Biology · Molecular Basis of Inheritance · NEET
A nucleoside is a nitrogenous base joined to a pentose sugar only (base + sugar). A nucleotide is a nucleoside PLUS a phosphate group (base + sugar + phosphate). So a nucleotide = nucleoside + phosphate. Example: adenosine is a nucleoside; adenylic acid (AMP) is a nucleotide. NEET often gives a list and asks you to sort bases, nucleosides, and nucleotides, so this exact difference is tested.
A nucleotide has exactly three parts: (1) a nitrogenous base, (2) a pentose sugar (ribose in RNA, deoxyribose in DNA), and (3) a phosphate group. This is a straight NCERT line and a very common one-mark NEET fact.
The nitrogenous base is linked to the OH of the 1' carbon (1' C) of the pentose sugar through an N-glycosidic linkage. This forms a nucleoside, such as adenosine, guanosine, cytidine, or uridine. In DNA the deoxy forms are made (deoxyadenosine, etc.).
When a phosphate group is linked to the OH of the 5' carbon (5' C) of a nucleoside through a phosphoester linkage, a nucleotide is formed. If the sugar is deoxyribose, it is called a deoxynucleotide. So two different linkages build the unit: N-glycosidic (base to sugar) and phosphoester (sugar to phosphate).
The backbone is made of alternating sugar and phosphate groups. The nitrogenous bases are linked to the sugar and project out from the backbone. Nucleotides join together by phosphodiester bonds (a phosphate links the 3' C of one sugar to the 5' C of the next) to form the long polynucleotide chain.
Adenosine is a NUCLEOSIDE (adenine base + ribose sugar, no phosphate). Add a phosphate and it becomes the nucleotide adenylic acid (AMP). Remember: names ending in -osine or -idine (adenosine, guanosine, cytidine, uridine) are nucleosides.
The correct statement regarding RNA and DNA, respectively is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. A nucleotide contains everything a nucleoside has (base + sugar) plus one extra phosphate group. So nucleotide = nucleoside + phosphate.
The base attaches at the 1' carbon of the sugar (N-glycosidic linkage). The phosphate attaches at the 5' carbon (phosphoester linkage). The free OH at the 3' carbon is used to join the next nucleotide.
The nucleotide is the monomer. Many nucleotides polymerise to form a polynucleotide chain, which is DNA or RNA.
Molecular Basis of Inheritance is a high-weightage chapter, and NEET regularly asks direct one-mark questions on nucleoside vs nucleotide, the three components, the two linkage types, and the sugar difference between DNA and RNA. These are easy, guaranteed marks if you know the exact definitions.
N-glycosidic linkage joins the nitrogenous base to the 1' carbon of the sugar (forms a nucleoside). Phosphoester linkage joins the phosphate to the 5' carbon of the sugar (forms a nucleotide).
