Nucleic Acids and Protein Synthesis: Question 5

Worked solution

Part (a): Writing the mRNA sequence

Transcription produces an mRNA strand that is complementary and antiparallel to the template strand, with uracil taking the place of thymine. Pairing each base of the template (read 3’ to 5’) with its complementary RNA base:

So the mRNA reads, from its 5’ end to its 3’ end:

5’-U A C G C G A A A C C U-3’

The mRNA’s 5’ end pairs with the template’s 3’ end (and the mRNA’s 3’ end pairs with the template’s 5’ end), because the newly made mRNA strand and the DNA template strand it was copied from always run antiparallel to one another.

Part (b): The role of RNA polymerase

RNA polymerase catalyses transcription. It binds to the DNA at the start of the gene and moves along the exposed template strand in the 3’ to 5’ direction. As it moves, it selects free RNA nucleotides that have base-paired, by complementary base pairing, with the template (with uracil pairing opposite adenine, rather than thymine). RNA polymerase then catalyses the formation of phosphodiester bonds between the sugar of one RNA nucleotide and the phosphate of the next, joining them together into a continuous mRNA strand that grows in the 5’ to 3’ direction. Once the gene has been fully copied, the mRNA strand separates from the DNA and RNA polymerase detaches.

Part (c): Translation at the ribosome

The mRNA produced in transcription travels to a ribosome, which reads it in successive, non-overlapping codons. For each codon, a tRNA molecule whose anticodon is complementary and antiparallel to that codon binds to the mRNA at the ribosome, held in place by hydrogen bonding between the codon and anticodon bases. Each tRNA carries a specific amino acid, determined by its anticodon, attached at its other end. As successive tRNAs deliver their amino acids at the ribosome, a peptide bond forms between each new amino acid and the growing chain, and the ribosome moves along the mRNA to the next codon. Repeating this process codon by codon builds up an ordered chain of amino acids, a polypeptide, with the amino acid sequence determined entirely by the order of codons in the mRNA.

Part (d): Effect of the substitution mutation

The mutation changes the second triplet on the template strand from CGC to CTC (the middle base, G, is replaced by T). Applying complementary base pairing to the new template triplet (C pairs with G, T pairs with A, C pairs with G) gives the new mRNA codon:

GAG

Codon GCG (the original) specifies the amino acid alanine, whereas codon GAG (after the mutation) specifies glutamic acid (glutamate). A different amino acid, so this is a missense mutation.

This substitution does not cause a frameshift. A frameshift happens only when bases are inserted or deleted in a number that is not a multiple of three, because that shifts which bases are grouped into each triplet for every codon that follows. A substitution simply swaps one base for another in the same position. The total number of bases is unchanged, so every triplet grouping after (and before) the mutated base still lines up exactly as it did before. Only the one affected codon changes; the reading frame for the rest of the gene is unaffected.

Final answers

• (a) mRNA (5’ to 3’): U A C G C G A A A C C U.
• (b) RNA polymerase moves along the template strand (3’ to 5’), joins complementary RNA nucleotides (U pairing with A) via phosphodiester bonds, building mRNA 5’ to 3’.
• (c) Ribosome reads mRNA codons; tRNA anticodons pair with codons, each tRNA delivering its specific amino acid; peptide bonds join successive amino acids into a polypeptide.
• (d) New codon = GAG, coding for glutamic acid (was alanine); not a frameshift, because a substitution does not change the total number of bases, so the triplet grouping downstream is unaffected.