15. Control of gene expression

Question 1

What effect would a substitution mutation in an intron have on the protein produced?

Answer 1

-Intron non coding/ only exons coding
-So not translated/no change in mRNA produced/ no effect on protein/ amino acids produced
OR
-Prevents changes/ splicing
-So faulty mRNA formed
-Get different amino acid sequence

Question 2

What is a transcription factor?

Answer 2

• A protein that moves from the cytoplasm to the DNA
• Binds to a specific gene/ promoter/ RNA polymerase
• Leads to/ blocks(pre)mRNA production/ blocks binding of RNA polymerase (to DNA)/ allows RNA polymerase to work

Question 3

CREB leads to the formation of a protein that removes electrons and protons from reduced NAD in the mitochondrion.
Huntington’s disease (HD) causes the death of neurones. People with HD produce a substance called huntingtin. Some scientists have suggested that binding of huntingtin to CREB may lead to the death of neurones.

Suggest how binding of huntingtin to CREB may lead to the death of neurones

Answer 3

• Binding to CREB prevents transcription/ mRNA formation
• Binding of huntingtin prevents production/ translation of protein (that removes electrons and protons from NAD)
• Fewer electrons to electron transport chain/ ETC stops
• Fewer protons for proton gradient
• Not enough ATP produced/ energy supplied to keep cells alive/ anaerobic respiration not enough to keep cells alive.

Question 4

The scientists used the polymerase chain reaction (PCR) to produce copies of the
cDNA (complimentary DNA). They added a DNA probe for allele A to the cDNA copies. This DNA probe
had a dye attached to it. This dye glows with a green light only when the DNA probe
is attached to its target cDNA.

Explain why this DNA probe will only detect allele A

Answer 4

• Probe complimentary to DNA of allele A
• Binds by forming base pairs/ hydrogen bonds
• So only this DNA labelled/ has green dye/ gives out green light

Question 5

How can cells be specialised?

Answer 5

They only have certain genes expressed which make them adapted for their function

• Some genes permanently expressed (eg those for respiration)
• Some genes permanently switched off/ not expressed
• Some genes switched on and off as they are needed

Question 6

What is a totipotent cell?

Answer 6

A cell that can mature into any body cell

Question 7

What are the two ways in which a gene can be prevented from being expressed?

Answer 7

• Preventing transcription (therefore preventing the production of mRNA)
• Breaking down mRNA before its genetic code can be translated

Question 8

What are the totipotent cells that exist in mature animals called?

Answer 8

Adult stem cells

Question 9

What are stem cells and where are (adult) stem cells found?

Answer 9

-They are differentiated dividing cells and need to constantly be replaced
-Found in:
>Inner lining of small intestine
>Skin
>Bone marrow

Question 10

How can plants be cloned using in vitro development?

Answer 10

-Most plant cells are totipotent so taking a cell, for example, from a carrot root, under the right conditions and chemical stimulation a whole new carrot plant can be grown that is a clone of the first plant.

Question 11

Describe how oestrogen can switch on a gene.

Answer 11

• Oestrogen diffuses across the phospholipid bilayer as it is lipid soluble
• Once in the cytoplasm it combines with a receptor site on the transcription factor because they are complementary shapes to one another
• Oestrogen binding changes the shape of the receptor molecule
• This causes the inhibitor on the DNA binding site of the transcriptional factor to be released
• Transcription factor enters the nucleus via a nuclear pore and combines with the DNA
• Combination of the T.F with DNA stimulates transcription to occur

Question 12

How does small interfering RNA affect gene expression? (Start with the formation of siRNA)

Answer 12

• Enzyme cuts large double stranded molecules of RNA into smaller ones (siRNA)
• One of the strands of the siRNA combines with an enzyme
• siRNA complementary base pairs to an mRNA molecule
• Enzyme cuts mRNA molecule into smaller sections
• This means the mRNA can no longer be translated into a polypeptide
• Gene is not expressed

Question 13

Give two potential scientific and medical uses of siRNA

Answer 13

• Use siRNA to block genes that cause genetic diseases to prevent the disease
• Used to identify the role of genes in a biological pathway- add siRNA to cells to block a certain gene and the effects, or lack of, could determine the role of the gene

Question 14

(2 marks)
Papaya plants reproduce sexually by means of seeds. Papaya plants grown from
seeds are very variable in their yield. Explain why.

Answer 14

1. Meiosis;
2. Independent assortment / crossing over;
3. (Fusion of) genetically different gametes / random
   fertilisation;

Question 15

(2 marks)

Describe and explain how expression of the target gene is affected by siRNA.

Answer 15

1. No longer able to make specific protein / cannot make
   whole protein / mRNA cannot be translated;
2. Because mRNA has been cut into pieces;

Notes:
1. Reference to transcribes negates this point.
2. Do not accept mRNA destroyed / do not accept
gene not expressed.
Reference to target gene broken down negates this
point.