There are three major types of Ribonucleic acid or RNA
- Messenger RNA
- Ribosomal RNA
- Transfer RNA
Messenger RNA (mRNA) is a molecule of RNA that encodes a chemical “blueprint” for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis, the ribosomes. In the ribosomes, the mRNA is translated into a polymer of amino acids: a protein. (This process is sometimes referred to as the central dogma of molecular biology.)
As in DNA, mRNA genetic information is encoded in the sequence of nucleotides, which are arranged into codons consisting of three bases each. Each codon encodes for a specific amino acid, except the stop codons, which terminate protein synthesis. This process of translation of codons into amino acids requires two other types of RNA: Transfer RNA (tRNA), that mediates recognition of the codon and provides the corresponding amino acid, and ribosomal RNA (rRNA), that is the central component of the ribosome’s protein-manufacturing machinery.
Ribosomal ribonucleic acid (rRNA) is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity. The tRNAs bring the necessary amino acids corresponding to the appropriate mRNA codon.
Transfer RNA (tRNA) is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the four-letter genetic code (ACGU) in messenger RNA (mRNA) with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by considering its three-dimensional structure. One end of the tRNA carries the genetic code in a three-nucleotide sequence called the anticodon. The anticodon forms three base pairs with a codon in mRNA during protein biosynthesis. The mRNA encodes a protein as a series of contiguous codons, each of which is recognized by a particular tRNA. On the other end of its three-dimensional structure, each tRNA is covalently attached to the amino acid that corresponds to the anticodon sequence. This covalent attachment to the tRNA 3’ end is catalyzed by enzymes called aminoacyl-tRNA synthetases. Each type of tRNA molecule can be attached to only one type of amino acid, but, because the genetic code contains multiple codons that specify the same amino acid, tRNA molecules bearing different anticodons may also carry the same amino acid.
During protein synthesis, tRNAs are delivered to the ribosome by proteins called elongation factors (EF-Tu in bacteria, eEF-1 in eukaryotes), which aid in decoding the mRNA codon sequence. Once delivered, a tRNA already bound to the ribosome transfers the growing polypeptide chain from its 3’ end to the amino acid attached to the 3’ end of the newly-delivered tRNA, a reaction catalyzed by the ribosome.
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).