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GATE 2024 BT Q.24: solution and answer

GATE 2024 · BTQ.24 · 1 mark · MCQ

An element that is present in a nucleotide but not in a nucleoside is ________.

  1. (A)

    carbon

  2. (B)

    nitrogen

  3. (C)

    oxygen

  4. (D)

    phosphorous

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Answer and solution

ANSWER

The element present in a nucleotide but not in a nucleoside is phosphorus, option (D).

Official answer: DSource: Official answer key, GATE 2024 BT (IISc Bengaluru)

  1. A nucleoside is a nitrogenous base joined to a pentose sugar (ribose or deoxyribose) by an N-glycosidic bond.

  2. A nucleotide is a nucleoside with one or more phosphate groups esterified to the sugar, usually at the 5' carbon.

  3. So the only extra component a nucleotide has over its nucleoside is the phosphate group. Its new atoms are phosphorus and additional oxygen.

  4. Carbon: present in both the base and the sugar of a nucleoside, so it does not separate the two. Option (A) is wrong.

  5. Nitrogen: present in the base, which a nucleoside and a nucleotide both carry. Option (B) is wrong.

  6. Oxygen: the sugar of a nucleoside already contains oxygen (for example the ring oxygen and the hydroxyl groups). The phosphate adds more, but the element is not exclusive to the nucleotide. Option (C) is wrong.

  7. Phosphorus occurs only in the phosphate group, so it is present in a nucleotide and absent from a nucleoside. Option (D) is correct.

KEY IDEA

Nucleoside = base + pentose sugar. Nucleotide = nucleoside + phosphate group(s). Phosphorus is the one element that comes only from the phosphate.

WATCH OUT FOR

Choosing oxygen because phosphate groups are rich in oxygen. Sugars already contain oxygen, so oxygen is in the nucleoside as well. Only phosphorus is unique to the nucleotide.

From the GATE 2024 BT paper. Every question of this paper · Download the full paper · All 12 Biomolecules questions, with notes

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