In prokaryotes, transcription results directly in the production of mRNA…

The idea

Prokaryotic genes are continuous because their DNA contains no introns. Therefore, transcription in a bacterium directly produces functional mRNA. There is no pre-mRNA stage and no splicing is required. Because prokaryotes lack a nuclear envelope, ribosomes can begin translating the mRNA while it is still being transcribed. The bacterial DNA is a circular molecule lying free in the cytoplasm, unassociated with histones. Bacteria are used in recombinant DNA technology to produce human proteins because the genetic code is universal, meaning the same codons specify the same amino acids across organisms. However, bacteria cannot splice pre-mRNA, so they fail to produce functional proteins if given unmodified eukaryotic genes containing introns.

What earns the marks

• Stating that prokaryotic DNA does not contain introns, meaning there is no pre-mRNA stage.
• Explaining that functional mRNA is produced directly from transcription without the need for splicing.
• Explaining that transcription and translation can occur simultaneously in the cytoplasm because there is no nuclear envelope.
• Explaining that a bacterium cannot splice pre-mRNA, so it cannot remove introns from an unmodified eukaryotic gene.
• Stating that the genetic code is universal, meaning the same codons code for the same amino acids in all organisms.

Common mistakes and how to put them right

• Claiming prokaryotes make pre-mRNA and splice it quickly.

  Put it right: They do not produce pre-mRNA at all because their genes lack introns.

• Assuming that all transcribed bases in prokaryotes code for amino acids.

  Put it right: Prokaryotic mRNA still contains untranslated regions (UTRs) and ribosome-binding sites.

• Writing that 'DNA is universal'.

  Put it right: This is imprecise. The correct biological principle is that the genetic code is universal.

• Offering 'the genetic code is degenerate' as the reason a bacterium can express a human gene.

  Put it right: Degeneracy refers to multiple codons coding for one amino acid, whereas universality allows cross-species expression.

• Stating that the mechanisms of transcription and translation are universal.

  Put it right: While the genetic code is universal, the enzymes (like RNA polymerase) and ribosomes (70S vs 80S) differ between prokaryotes and eukaryotes.

Exam tips

• In recombinant DNA questions, specify that the 'genetic code' is universal, meaning the same codons code for the same amino acids.
• When asked why a bacterium cannot make a functional human protein from an unmodified gene, state that it cannot splice pre-mRNA, so introns remain.
• Remember the workaround for expressing eukaryotic genes in bacteria: cDNA made by reverse transcriptase from mature mRNA already has its introns removed.