Question

Both DNA and RNA are synthesized by covalently linking a nucleoside triphosphate to the previous nucleotide, constantly adding to a growing chain. In the case of DNA, the new strand becomes part of a stable helix. The two strands are complementary in sequence and antiparallel in directionality. What is the principal force that holds these two strands together?

(a) ionic interactions
(b) hydrogen bonds
(c) covalent bonds
(d) van der Waals interactions

Answer 1

The two strands of DNA are held together primarily by hydrogen bonds forming between the complementary nitrogenous bases. These hydrogen bonds are weaker than the covalent bonds within the nucleotide molecules, allowing for the two DNA strands to 'unzip' easily during DNA replication.

Step-by-step explanation:

The principal force that holds the two strands of DNA together is due to the hydrogen bonds that form between the complementary nitrogenous bases. In the DNA double helix, adenine always pairs with thymine, and cytosine always pairs with guanine, each pairing held together by these bonds. These bonds allow the two strands to be anti-parallel in nature, meaning the 3' end of one strand faces the 5' end of the other strand, resulting in a stable helical structure.

The backbone of DNA consists of a sugar-phosphate structure, and the nitrogenous bases extend from this backbone into the interior, where they engage in hydrogen bonding. Although covalent bonds hold the atoms within each individual nucleotide strong together, it's the hydrogen bonds between the nucleotides of opposing strands that stabilize the overall structure of the DNA molecule.

During DNA replication, these hydrogen bonds can be relatively easily 'unzipped', allowing each strand to function as a template for the creation of a new complementary strand, thanks to the semi-conservative nature of replication, facilitated by the enzyme DNA polymerase.