Final answer:
The expected proportion of guanine in a sample of double-stranded DNA with a thymine content of 27% is 23%, according to Chargaff's rules.
Step-by-step explanation:
If a sample of double-stranded DNA has a thymine content of 27%, the expected proportion of guanine is 23%. This is based on Chargaff's rules, which dictate that in DNA, adenine (A) pairs with thymine (T) and guanine (G) pairs with cytosine (C). Thus, the content of adenine will equal the content of thymine, and the content of guanine will equal the content of cytosine.
As we know the thymine content is 27%, the adenine content must also be 27%. Therefore, the remaining percentage must be divided equally between guanine and cytosine, which makes up the other two nucleotide bases. Given that the total percentage for all four bases must equal 100%, we can deduce that guanine and cytosine each make up 23% of the DNA (100% - 27% - 27% = 46%; 46% / 2 = 23%).