Dna is isolated from a virus and found to contain 27% a, 27% t, 23% c and 23% g. the dna is likely to be:

This viral DNA is most likely to be a double-stranded molecule. We can make this conclusion based on the identical concentrations of the base pairs.
The four nitrogen bases that make up the DNA sequence always pair with their complement bases, meaning that adenine always pairs with thymine and guanine always pairs with cytosine. So, if there is 27% of adenine, there must be an identical amount of thymine in a double-stranded molecule.

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