D1.2 Protein synthesis

The question asks how one gene can produce multiple distinct proteins, which is a eukaryote-specific phenomenon linked to post-transcriptional processing.

In eukaryotes, the pre-mRNA contains both exons (coding regions) and introns (non-coding regions). During RNA processing, introns are removed and exons are joined. Crucially, different combinations of exons can be retained in different tissues — this is alternative splicing.

Alternative splicing means a single gene can produce multiple distinct mRNA sequences, each encoding a different polypeptide. This is one reason the proteome is larger than the number of protein-coding genes.

The option describing different segments of pre-mRNA being joined in varying combinations correctly describes alternative splicing and is the best answer.

We need to find the mechanism that allows one gene → multiple proteins in different tissues.

'The gene is replicated multiple times in different tissues' is incorrect. Gene copy number does not change between tissues in normal cells, and having more copies of the same gene would produce more of the same protein, not different proteins.

'The ribosome reads the mRNA in different reading frames' is incorrect. The ribosome always begins at AUG and reads in one fixed frame; reading frame shifts are caused by mutations, not normal tissue-specific regulation.

'Introns are translated instead of exons' is incorrect. Introns are non-coding sequences that are spliced out and degraded; they are not translated into protein.

'Different segments of the pre-mRNA transcript can be joined in varying combinations during RNA processing' correctly describes alternative splicing — the mechanism by which one gene generates multiple protein products.

The question asks which ribosomal binding site holds the tRNA attached to the growing polypeptide chain during elongation.

The ribosome has three tRNA binding sites: A site (Aminoacyl) — holds the incoming charged tRNA; P site (Peptidyl) — holds the tRNA attached to the growing polypeptide; E site (Exit) — where the empty tRNA exits.

The P site is named 'Peptidyl' precisely because it holds the peptidyl-tRNA — the tRNA that carries the growing peptide chain. After a peptide bond forms, the ribosome translocates and the tRNA in the P site moves to the E site.

The correct answer is the P site.

We need to identify which ribosomal site holds the tRNA carrying the growing polypeptide chain.

The A site is incorrect. It holds the incoming aminoacyl-tRNA (a tRNA freshly loaded with one amino acid), not the growing polypeptide chain.

The E site is incorrect. It is the exit site where the empty tRNA (after it has donated its amino acid) leaves the ribosome.

'Signal site' does not exist as a ribosomal tRNA binding site — this is a distractor referencing signal sequences, which relate to protein targeting, not ribosome structure.

The P site (Peptidyl site) is correct — it holds the tRNA attached to the growing polypeptide chain.