Topic 6: Nucleic Acids and Protein Synthesis

Question 1

Definitions I:
(a) Nucleotide
(b) DNA
(c) mRNA

Answer 1

(a) Nucleotide: Sugar, phosphate, base unit. (1 mark)
(b) DNA: Double helix of nucleotides, hereditary. (1 mark)
(c) mRNA: Single strand carrying gene code. (1 mark)

Question 2

Describe the structure of a nucleotide like ATP in a muscle cell. [2 Marks]

Answer 2

ATP: Ribose sugar, phosphate groups (1 mark); Adenine base, energy carrier (1 mark). (2 marks)

Question 3

State the types of bases in DNA and their structures. [2 Marks]

Answer 3

Purines: Adenine, guanine, double rings (1 mark); Pyrimidines: Cytosine, thymine, single rings (1 mark). (2 marks)

Question 4

Outline the structure of DNA in a human nucleus. [3 Marks]

Answer 4

Double helix, antiparallel strands (1 mark); A-T (2 H-bonds), C-G (3 H-bonds) (1 mark); Phosphodiester links (1 mark). (3 marks)

Question 5

In a liver cell, explain semi-conservative DNA replication during S phase. [4 Marks]

Answer 5

DNA unwinds, polymerase adds 5′ to 3′ (1 mark); Leading continuous, lagging in fragments (1 mark); Ligase joins Okazaki (1 mark); Semi-conservative: one old, one new strand (1 mark). (4 marks)

Question 6

Describe the structure of mRNA in protein synthesis. [2 Marks]

Answer 6

Single strand, ribose, uracil bases (1 mark); Carries codons from DNA (1 mark). (2 marks)

Question 7

State the role of a gene in coding for a muscle protein. [1 Mark]

Answer 7

Gene: Nucleotide sequence coding for protein. (1 mark)

Question 8

Explain the universal genetic code in insulin production. [2 Marks]

Answer 8

Triplets code amino acids universally (1 mark); Start/stop codons control synthesis (1 mark). (2 marks)

Question 9

In a pancreas cell, describe transcription to form mRNA. [3 Marks]

Answer 9

RNA polymerase transcribes template strand (1 mark); Forms primary transcript (1 mark); Nucleus process, mRNA exits (1 mark). (3 marks)

Question 10

Outline translation in the cytoplasm of a skin cell. [3 Marks]

Answer 10

mRNA binds ribosome, tRNA brings amino acids (1 mark); Codons match anticodons (1 mark); Peptide bonds form polypeptide (1 mark). (3 marks)

Question 11

State the roles of transcribed and non-transcribed DNA strands. [2 Marks]

Answer 11

Transcribed (template): Used for mRNA (1 mark); Non-transcribed: Not copied, structural (1 mark). (2 marks)

Question 12

Explain how a primary transcript becomes mRNA in a nerve cell. [2 Marks]

Answer 12

Introns removed, exons spliced (1 mark); Forms mature mRNA in nucleus (1 mark). (2 marks)

Question 13

In haemoglobin, explain how substitution mutations alter the polypeptide. [3 Marks]

Answer 13

Substitution swaps base, e.g., sickle cell (1 mark); May change amino acid (1 mark); Alters protein function (1 mark). (3 marks)

Question 13

Define a gene mutation and its effect on a polypeptide. [2 Marks]

Answer 13

Mutation: Base pair change in DNA (1 mark); May alter polypeptide sequence (1 mark). (2 marks)

Question 14

Analyse how insertion and deletion mutations affect a protein’s structure. [4 Marks]

Answer 14

Insertion adds base, shifts frame (1 mark); Deletion removes, shifts frame (1 mark); Both disrupt sequence (1 mark); Produce faulty protein (1 mark). (4 marks)

Question 15

Describe the roles of tRNA and ribosomes in translation. [3 Marks]

Answer 15

tRNA carries amino acids, anticodons match (1 mark); Ribosomes bind mRNA, link peptides (1 mark); Form polypeptide chain (1 mark). (3 marks)