Genes & Genetic code

Question 1

Describe one advantage an done disadvantage of genetic screening a foetus (2)

Answer 1

advantage:
- prevents child dying late in pregnancy

disadvantage:
- high cost

Question 2

Explain why people with a gene mutation cannot convert phenylalanine to tyrosine (4)

Answer 2

• change in DNA sequence
• change in primary structure
• different R group
• phenylalanine doesn’t fit in enzyme’s active site

Question 3

Explain why a gene mutation involving the replacement of one base with another has less effect than the loss of a base (2)

Answer 3

• loss caused the whole amino acid sequence to change
• replacement only changes one amino acid

Question 4

Explain one ethical issue relating to the use of prenatal genetic screening (2)

Answer 4

• may result in a choice of an abortion
• unethical to cause death of a foetus

Question 5

Define: recessive allele (3)

Answer 5

• different form of a gene
• both alleles need to be present in order for a recessive phenotype to be expressed

Question 6

Explain how deletion and substitution have different effects in protein structure (4)

Answer 6

• deletion could affect every codon
• deletion more likely to affect position of start codon
• substitution may code for same amino acid
• due to degenerate nature of genetic code

Question 7

Explain how a frame-shift mutation could result in the production of lepton with a variety of shorter primary structures (2)

Answer 7

• changes in the triplet code
• coding for a shorter sequence of amino acids

Question 8

Define: allele

Answer 8

Alternative forms of the same gene

Question 9

The mutated CFTR protein has one missing amino acid compared to a functioning one. Explain how this results in a non-functioning protein (4)

Answer 9

• a mutation causes a different sequence of R groups
• which changed the position of bonds between the R groups
• this causes a different tertiary structure
• which affects the movement of chloride ions through the cell membrane

Question 10

Explain why people with cystic fibrosis may develop lung infections (4)

Answer 10

• CF produces thick, sticky mucus
• because the cells transport less water
• therefore the cilia lining are unable to move mucus out of airways
• resulting in microorganisms getting trapped and causing lung infections

Question 11

Explain why plasma membranes are described as having a fluid mosaic structure (2)

Answer 11

• the membrane is fluid because molecules can move within the membrane by diffusion
• the membrane looks mosaic if viewed from above due to both proteins and phospholipids arranged in a scattered pattern