6. Nucleic Acids & Protein Synthesis

Question 1

4 structural features of DNA that make it a stable molecule

Answer 1

1. complementary base pairing holds strands together
2. because of MANY hydrogen bonds
3. sugar-phosphate backbone with covalent phosphodiester bonds
4. double helix structure protects bases

Question 2

Suggest why a mutation in which 1 nucleotide of a triplet code is altered often makes no difference to the protein molecule coded by the DNA. [2m]

Answer 2

• many amino acids have more than 1 triplet code

- so sequence of amino acids is unchanged

Question 3

Suggest why the addition or deletion of 3 nucleotides in the DNA sequence of a gene often has less effect on the encoded protein than the addition or deletion of a single nucleotide. [4m]

Answer 3

• adding/deleting 3 nucleotides may add or remove the coding for 1 amino acid
• this may not affect the final shape of the protein
• adding/deleting 1 nucleotide affects the arrangement of all subsequent triplets
• this “frame shift” may alter the coding of all amino acids following the addition/deletion
• a triplet may be altered to a stop signal

Question 4

The mechanisms of antibiotic resistance involve proteins.
eg. enzymes to break down antibiotics
Explain why antibiotic resistance arises as a result of mutation. [2m]

Answer 4

• mutation involves change in nucleotide sequence of DNA
• different protein structures synthesised
• so range of functions possible

Question 5

More mRNA molecules than tRNA molecules are synthesised in cells.
Suggest a reason for this. [1m]

Answer 5

• mRNA less stable compared to tRNA (b/c mRNA has no hydrogen bonds)
• tRNA can be reused for a longer time

Question 6

Describe the role of ribosomes in protein synthesis. [3m]

Answer 6

• involved in translation process
• attaches and moves along mRNA
• provides 2 binding sites for tRNAs carrying amino acid to bind to mRNA
• binding between anticodon on tRNA and codon on mRNA
• catalyse formation of peptide bond to form polypeptide by enzyme peptidyl transferase
• allows assembly of amino acids into sequence / primary structure

Question 7

Gene

Answer 7

A sequence of nucleotides that forms part of a DNA molecule

- a polypeptide is coded for by a gene

Question 8

Explain how a substitution mutation could result in a change in the amino acid sequence of a polypeptide. [2m]

Answer 8

• diff. mRNA codon formed during transcription
• diff. tRNA with diff. amino acid binds to ribosome
• diff. amino acid added to growing polypeptide chain

Question 9

3 reasons why parts of tRNA are double stranded

Answer 9

1. COMPLEMENTARY base pairing between different parts of a single stranded tRNA
2. contribute to stability of tRNA
3. allow tRNA to have shape that fits to the sites at ribosome