Let’s do an easy one, shall we? Ok! Here’s the question:
The cDNA fragment that includes the ricin gene is 5.7 kilobases. If the entire fragment codes for the ricen polypeptide,the approximate number of amino acids in the poly peptide would be: (enter some weird numbers with lots of zeros here).
Well, once again the GRE just loves trying to confuse people with scary names and things. In this case, it throws in that whole ricin thing to throw you off. You can really just take that out of this question, so it reads something like “The cDNA fragment is 5.7 kilobases. How many amino acids does this code for?”
Alright, this is another one of those you-have-to-know-it questions. How much DNA does it take to code for a single amino acid? First, some very basic background. Amino acids are the building blocks of protein, and really what DNA codes for. Remember when we talked about DNA? DNA strands are studded with genes. Genes are simply lengths of DNA that code for certain proteins. Since the lengths of DNA make proteins, parts of the genes must code for the building blocks of proteins, or amino acids.
The next logical question is what percentage of each length of DNA codes for each amino acid? Ok, I’ll just tell you: 3 base pairs. Yep, that’s it. 3. Once you know how many base pairs are in a gene, then you just divide by three and that gives you the number of amino acids the gene codes for. How many base pairs are in the gene the question is asking about? 5.7 kilobases. Once again, don’t be afraid of words here. “Kilo” simply means 1000, while “bases” means, well, bases. So 5.7 kilobases is 5700 bases or base pairs. Divide that by three, and you get the nice round number of 1900. There you go!