Which is true about the pairing of bases in the DNA molecule?
a. purines always pair with pyrimidines
b. a single ring base pairs with another single ring base
c. a double ring base pairs with another double ring base
d. purines pair with purines and pyrimidines with pyrimidines
To determine which statement about the pairing of bases in the DNA molecule is true, we need to review the structure of DNA.
DNA is made up of two strands that form a double helix structure. Each strand consists of a chain of nucleotides. Each nucleotide is composed of a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base.
There are four different nitrogenous bases found in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). Adenine and guanine are larger nucleotides called purines, while thymine and cytosine are smaller nucleotides called pyrimidines.
In DNA, the bases always form pairs. Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This pairing is based on the hydrogen bonding between the bases. Adenine forms two hydrogen bonds with thymine, while guanine forms three hydrogen bonds with cytosine.
With this information, we can now determine which statement is true:
a. purines always pair with pyrimidines - This statement is true. Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). Purines always pair with pyrimidines in DNA.
b. a single ring base pairs with another single ring base - This statement is not true. A single ring base (e.g., thymine or cytosine) pairs with a double ring base (e.g., adenine or guanine).
c. a double ring base pairs with another double ring base - This statement is not true. A double ring base (e.g., adenine or guanine) pairs with a single ring base (e.g., thymine or cytosine).
d. purines pair with purines and pyrimidines with pyrimidines - This statement is not true. Purines (adenine and guanine) always pair with pyrimidines (thymine and cytosine), not with other purines or pyrimidines.
Therefore, the correct statement is: a. purines always pair with pyrimidines.