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Other RNA types

SnRNA – small nuclear RNA

These are short RNA molecules (around 150 nucleotides in length) found in the nucleus of eukaryotic cells. They were actually discovered by accident in the 1960s while doing some experiments with gel electrophoresis, and several functions have been connected with them up to now. The most important one is the processing of pre-mRNA, but they also help in the stability of telomeres and regulation of RNA polymerase II and some transcription factors. Subtype of snRNAs are snoRNAs (small nucleolar RNA), found in the nucleolus, and they are responsible for the synthesis and modification of rRNAs, tRNAs and snRNAs themselves.

The main function of the snRNA, processing of the pre-mRNA, is actually performed along with SM proteins (Sec1/Munc18-like proteins). Together, they form the complexes known as snRNPs (small nuclear ribonucleoproteins). More snRNPs join together to form the spliceosome – complex which actually process the primary transcript of RNA into mature messenger RNA. SnRNAs involved in the processing of pre-mRNA are U1, U2, U4, U5 and U6.

Splicing of pre-mRNA

Spliceosome basically removes the introns (non-coding parts of the DNA/RNA) and ligates the exons (coding parts) back together. It performs this by recognizing specific sequences on the transcript. These are 5’ end splice, 3’ end splice, branch point and polypyrimidine tract.

U1 snRNA attaches the first to the GU nucleotide sequence at the 5’ end splice site, following by U2, which attaches to the branch point. After the U4 and U6 attach, U1 gets removed. U5 also attaches a bit more upstream. U4, U5, and U6 together form the lariant (loop) form of the intron, and they cut it out acting as nuclease.

Alternative splicing

During the splicing of pre-mRNA, alternative splicing also occurs, which basically refers to the arranging of DNA exons in different ways (or completely omitting some) in order to get different mRNAs (and different proteins) from the same gene.