Biology · Molecular Basis of Inheritance · NEET
DNA is a polymer, which means a long chain made of many small repeating units. The small unit is called a nucleotide. So DNA = many nucleotides joined together. In NEET, remember NCERT's line: 'nucleic acids are polymers of nucleotides.' DNA and RNA are the two types of nucleic acids.
Every nucleotide has exactly three parts: (1) a nitrogenous base, (2) a pentose sugar (a 5-carbon sugar; in DNA it is deoxyribose), and (3) a phosphate group. Use the hook B-S-P (Base, Sugar, Phosphate). If any one part is missing, it is not a complete nucleotide.
A nucleoside = nitrogenous base + sugar only (no phosphate). A nucleotide = nucleoside + phosphate (base + sugar + phosphate). So the ONLY difference is the phosphate group. Simple trick: nucleoTide has the phosphaTe (both have a T). NEET loves this small trap.
Yes. The phosphate group is one of the three parts of a nucleotide. It is what makes DNA acidic and what links one nucleotide to the next to form the sugar-phosphate backbone. Without phosphate the unit is only a nucleoside, not a nucleotide.
The monomer (single building block) of DNA is the deoxyribonucleotide. Many such nucleotides join to form the DNA polymer. This is why DNA is called a polynucleotide. RNA's monomer is the ribonucleotide (it uses ribose sugar instead of deoxyribose).
Inside a nucleotide, the nitrogenous base joins the 1' carbon of the sugar (this bond is an N-glycosidic bond), and the phosphate group joins the 5' carbon of the same sugar (a phosphoester bond). So the sugar sits in the middle, holding the base on one side and the phosphate on the other.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
DNA contains deoxyribose sugar. The prefix 'deoxy' means it has one less oxygen than ribose (no OH at the 2' carbon). RNA contains ribose. This is a common NEET point, so remember: D-NA has De-oxyribose.
DNA has four nitrogenous bases: adenine (A), guanine (G), cytosine (C) and thymine (T). In RNA, thymine is replaced by uracil (U). The base is one of the three parts of every nucleotide.
'Deoxyribo' comes from the deoxyribose sugar it contains, 'nucleic' because it is found in the nucleus, and 'acid' because the phosphate groups make it acidic. So the full name directly tells you its parts.
A single nucleotide is a monomer (one building block). When many nucleotides join, they form the polymer called a polynucleotide, which is DNA or RNA. This is important for NEET definition questions.
