Question

Describe the structure and function of miRNA, snRNA, and lncRNA, which represent the three major types of noncoding RNAs; what are four mechanisms by which lncRNA molecules are thought to regulate cellular processes?

Answer 1

miRNA, snRNA and lncRNA are three types of small RNAs found in eukaryotic cells. Of these three types of RNAs miRNAs(micro RNAs) and lncRNAs (long non-coding RNAs) are gene regulators. Micro RNAs and long non-coding RNAs control gene expression by complementary base pairing with their target mRNAs. They can prevent translation of the target mRNA or promotes degradation of the target mRNA. Some miRNAs can cause changes in chromatin structure by remodelling chromatin and some promotes formation of hetrochromatin thereby preventing the expression of their target genes.

miRNAs are synthesized as long precursor RNAs called pri-miRNAs, that are self-complementary and can automatically fold into a double-stranded hairpin structure, usually with some imperfect base pairing. The pri-miRNA is then processed in a two-step reaction. The first step is catalyzed by Drosha in the eukaryotic cell nucleus. Drosha reduces (cleaves) the pri-RNA to about a 70 bp precursor fragment, pre-miRNA (precursor miRNA) which is the exported to the cytoplasm where the second step in its processing which  is catalyzed by Dicer to produce a short double-stranded -22 base pair fragment. This 22 bp long fragment then associates with certin proteins, particularly the proteins of the agronaut family to form an RNA induced silencing complex (RISC). Tis complex is responsible for silencing their target genes.

Small nuclear RNAs (snRNAs) are small RNAs found in the nucleus of the eukaryotic cell. In the nucleoplasm they are bound to certain proteins and exist as snRNP. Some of them are capped like mRNA. They are required for the processing, particularly splicing of the hnRNA. hnRNA (precurosor mRNA) is converted into mature translatable mRNAs by splicing. The splicing of the hnRNA is carried out by spliceosomes formed of snRNAs and certain protens.