Question

Cell Biology. Please explain why this is the answer.

You are a virologist interested in studying the evolution of viral genomes. You are studying two newly isolated viral strains and have sequenced their genomes. You find that the genome of strain 1 contains 25% A, 55% G, 20% C, and 10% T. You report that you have isolated a virus with a single-stranded DNA genome. Based on what evidence can you make this conclusion?

Double-stranded genomes have equal amounts of A and T.

Answer 1

According to chargaff rule of double stranded nucleic acid,

Amount of adenine is equal to amount of thymine in DNA. This is because they both pair up with each other by two hydrogen bonds.

Amount of adenine is equal to amount of uracil in RNA. This is because they both pair up with each other by two hydrogen bonds.

Amount of cytosine is equal to amount of guanine in DNA and RNA both. This is because both of them pair up with each other by 3 hydrogen bonds.

So in a double stranded nucleic acid molecule, we will always see, A=T and G=C.

There are no chances expect for mutation that mispairing occurs.

But if in case you are given a data which contains different amount of adenine and thymine/uracil and different amount of guanine and cytosine, then it is for sure single stranded nucleic acid molecule. Because in single stranded nucleic acid, there is no other strand to undertake complementary base pairing and therefore the amount of all the bases is different.

In case you want to differentiate between RNA and DNA, by using a given data, then only look at uracil and thymine. Uracil will be present in RNA and thymine will be present in DNA.

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