The correct answer is option 1: Nucleotide. Nucleotides are the subunits or building blocks of DNA. They are composed of three main components: a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base. The nitrogenous bases in DNA include adenine (A), thymine (T), cytosine (C), and guanine (G). These nitrogenous bases pair up with each other to form the "rungs" of the DNA ladder, with adenine always pairing with thymine and cytosine always pairing with guanine. The sugar-phosphate backbone of the DNA molecule is formed by linking the sugar of one nucleotide to the phosphate group of the next nucleotide. Together, these nucleotides form a long chain, which makes up the structure of DNA.
Option 2, nucleosome, refers to the basic unit of DNA packaging in cells. It consists of DNA wrapped around histone proteins.
Option 3, nucleoside, is similar to a nucleotide but lacks the phosphate group.
Option 4, polypeptide, refers to a chain of amino acids, which is the building block of proteins. It is not directly related to DNA.