Jan 2020

Question 1

1.3. Tetracycline is an antibiotic that attaches to ribosomes. Suggest how tetracycline stops bacteria from dividing. (3)

Answer 1

(1) Ribosomes identified as site of protein synthesis
(2) tRNA not able to bind (to ribosomes) so prevents translation/production of proteins
(3) No/less enzymes so unable to catalyse reactions for synthesis of materials needed for division OR No/less (structural) proteins so prevents formation of new cell membrane/wall

Question 2

4.2. Describe the mechanisms that cause air to move into the lungs. (3)

Answer 2

(1) Contraction of diaphragm (muscles) and flattens the diaphragm
(2) Contraction of (external) intercostal muscles and raises ribcage
(3) Volume of the lungs/thoracic cavity increases, and the pressure decreases

Question 3

6.1. What is meant by the term tertiary structure? (1)

Answer 3

(Further) folding/coiling of a (secondary structure) protein/polypeptide due to interactions between R-groups

Question 4

6.4. Scientists investigate the effect of disulfide bonds on the structural strength of a human cardiac muscle protein called titin.
The final length of the stretched titin molecules without the disulfide bonds was longer than the titin with disulfide bonds. Why? (2)

Answer 4

(1) Disulfide bonds too strong (to break with this method)

(2) (Titin molecules) not fully stretched to (primary/secondary structure) / disulfide bonds hold molecule together / prevents/reduces stretching

Question 5

6.5. Scientists investigate the effect of disulfide bonds on the structural strength of a human cardiac muscle protein called titin.
Describe briefly 4 ways the scientists could’ve ensured valid and accurate results. (4)

Answer 5

any 4 of:
(1) Use a large number of samples/titin molecules for each group
(2) Keep pH constant/use a buffer
(3) Keep temperature constant /use a water bath
(4) Apply the same force/apply force for same length of time
(5) Same number of disulfide bonds
(6) Measure results to same level of precision
(7) Use titin molecules from same source/same species /same section of the heart
(8) Same size/thickness of titin

Question 6

7.1. Explain why cells need to undergo 2 divisions in meiosis to produce sperm and egg cells. (3)

Answer 6

(1) First division separates homologous chromosomes, so each daughter cell receives one chromosome from each pair

(2) Second division separates chromatids so produces haploid
(daughter) cells

(3) At fertilisation the diploid number is restored / a constant number of chromosomes is maintained across generations

Question 7

7.4. Explain how the process of crossing over results in genetic variation. (4)

Answer 7

(1) Homologous pairs of chromosomes associate / form a bivalent
(2) Chiasma(ta) form
(3) (Equal) lengths of (non-sister) chromatids / alleles are exchanged
(4) Producing new combinations of alleles

Question 8

8.3. When they give you a table of organisms and their amino acid sequence for a protein, and they tell you: “Any conclusions made based on the table ONLY isn’t valid. Explain why” What would you answer? (3)

Answer 8

any 3 of:
(1) The code is degenerate/more than one triplet for each amino acid
(2) Data only shows part of the amino acid sequence
(3) Data only shows the amino acid sequence for one protein
(4) Differences may be found within introns/ non-coding regions