Even within a gene only some sequences, called exons, code for amino acid…

The idea

A eukaryotic gene is not one continuous coding stretch. Within the gene, exons are the sections whose base sequence is read as the amino acid sequence of the polypeptide; remember exons are expressed. The other sections, introns, do not code for amino acids. When the gene is transcribed, RNA polymerase copies the whole gene, exons and introns alike, producing pre-messenger RNA. Splicing then removes the introns and joins the exons, giving mature messenger RNA, which leaves the nucleus through a nuclear pore to a ribosome in the cytoplasm. Because splicing can join different combinations of exons from the same pre-messenger RNA, one gene can give rise to more than one messenger RNA and therefore more than one polypeptide. Prokaryotic genes are generally continuous, with no introns, so they need no splicing.

What earns the marks

• Exons being the sections of a gene whose base sequence codes for amino acids.
• Introns being non-coding sections within the gene.
• Pre-messenger RNA containing both exons and introns, because the whole gene is transcribed.
• Splicing removing introns and joining exons to form mature messenger RNA.
• Contrasting this with prokaryotic genes, which are generally continuous and need no splicing.

Common mistakes and how to put them right

• Saying messenger RNA is transcribed from the exons only, so the pre-messenger RNA stage disappears.

• Confusing an exon with a codon.

  Put it right: An exon is a section of a gene, a codon is three bases on messenger RNA.

• Saying splicing happens at the ribosome or in the cytoplasm rather than in the nucleus.

• Claiming that removing the introns changes the amino acids coded for by the exons.

• Applying splicing to a bacterial gene in a recombinant DNA question.

  Put it right: Bacteria cannot splice eukaryotic introns.

Exam tips

• Remember that exons are expressed; the mnemonic has rescued a great many marks.
• Use pre-messenger RNA and mature messenger RNA as separate terms in any eukaryotic transcription answer.
• If a question puts a eukaryotic gene into a bacterium, say the bacterium cannot splice, so complementary DNA made from mature messenger RNA must be used.