If you’re first thought on reading this is panic. Don’t worry there are a clues to guide you to the answer. As with almost every problem the first step is to read the question carefully and look at the number of marks available. I’ll break it down below:
Humans and grasshoppers have very similar percentages of each base in their DNA but they are very different organisms
Use your knowledge of DNA structure and function to explain how this is possible (2 Marks)
Okay so we know our answer is worth two marks so we have to say two things which must explain how similar nitrogenous base percentages (Adenine, Thymine, Cytosine and Guanine) can produce different organisms. The question helpfully gives us a pointer about how it should be answered. We must say something firstly about DNA Structure and secondly Function.
So let’s start with structure. We should know that DNA is composed of two strands of nitrogenous bases running in opposite directions. We also know that the order of bases in these strands are fixed. This provides a possible solution to the problem. They may have the same percentages of bases but the sequence of the DNA could well be different in grasshoppers than humans.
Let’s try linking this to function, we should know that DNA is acts as template for making proteins in the cell. These proteins keep the cell alive and allow it to perform its functions. Since grasshoppers and humans are so different it makes sense that they might have different proteins.
We have now actually got enough to answer the question, according to the mark-scheme which we have worked out from basic material.
Grasshoppers and humans have a different sequence of bases which therefore produce different proteins.
If we wanted to make our answer better to be extra safe, we could add in some more information about the step in the middle between the genetic sequence and the protein. Proteins are made out of a chain amino acids just as DNA is a chain of nitrogenous bases. The length of DNA which codes for a particular protein is a gene. The issue in translating the DNA into protein is that there are more amino acids (24 different kind) than bases (4 different kind). To solve this DNA works as a triplet code, it takes three amino acids to specify each amino acid. Three bases are called a codon.
So a better answer would be:
Grasshopper and Human DNA have very different sequences of nitrogenous bases; this results in their genes having different codons relative to each other. They thus produce different proteins with divergent series of amino acids. (These proteins have different functions and result in different animals- This sentence isn't credited in the mark scheme but shows you understand the rationale fully rather than learning the markscheme, can be useful for earning the benefit of the doubt in some cases)
This answer is perfect for A-levels but is actually pretty simplistic, this question could easily fill a book or even an encyclopaedia. Consideration in real life would include non-coding DNA, epigenetics, chromosomal structure, developmental pathways etc. These are the kinds of things which may be touched on in Oxbridge interviews. If you’re interested in finding out more, please feel free to arrange a tutorial with me, and I’d be happy to go through this question with the detail it disserves.