Final answer:
According to Chargaff's rules, the number of adenine bases in a DNA strand equals the number of thymine bases. For a gene with 399 adenines, there will also be 399 thymines.
Step-by-step explanation:
The question pertains to the molecular components of DNA, specifically the base pairing rules discovered by Erwin Chargaff. Chargaff's rules indicate that in a double-stranded DNA molecule, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C). Since you've mentioned that a gene has 245 guanines (G) and 399 adenines (A), according to Chargaff's rules, there would be an equal number of cytosines to guanines and an equal number of thymines to adenines.
Therefore, if there are 399 adenines in the gene, there will also be 399 thymines (T). The correct answer to the question, "How many thymines will there be in this gene?" is a) 399.