In DNA structure, how do the nitrogenous bases bond together to hold the two strands of the double helix in place?

• A. By hydrogen bonds between complementary base pairs (A with T, and G with C)
• B. By covalent phosphodiester bonds between opposing bases across the two strands
• C. By peptide bonds formed between adjacent bases along the same strand
• D. By ionic bonds between the bases and the sugar-phosphate backbone

Answer: (A)

Verified step by step guidance

1. Understand that DNA is composed of two strands forming a double helix, where the strands are held together by interactions between nitrogenous bases.
2. Recall that the nitrogenous bases pair specifically: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). This is known as complementary base pairing.
3. Recognize that the bonds holding these complementary bases together across the two strands are hydrogen bonds, which are relatively weak but numerous, providing stability to the DNA structure.
4. Differentiate these hydrogen bonds from covalent phosphodiester bonds, which connect nucleotides within the same strand, not between strands.
5. Note that peptide bonds are involved in protein structure, not DNA, and ionic bonds do not play a primary role in bonding bases to the sugar-phosphate backbone.