behaviour of the chromosomes during prophase. [2]
(b) Describe the behaviour of the chromosomes during metaphase. [2]
(c) Describe what happens to the centromeres and to the sister chromatids of each chromosome during anaphase. [2]
(d) State two events, other than the movement of chromosomes, that occur during telophase. [2]
Question 4
Skin cells at the edge of a healing wound divide repeatedly to replace the tissue that was lost.
Which statement correctly explains why mitosis, rather than meiosis, is the appropriate type of cell division for repairing this tissue?
Question 5
Cell cycle checkpoints are points in the cell cycle at which the cycle can be halted if a problem is detected, before the cell is allowed to continue to the next stage. Researchers compared normal skin cells with cells taken from a skin tumour.
(a) State what is meant by a cell cycle checkpoint, and name one point in the cell cycle at which a checkpoint occurs. [2]
(b) Explain how the loss of normal checkpoint control in a cell can lead to the formation of a tumour. [3]
(c) The researchers found that the tumour cells had high levels of telomerase activity, while the normal skin cells had very low telomerase activity. Explain how this difference in telomerase activity contributes to the tumour cells being able to divide many more times than normal skin cells. [2]
Question 6
A biologist views a stained chromosome from a human cell under an electron microscope.
Which statement correctly describes how the DNA of a chromosome is organised?
Question 7
Stem cells in the bone marrow divide repeatedly by mitosis throughout a person's life. Some of the daughter cells remain in the bone marrow as stem cells, while others go on to differentiate into new red blood cells, replacing red blood cells that have worn out.
(a) State what is meant by the term stem cell. [2]
(b) Explain, in terms of the chromosomes present at the start of mitosis and their behaviour during the division, why the two daughter cells produced when a bone marrow stem cell divides are genetically identical to each other and to the parent stem cell. [3]
(c) Suggest why it is important that the new red blood cells produced in this way are genetically identical to the person's other body cells, rather than being genetically varied. [2]
Question 8
A student prepared a stained root tip squash from an onion (Allium cepa) and examined it under a light microscope. In one field of view, the student counted 250 cells in total. Of these, 20 cells had visibly condensed chromosomes, and the remaining cells showed no visible individual chromosomes.
(a) Define the term mitotic index. [1]
(b) Calculate the mitotic index for this field of view, showing your working. [2]
(c) The 230 cells with no visible individual chromosomes could be in the G1, S or G2 phase of interphase. Explain why a cell in any of these three phases looks the same under a light microscope. [2]
(d) State two visible features that would allow the student to identify a cell as being in metaphase, rather than a cell in interphase. [2]
Question 9
Genes that normally help to control the cell cycle can, following mutation, contribute to the uncontrolled cell division seen in tumours. Two categories of such genes are proto-oncogenes (which can become oncogenes) and tumour-suppressor genes.
Which statement correctly distinguishes the typical effect of a mutation in each type of gene?
Question 10
Two genes involved in the normal control of the cell cycle are RAS, a proto-oncogene whose product normally helps to relay a signal that stimulates a cell to divide, and p53, a tumour-suppressor gene whose product normally halts the cell cycle at a checkpoint if DNA damage is detected. Mutations affecting either gene are commonly found in tumour cells.
(a) Explain, in terms of the normal function of RAS, how a mutation converting this proto-oncogene into an oncogene can promote uncontrolled mitosis. [3]
(b) Explain, in terms of the normal function of p53, how a mutation that inactivates this tumour-suppressor gene can allow a cell to continue dividing by mitosis when it should not. [3]
(c) State one similarity between the overall effect that a RAS oncogene mutation and a p53 mutation each have on the control of the cell cycle. [1]