Which of the following statements is not true about RNA?

We start by recalling the elementary facts about ribonucleic acid, popularly abbreviated as RNA. RNA is a biopolymer built from ribonucleotides, each consisting of a ribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, guanine, cytosine, or uracil). In most organisms, RNA performs several crucial biological tasks such as carrying genetic information from DNA to ribosomes (messenger RNA, mRNA), forming the core of ribosomal structure and catalyzing protein synthesis (ribosomal RNA, rRNA), and transporting specific amino acids to the ribosome during protein assembly (transfer RNA, tRNA).

Now, let us verify every option one by one.

Option A: “It controls the synthesis of protein.” Messenger RNA indeed carries the genetic code from DNA to the ribosome, and along with rRNA and tRNA, it directs the process of translation. Hence RNA does control protein synthesis. Therefore Option A is a true statement about RNA.

Option B: “It usually does not replicate.” Unlike DNA, RNA molecules are not self-replicating under normal cellular conditions. They are synthesized afresh from a DNA template by the enzyme RNA polymerase. Only in certain RNA viruses does RNA replication occur. Thus, in the usual context of cellular biology, RNA indeed “does not replicate.” Hence Option B is also true.

Option D: “It is present in the nucleus of the cell.” Inside eukaryotic cells, a large quantity of RNA (for example, heterogeneous nuclear RNA: hnRNA and various small nuclear RNAs: snRNAs) is synthesized and processed in the nucleus before being exported to the cytoplasm. Therefore RNA is certainly present in the nucleus. So Option D is true as well.

Option C: “It has always double standard $$\alpha$$-helix structure.” The classic double helical $$\alpha$$-helix (commonly referred to as the double-stranded helix) is the hallmark of DNA, not RNA. The vast majority of RNA molecules are single-stranded. While an RNA strand can fold back on itself to form hairpins, loops, or short double-stranded regions, these local secondary structures do not amount to a permanent, extensive, double-stranded $$\alpha$$-helix throughout the entire molecule. Therefore the declaration that RNA “always” possesses a double-stranded $$\alpha$$-helix is factually incorrect.

Because Option C is not true about RNA, whereas the other three statements are true, Option C is the odd one out.

Hence, the correct answer is Option C.