Regulation of gene expression and cell specialization
Non-coding RNA (ncRNA)
Voiceover: What is a non-coding RNA? A non-coding RNA, or an ncRNA, as it is abbreviated, is a functional RNA molecule that actually skips this last step and is not translated into a protein. In other words, they just go directly from transcription into an RNA molecule and then go off to perform any number of vital functions within the cell. There are many examples of non-coding RNAs, including micro RNAs, ribosomal RNAs, transfer RNA, the list goes on and on. As we go through each of these different types and examples of non-coding RNAs, you'll start to see that there's sort of an emerging theme, here. That is that most of these non-coding RNAs participate in either transcription or translation in one capacity or another. Let's start off with micro RNAs. Micro RNAs, or miRNAs, function in transcriptional and post transcriptional regulation of gene expression. They do this by base paring with complementary sequences within mRNA, or messenger RNA, molecules. This usually results in gene silencing through translational repression or target degradation. In essence, the mRNA to which these micro RNAs bind are prevented from being translated or they are sent on a pathway for degradation. The next set of non-coding RNAs that we'll be talking about are all involved in translation. The first of which is ribosomal RNA. Ribosomes are the cellular machinery used to translate mRNA into proteins. It is made up of one type of RNA molecule, ribosomal RNA. Transfer RNAs are an adapter molecule that links the codons in an mRNA strand to the corresponding amino acids. This is another type of non-coding RNA that you'll see in translation. The thrid type is called snow RNA, which stands for small nucleolar RNA. It's a class of small RNA molecules that guide covalent modifcations of ribosomal RNA, transfer RNA, and small nuclear RNAs, primarily through methylation, which is the addition of methyl groups, or pseudouridylation, which is the addition of an isomer of the nucleoside uridine. Another class of non-coding RNAs are the small nuclear RNAs, or snRNAs, not to be confused with the snow RNAs, the small nucleolar RNAs that we just talked about. Small nuclear RNAs get their name from the fact that the average length of these RNA molecules is approximately 150 nucleotides. Their primary function is in the processing of pre-mRNA in the nucleus. They also aid in the regulation of transcription factors or a particular RNA polymerase, RNA polymerase two, as well as maintaining telomeres, which are the regions of repetitive nucleotide sequences at the end of a chromotid, which protects the end of the chromosome from deterioration during chromosomal replication. SnRNA can be associated with a set of specific proteins and form complexes that are called small nuclear ribonucleic proteins, or snRNPs or sometimes people just call them snRPs. There is a special snRP complex called the spliceosome, made up of five small nuclear RNAs and over 150 proteins that is responsible for splicing, or removing, the introns contained in messenger RNA, which is a major step in the post transcriptional modification that takes place in the nucleus of eukaryotes. The way the the spiceosome does this is that it binds to specific sequences in the pre-messenger RNA strand and performs two sequential transesterification reactions that splice out the intron and then [lagate] the two exons to form a mature mRNA. Now you know a little bit more some examples of non-coding RNAs and some of the functions that they perform within the cell.
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