Biomolecules — NEET UG practice

1. Recall the purpose of sodium fusion extract.

Sodium fusion converts the nitrogen, sulphur and halogens present in an organic compound into simple, detectable ionic forms such as $NaCN$, $Na_2S$ and sodium halides.

2. Recall what sodium nitroprusside detects.

Sodium nitroprusside test specifically detects the presence of sulphur in the compound, normally giving a violet colour when sulphide ($S^{2-}$) alone is present.

3. Recall the special case when both nitrogen and sulphur are present together.

Given that the molecule contains both N and S, they can combine during fusion, therefore the test instead gives a characteristic blood-red colour rather than the usual violet.

4. Match this behaviour to the given amino acids.

Among serine, leucine, threonine and methionine, only methionine contains a sulphur atom (in its side chain) along with the nitrogen already present in every amino acid.

Hence, the answer is D. methionine.

1. Recall a rich dietary source of vitamin A.

Carotenoids present in carrots are converted in the body to vitamin A, so A matches IV.

2. Recall a rich dietary source of vitamin $B_{12}$.

Vitamin $B_{12}$ is mainly obtained from animal products, so meat is its source, giving B matches I.

3. Recall a rich dietary source of vitamin E.

Vitamin E, being fat-soluble, is abundant in vegetable oils such as sunflower oil, so C matches II.

4. Recall a rich dietary source of vitamin K.

Vitamin K is synthesised by and found abundantly in green leafy vegetables, so D matches III.

Given the matches A-IV, B-I, C-II, D-III, hence the answer is C.

1. Recall the strandedness and bases of RNA.

RNA is normally single-stranded and its four bases include uracil, not thymine, so any option pairing RNA with thymine (as in option A) is incorrect.

2. Recall whether RNA can be double-stranded.

Since RNA is single-stranded rather than double-stranded, option C, which calls it double-stranded, is also incorrect.

3. Recall the strandedness and bases of DNA.

DNA has a double-strand helix structure made of two polynucleotide chains, and its four bases are adenine, guanine, cytosine and thymine.

4. Check the remaining option describing DNA.

Option D wrongly calls DNA single-stranded and pairs it with uracil, so it is incorrect, while option B correctly states DNA is double-stranded and contains thymine.

Hence, the answer is B.