Example oxford biochemistry interview question - why does dna have 4 bases?
Updated: Sep 18
Why does DNA have 4 bases?
This is a classic example Oxford biochemistry interview question.
Here's what the interviewers are actually looking for. They're looking for you to answer the 'question before the question'. In this case, what is the structure of DNA?
This is where you stay calm and mention the knowledge that you know from your A-Level and IB studies. This includes the complementary strands and the triplet codon system.
At this point the tutors are looking for interpretation of this information. On their official marking scheme this comes under: problem solving ability, description/analysis and interpretation of novel information. A good answer looks like this - a process of elimination: Its obvious that you can't have a complementary system with an odd number of bases - so three bases is eliminated
2 bases can only give you 8 codons in a triplet codon system meaning there is a lack of genetic diversity - and not enough codons for 20 amino acids.
Okay, so why not 6? Even with 4 bases, which give 20 amino acids we have a large number of degenerate codons - and the purpose of this is to protect against mutations impacting at the protein functional level.
So simply put, there may have been 6 bases at some point in early evolutionary history, but only the four most chemically stable bases were chosen.
What does a weak answer look like?
If you rush straight to justifying why DNA has 4 bases without understanding the need for complimentarity, sufficient genetic diversity and molecular evolution constraints, you would not be showing the critical thinking skills required.
How might the interviewer extend this question?
You might be asked to justify why (or why not) - 20 amino acids are enough.
Have a think about the type of chemistry that amino acids can do and the chemistry that they lack.
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