The structure of DNA is held together by
A.purines pairing with pyrimidines
B.hydrogen bonds between adenine and thymine
C.attraction between cytosine and guanine
D.all of the above
I'm pretty sure that it's B
The correct answer is D. all of the above.
To explain why, let's break down each option:
A. Purines pairing with pyrimidines:
The structure of DNA is made up of two strands that are twisted around each other to form a double helix shape. The two strands are held together by hydrogen bonds between nitrogenous bases. The nitrogenous bases in DNA consist of two types: purines (adenine and guanine) and pyrimidines (thymine and cytosine). These nitrogenous bases have a complementary pairing rule: purines always pair with pyrimidines. Adenine always pairs with thymine (forming two hydrogen bonds), and cytosine always pairs with guanine (forming three hydrogen bonds). This pairing of purines with pyrimidines is an essential feature of DNA's structure and ensures that the two DNA strands are complementary to each other.
B. Hydrogen bonds between adenine and thymine:
As mentioned earlier, hydrogen bonds play a crucial role in maintaining the structure of DNA. Specifically, hydrogen bonds form between the nitrogenous bases adenine and thymine. Adenine and thymine are connected by two hydrogen bonds, which further stabilize the DNA double helix structure.
C. Attraction between cytosine and guanine:
Similarly, the attraction between the two other nitrogenous bases, cytosine and guanine, also contributes to the structural stability of DNA. Cytosine and guanine are connected by three hydrogen bonds, which provide additional strength and stability to the double helix structure.
Therefore, the structure of DNA is held together by purines (adenine and guanine) pairing with pyrimidines (thymine and cytosine), as well as the hydrogen bonds formed between adenine and thymine and the attraction between cytosine and guanine. Hence, all of the given options are correct.