Gene expression and cancer

Question 1

Describe how tumours and cancers form

Answer 1

● Mutations in DNA / genes controlling mitosis can lead to
uncontrolled cell division
● Tumour formed if this results in mass of abnormal cells
○ Malignant tumour = cancerous, can spread by metastasis
○ Benign tumour = non-cancerous

Question 2

Compare the main characteristics of benign and malignant tumours

Answer 2

Benign tumours
- Usually grow slowly (cells divide less often)
- Cells are well differentiated / specialised
- Cells have normal, regular nuclei
- Well defined borders and often surrounded by a
capsule so do not invade surrounding tissue
- Do not spread by metastasis (as cell adhesion molecules stick cells together)
- Can normally be removed by surgery and they rarely return

Malignant tumours
- Usually grow faster (cells divide more often)
- Cells become poorly differentiated / unspecialised
- Cells have irregular, larger / darker nuclei
- Poorly defined borders and not encapsulated so can invade surrounding tissues (growing projections)
- Spread by metastasis - cells break off and spread to
other parts of the body, forming secondary tumours
(due to lack of adhesion molecules)
- Can normally be removed by surgery combined with
radiotherapy / chemotherapy but they often return

Question 3

Describe the function of tumour suppressor genes

Answer 3

Code for proteins that:
● Inhibit / slow cell cycle (eg. if DNA damage detected)
● OR cause self-destruction (apoptosis) of potential
tumour cells (eg. if damaged DNA can’t be repaired)

Question 4

Explain the role of tumour suppressor genes in the development of tumours

Answer 4

● Mutation in DNA base sequence → production of non-functional protein
○ By leading to change in amino acid sequence which changes protein tertiary structure
● Decreased histone acetylation OR increased DNA methylation → prevents production of protein
○ By preventing binding of RNA polymerase to promoter region, inhibiting transcription
● Both lead to uncontrolled cell division (cell division cannot be slowed)

Question 5

Describe the function of proto-oncogenes

Answer 5

Code for proteins that stimulate cell division
(eg. through involvement in signalling pathways
that control cell responses to growth factors)

Question 6

Explain the role of oncogenes in the development of tumours

Answer 6

An oncogene is a mutated / abnormally expressed form of the corresponding proto-oncogene.
● Mutation in DNA base sequence → overproduction of protein OR permanently activated protein
○ By leading to change in amino acid sequence which changes protein tertiary structure
● Decreased DNA methylation OR increased histone acetylation → increases production of protein
○ By stimulating binding of RNA polymerase to promoter region, stimulating transcription
● Both lead to uncontrolled cell division (cell division is permanently stimulated)

Question 7

Suggest why tumours require mutations in both alleles of a tumour
suppressor gene but only one allele of an oncogene

Answer 7

● One functional allele of a tumour suppressor gene can produce enough protein to slow the cell cycle
OR cause self-destruction of potential tumour cells → cell division is controlled
● One mutated oncogene allele can produce enough protein to lead to rapid / uncontrolled cell division

Question 8

Explain the relevance of epigenetics in cancer treatment

Answer 8

Drugs could reverse epigenetic changes that caused cancer, preventing uncontrolled cell division. For example:
● Increasing DNA methylation OR decreasing histone acetylation of oncogene
○ To inhibit transcription / expression
● Decreasing DNA methylation OR increasing histone acetylation of tumour suppressor gene
○ To stimulate transcription / expression

Question 9

Explain the role of increased oestrogen concentrations in the development
of some (oestrogen receptor-positive) breast cancers

Answer 9

1. Some breast cancers cells have oestrogen receptors, which are inactive transcription factors
2. If oestrogen concentration is increased, more oestrogen binds to oestrogen receptors,
   forming more oestrogen-receptor complexes which are active transcription factors
3. These bind to promoter regions of genes that code for proteins stimulating cell division
4. This increases transcription / expression of these genes, increasing the rate of cell division

Question 10

Suggest how drugs that have a similar structure to oestrogen help treat
oestrogen receptor-positive breast cancers

Answer 10

● Drugs bind to oestrogen receptors (inactive transcription factors), preventing binding of oestrogen
● So no / fewer transcription factors bind to promoter regions of genes that stimulate the cell cycle