Section 8 - Unit 20: Gene Expression

Question 1

Oestrogen is a hormone that affects transcription. It forms a complex with a receptor in the cytoplasm of target cells. Explain how an activated oestrogen receptor affects the target cell (2 marks)

Answer 1

• Receptor / transcription factor binds to promoter which stimulates RNA
  polymerase
• Transcribes gene / increase transcription

Question 2

Oestrogen only affects target cells. Explain why oestrogen does not affect other cells in the body (1 mark)

Answer 2

• Other cells do not have the oestrogen receptors

Question 3

Give two characteristic features of stem cells (2 marks)

Answer 3

• Will keep dividing

- Undifferentiated

Question 4

Explain how the methylation of tumour suppressor genes can lead to cancer (3 marks)

Answer 4

• Methylation prevents transcription of gene
• Protein not produced that prevents cell division
• No control of mitosis

Question 5

Describe how altered DNA may lead to cancer (6 marks)

Answer 5

• (DNA altered by) mutation
• (mutation) changes base sequence
• of gene controlling cell growth / oncogene / that monitors cell division
• of tumour suppressor gene
• change protein structure / non-functional protein / protein not formed
• (tumour suppressor genes) produce proteins that inhibit cell division
• mitosis
• uncontrolled / rapid / abnormal (cell division)
• malignant tumour

Question 6

Explain how examining mRNA enables scientists to discover whether cancer is present (3 marks)

Answer 6

• mRNA base sequence has changed
• gene / DNA structure is different / has mutated
• cancer gene active / tumour suppressor gene inactive

Question 7

What are totipotent cells, where are they found and how are they totipotent? (3)

Answer 7

• Can divide and produce any type of body cell
• During development only part of DNA is translated so cell eventually becomes specialised
• Found in the very early stages of embryos

Question 8

Pluripotent cells - what are they and what are they used for (2)

Answer 8

• Can divide in unlimited numbers into almost any type

- Used in treating human disorders however, they can sometimes divide rapidly to form tumours

Question 9

Multipotent and unipotent cells - what are they and where are they found (2)

Answer 9

• Found in mature mammals

- Divide to form a limited number of different cell types

Question 10

Induced pluripotent cells (2)

Answer 10

• Produced from adult somatic cells

- Made into pluripotent cells by switching off transcription factors which make the cell specialised

Question 11

Define epigenetics (2)

Answer 11

• Heritable changes in gene function

- Without changes to the base sequence of DNA

Question 12

State two changes which inhibit transcription (2)

Answer 12

• Increased methylation of DNA

- Decreased acetylation of associated histones

Question 13

Explain how increased methylation of DNA inhibits transcription (3 marks)

Answer 13

• Methyl groups (+ve charge) bind to DNA (-ve charge)
• Causing chromatin to be more condensed
• So transcription factors cannot bind to the DNA

Question 14

Explain how decreased acetylation inhibits transcription (4 marks)

Answer 14

• Acetyl groups (-ve charge) are removed from histones, increasing the +ve charge on the histones
• This increases attraction to phosphate groups on DNA
• Causing chromatin to be more condensed
• So transcription factors cannot bind to the DNA

Question 15

Explain why stem cells are effective in replacing faulty blood cells (3 marks)

Answer 15

• Healthy blood cells produced
• Not faulty cells
• Stem cells divide and replicate

Question 16

Explain how an inhibitor of DNA methyltransferase (an enzyme which adds methyl to cytosine bases on tumour suppressor genes) stop tumour formation (3 marks

Answer 16

• Decreased methylation
• DNA histone complex less tightly wound
• So tumour suppressor genes transcribed
• And mitosis controlled

Question 17

The mRNA for a gene is called the sense strand. An antisense strand is added and the transcription of this gene is stopped. Explain how (4 marks)

Answer 17

• Antisense mRNA is complementary to sense mRNA
• Antisense mRNA would bind to sense mRNA
• Ribosomes can’t bind
• So less translation of mRNA

Question 18

Explain how cells become specialised [3 marks]

Answer 18

• Only some of the genes are expressed
• So only part of the DNA is translated into proteins
• The proteins translated are the ones required to carry out the special function

Question 19

Define self-renewal [1 mark]

Answer 19

• The process in which stem cells make copies of themselves through mitosis

Question 20

Explain how hormones stimulating transcription work [5 marks]

Answer 20

• They are lipid soluble so can diffuse through the phospholipid bilayer
• In the cytoplasm, they bind to a complementary receptor protein
• This changes the tertiary structure of the protein, causing it to become activated as a transcription factor
• This can now enter the nucleus through the nuclear pores and bind to the promoter region of DNA
• Stimulating RNA polymerase to carry out transcription

Question 21

What is the epigenome [1 mark]

Answer 21

• Set of chemical modifications to the DNA and DNA-associated proteins in the cell, which alter gene expression, and are heritable

Question 22

State two factors which cause genetic variation [2 marks]

Answer 22

• Crossing over

- Random fertilisation