Since the genetic code is universal, as are transcription and translation…

The idea

The genetic code is universal: the same DNA triplet and mRNA codon code for the same amino acid in almost every organism. Transcription and translation mechanisms are also universal. RNA polymerase transcribes mRNA, and ribosomes assemble amino acids as dictated by codons. Thus, a human gene transferred into a bacterium can be transcribed and translated to produce a human amino acid sequence. However, limitations exist. Bacteria lack spliceosomes and cannot remove introns from eukaryotic pre-mRNA, so cDNA made from mature mRNA is used instead of chromosomal DNA. Furthermore, bacteria lack eukaryotic promoter recognition and organelles like the Golgi apparatus, meaning they cannot perform post-translational modifications such as glycosylation.

What earns the marks

• The genetic code is universal, meaning the same triplets or codons code for the same amino acids in almost all species.
• The mechanism of transcription is fundamentally the same in both organisms.
• The mechanism of translation is the same, as bacteria possess ribosomes to translate the mRNA.
• Bacteria cannot splice pre-mRNA because they lack spliceosomes, so introns are not removed.
• Bacteria lack a Golgi apparatus, so the protein cannot undergo post-translational modifications like glycosylation.
• Bacteria lack eukaryotic promoter recognition, which can prevent the transcription of unmodified eukaryotic genes.

Common mistakes and how to put them right

• Writing 'DNA is universal' on its own, which is not accepted unless qualified as the genetic code.

• Stating 'the genetic code is degenerate' as the reason a bacterium can use a human gene.

  Put it right: This is incorrect and rejected.

• Claiming bacteria lack transcription factors entirely, rather than lacking specific eukaryotic promoter recognition.

• Forgetting introns and claiming a bacterium can express any human gene taken straight from a chromosome.

Exam tips

• Two distinct marks are often available for stating the code is universal and the mechanisms are universal; write both.
• When asked why a bacterium might fail to make a functional human protein, name a missing structure (e.g., Golgi apparatus) and its function (e.g., glycosylation).