Nucleic Acids and Protein Synthesis: Question 8

Worked solution

Part (a): Coding strand versus template strand

Every gene consists of two strands of DNA, which have different roles in transcription:

• The coding (sense) strand has the same base sequence as the mRNA molecule that will be produced, apart from one difference. Everywhere the coding strand has thymine, the mRNA has uracil instead. The coding strand is not read directly by RNA polymerase.
• The template (antisense) strand is complementary and antiparallel to the mRNA. This is the strand that RNA polymerase actually reads, moving along it and pairing free RNA nucleotides with the exposed template bases by complementary base pairing (uracil pairing with adenine).

RNA polymerase therefore uses the template (antisense) strand directly as the strand it reads to synthesise the mRNA; the coding strand is not copied directly, but its sequence can be read off from the mRNA sequence because the two are (almost) identical.

Part (b): Processing of the pre-mRNA

The molecule produced directly by transcription is the pre-mRNA (primary transcript). This contains alternating stretches of exons (sequences that code for the protein) and introns (non-coding sequences interspersed between the exons). Before the mRNA leaves the nucleus, it is processed by splicing: the introns are cut out and the remaining exons are joined together to form the continuous, mature mRNA molecule. Only after this splicing has taken place does the mature mRNA pass out of the nucleus, through a nuclear pore, into the cytoplasm, where it can associate with a ribosome for translation.

Part (c): Why introns must be removed

A ribosome does not “know” which parts of an mRNA came from an exon and which came from an intron (it simply reads the mRNA in consecutive, non-overlapping codons from the start of the coding sequence, and a tRNA delivers the amino acid matching each codon in turn. If the introns had not been removed, the ribosome would translate their base sequence exactly as it translates the exon sequence, reading intron codons as though they specified real amino acids. This would insert additional amino acids, in the wrong sequence, into the growing polypeptide, and could easily introduce a premature stop codon partway through the message. Either outcome would produce a polypeptide with the wrong amino acid sequence (or a shortened one), which would very likely fail to fold and function correctly. Removing the introns during splicing ensures that only the exon sequences) which together specify the correct, functional protein, are translated.

Final answers

• (a) Coding (sense) strand = same sequence as mRNA (T instead of U); template (antisense) strand = complementary/antiparallel to mRNA and is the strand RNA polymerase actually reads.
• (b) The pre-mRNA is spliced in the nucleus: introns are removed and exons are joined together to form mature mRNA, which then leaves the nucleus.
• (c) Left-in introns would be translated codon by codon just like exons, inserting wrong amino acids (and/or a premature stop codon) and producing a non-functional protein.