XNA is a laboratory-made nucleic acid. XNA is made of nucleotides in which one component has been replaced by chemical X. The chemical X is organic but is not found in nature. The part of the molecule responsible for coding is not changed.
Which component of a DNA or RNA nucleotide has been replaced by the organic chemical X?
▶️ Answer/Explanation
Ans: A
Since the coding part (bases) remains unchanged, the modification must be in either the sugar or phosphate. The phosphate group (B) is inorganic, so the replacement must be the five-carbon sugar (A). This creates XNA (xeno nucleic acid) while maintaining the genetic information in the bases.
The diagram shows part of a nucleic acid molecule.
What is the name of bond X?
▶️ Answer/Explanation
Ans: D
The bond labeled X connects nucleotides in a nucleic acid chain. This is a phosphodiester bond which forms between the phosphate group of one nucleotide and the sugar of another. Glycosidic bonds (A) link sugars to bases, hydrogen bonds (B) connect base pairs, and peptide bonds (C) form between amino acids.
During the production of protein molecules, only one strand from the DNA double helix is used.
Which name is given to the DNA strand that is used to produce a new protein?
▶️ Answer/Explanation
Ans: C
The DNA strand used for transcription is called the template strand (C). The non-transcribed strand (A) is not used, while leading/lagging strands (B/D) are terms for DNA replication, not transcription. Thus, the correct answer is C (template strand).
Which row correctly describes cytosine?
▶️ Answer/Explanation
Ans: C
Cytosine is a pyrimidine (single-ring structure) that forms three hydrogen bonds with guanine. Option A/D incorrectly labels it as purine, and B has the wrong hydrogen bond count. Thus, the correct description is C (single ring, three bonds, pyrimidine).