Final answer:
According to Chargaff's rules, in DNA, the percentage of guanine equals that of cytosine, and the percentage of adenine equals that of thymine. Since 29% of the DNA is guanine, there will also be 29% cytosine and the remaining 42% is evenly split between adenine and thymine, meaning thymine makes up 21% of the DNA.
Step-by-step explanation:
The question pertains to understanding the nucleotide base pairing in DNA. According to Chargaff's rules, in DNA, the percentage of guanine (G) is always equal to the percentage of cytosine (C), and the percentage of adenine (A) is always equal to the percentage of thymine (T). Given that a species has 29% of its DNA composed of guanine (G), the same percentage, 29%, will be composed of cytosine (C) as G pairs with C. Because the total percentage of G and C combined is 58%, and since DNA is made up of a total of 100%, this leaves 42% of the DNA composed of adenine (A) and thymine (T) combined. Having learned that adenine and thymine are present in equal amounts as well, we can divide the remaining percentage by two to determine their individual percentages. Therefore, the percentage of thymine (T) would be 21%, which corresponds to option b. Hence, the correct answer to the question, 'A species has 29% of its DNA composed of guanine (G), what percentage does the nitrogen base thymine (T) equal?' is b) 21%.