How many types of RNA are there?

Oh God, this might get a little confusing.

OK, first up, let's restrict this discussion to humans; there are certain types of RNA that are found only in plants or prokaryotes, so we'll ignore them.

Second, let's get going by listing the three main types of RNA - the types that you'll have to spit out in an exam question. They are:

• Messenger RNA (mRNA) - DNA is snobbish - it doesn't deal directly with amino acids to make a protein. If a gene wants to be transcribed, it first copies itself into RNA form, and this RNA form is known as mRNA.
• Transfer RNA (tRNA) - Transfer RNA does the clever job getting the right amino acid to the right spot on the mRNA. For instance, if the mRNA codes for the amino acid alanine, it must rely on tRNA to get it there. There are specific tRNAs for each amino acid. The alanine one will hold alanine with one hand, and then bind to the appropriate part of the mRNA with the other hand.
• Ribosomal RNA (rRNA) - Ribosomes are the protein factories of the cell - they are where the whole process of creating proteins out of mRNA takes place. Ribosomes are composed of rRNA and other proteins, but the rRNA accounts for the catalytic site that does this magic.

If you really want to know more, there are other types of RNA, each of which does something really cool. They are basically two groups in humans:

• RNA that regulate gene expression - Many of the RNAs in this category participate in the fascinating process of RNA interference. This probably evolved as a defence against viral infections, and is initiated when a microRNA (miRNA) strand binds to, and then helps destroy, a double-stranded RNA molecule (the latter characteristic of certain viruses). Other types of regulatory RNAs can down- or up-regulate gene transcription.
• RNAs that modify other RNA - the classic example here is shown when the introns are spliced out of the initial mRNA transcript. This process is mediated by splicosomes, which are a complex of proteins and small nuclear RNA (snRNA).