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Correct Answer: a = t
dna, or deoxyribonucleic acid, is the molecule that carries the genetic instructions used in growth, development, functioning, and reproduction of all known living organisms and many viruses. dna molecules consist of two strands that wind around each other to form a double helix.
the structure of dna is composed of nucleotides, each containing a sugar (deoxyribose), a phosphate group, and a nitrogenous base. there are four types of nitrogenous bases in dna: adenine (a), thymine (t), cytosine (c), and guanine (g). these bases form specific pairs with each other: adenine pairs with thymine, and cytosine pairs with guanine.
the base pairing rules are crucial for the structure of dna. adenine (a) forms hydrogen bonds with thymine (t), and they pair up in a one-to-one ratio (a=t). similarly, guanine (g) pairs with cytosine (c) also in a one-to-one ratio (g=c). these pairings ensure the stability of the dna structure and are essential for the process of dna replication, where the double helix unzips and each strand serves as a template for creating a new complementary strand.
given these base pairing rules, it is correct to say that in any given molecule of dna, the quantity of adenine will equal the quantity of thymine (a=t), and the quantity of guanine will equal the quantity of cytosine (g=c). however, there is no necessary equivalence between the amount of purines (adenine and guanine) and pyrimidines (thymine and cytosine) overall. similarly, there are no direct base pairing relationships between adenine and cytosine or thymine and guanine. therefore, statements suggesting such relationships (like a=c, c+g=a+t, c=t, or a=g) are incorrect.
in summary, the correct statement about a standard molecule of dna, considering the rules of base pairing, is that the number of adenines (a) must equal the number of thymines (t), and the number of guanines (g) must equal the number of cytosines (c). other relationships between these bases are not supported by the base pairing rules. thus, the correct option in the context provided would be the one that simply states "a=t" and "g=c" and explains these pairings without implying any other relationships between the different bases.
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