Non-coding RNA

A non-coding RNA (ncRNA) is an RNA molecule that is transcribed from DNA but not translated into a protein. These RNAs are functional in their RNA form - examples include transfer RNA (tRNA) and ribosomal RNA (rRNA), which are essential in protein synthesis but do not code for proteins, as well as regulatory RNAs like microRNA that modulate gene expression.

• Major types of non-coding RNAs include <u>ribosomal RNA (rRNA)</u> and <u>transfer RNA (tRNA)</u> (both involved in translation machinery), <u>small regulatory RNAs</u> such as microRNAs (miRNA) and short interfering RNAs (siRNA) which can silence gene expression, and <u>long non-coding RNAs (lncRNA)</u> which often regulate genes at the transcriptional or epigenetic level.
• Only about 1-2% of the human genome's RNA transcripts are mRNAs that code for protein; the rest are non-coding RNAs. This means the majority of transcribed RNA in a cell does not become protein but often has regulatory or structural roles (disproving the idea that they are "junk").
• Non-coding RNA is not to be confused with "non-coding DNA." Non-coding RNA refers to RNA transcripts that function as RNA (they do not have open reading frames for protein). For example, XIST RNA is a long ncRNA that coats the X chromosome to inactivate it (an epigenetic function), and telomerase RNA is an ncRNA that is part of the enzyme telomerase.

• If a question asks for examples of RNA that aren't translated into protein -> answer with things like tRNA and rRNA, or mention regulatory RNAs (miRNA, lncRNA). For instance: "Which RNA molecule is not directly involved in coding for a protein sequence?" - Correct answers could be tRNA or rRNA (both are non-coding RNAs).
• When you see mention of gene regulation by RNAs (e.g., a small RNA that binds to an mRNA to prevent its translation) -> that's referring to non-coding RNAs (like microRNAs or siRNAs) functioning to control gene expression.
• A classic exam detail: the ribosome itself is a ribozyme (rRNA in the ribosome catalyzes peptide bond formation). This underscores that rRNA - a non-coding RNA - has enzymatic and structural function in translation without carrying genetic code for a protein.