Two strands of DNA are held together by _________.(a) Peptide bonds(b) Phosphodiester bonds(c) S-S bonds(d) Hydrogen bonds

The correct answer is (d) Hydrogen bonds.

Explanation: DNA consists of two strands that run in opposite directions and are held together by hydrogen bonds between complementary nitrogenous bases. These nitrogenous bases pair as follows:

Adenine (A) pairs with Thymine (T) through two hydrogen bonds.
Cytosine (C) pairs with Guanine (G) through three hydrogen bonds.
The hydrogen bonds are weak compared to covalent bonds, which allows the DNA strands to separate easily during processes like DNA replication and transcription.

To summarize:

Peptide bonds are found in proteins, linking amino acids together.
Phosphodiester bonds connect the nucleotides within the same strand of DNA.
S-S bonds are covalent bonds between sulfur atoms, often seen in proteins (like disulfide bridges).
Thus, DNA strands are held together by hydrogen bonds between the nitrogenous bases.