Nucleic Acids and Protein Synthesis: Question 6

Worked solution

Working through the options

“Universal” is one of three key properties of the genetic code (alongside triplet and degenerate), and it describes how the code is shared between organisms, not how many codons or genes an organism has.

With only a small number of rare exceptions (for example in some mitochondria), the meaning of each codon is the same in virtually every living organism: the codon AUG specifies methionine, the codon UUU specifies phenylalanine, and so on, whether the organism is a bacterium, a plant or a human. This is why a gene taken from one species and inserted into another (as in genetic engineering) can still be transcribed and translated correctly by the second organism’s cell machinery, its ribosomes and tRNAs interpret the codons in exactly the same way.

Checking each option:

• A, correctly describes universality as the same codon meaning being shared across organisms. Correct.
• B. Confuses universality of the code with genome size or gene number, which vary enormously between species and have nothing to do with the genetic code itself. Incorrect.
• C. This describes the opposite of universality: it would mean the same codon has different meanings in different organisms, which is not the case. Incorrect.
• D, incorrect: prokaryotes such as bacteria use, with only rare exceptions, the same genetic code as eukaryotes; this shared code is precisely what makes universality true.

Final answers

• A. The genetic code is universal because, with only rare exceptions, the same codon specifies the same amino acid in almost all organisms.