14. Small nucleolar RNAs used to process and chemically modify rRNAs are called
(1) ScaRNAs     (2) SiRNAs
(3) SnoRNA      (4) SnRNAs

The correct answer is (3) SnoRNA.

Small Nucleolar RNAs and rRNA Modification

Small nucleolar RNAs (snoRNAs) are a class of non-coding RNAs that play a central role in the chemical modification and processing of ribosomal RNAs (rRNAs) in eukaryotic cells. Their primary functions include guiding the 2′-O-methylation and pseudouridylation of specific nucleotides within rRNA, which are crucial for the correct folding, stability, and function of mature ribosomes.

There are two main classes of snoRNAs:

• C/D box snoRNAs: Guide methylation of rRNA.

• H/ACA box snoRNAs: Guide pseudouridylation of rRNA.

Each snoRNA acts as a guide by base-pairing with its target rRNA sequence, ensuring that the modification enzymes act at the correct nucleotide. These snoRNAs are typically found in the nucleolus, the site of rRNA synthesis and ribosome assembly.

Why Not the Other Options?

• ScaRNAs (Small Cajal body-specific RNAs): Involved in the modification of small nuclear RNAs (snRNAs), not rRNAs.

• SiRNAs (Small interfering RNAs): Involved in RNA interference and gene silencing, not rRNA modification.

• SnRNAs (Small nuclear RNAs): Components of the spliceosome, involved in pre-mRNA splicing, not rRNA processing.

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