By Dr Ananya Mandal, MD
There are various types and shapes of RNA. For example, the messenger RNA (mRNA) carries the information from the DNA to the ribosome in the cytoplasm where the actual protein synthesis (translation) takes place.
Types of RNA can be outlined as:
Messenger RNA (mRNA)
This is formed by the process of transcription. The DNA strand acts as a template in formation of this RNA strand. The mRNA carries the genetic code from the DNA into the cytoplasm to the ribosomes for translation of the RNA into a protein or polypeptide strand.
This helps the mRNA and the transfer RNA to come together to form the polypeptide chain.
Transfer RNA or tRNA
There are at least 20 species of tRNA and these act in bringing the bases to form the amino acids of the polypeptide chain.
non-coding RNAs or ncRNA
These are encoded by RNA genes and also derived from mRNA introns.
Transfer-messenger RNA (tmRNA)
This is found in many bacteria and plastids. These tag the proteins encoded by mRNAs that lack stop codons for degradation and prevents the ribosome from stalling.
Ribozymes (RNA enzymes)
Some RNAs are enzymes. It was widely believed for many years that only proteins could be enzymes. RNAs are now known to adopt complex tertiary structures and act as biological catalysts. Such RNA enzymes are known as ribozymes, and they exhibit many of the features of a classical enzyme, such as an active site, a binding site for a substrate and a binding site for a cofactor, such as a metal ion.
One of the ﬁrst ribozymes to be discovered was RNase P, a ribonuclease that is involved in generating tRNA molecules from larger, precursor RNAs. RNase P is composed of both RNA and protein; however, the RNA moiety alone is the catalyst.
These RNAs serve to regulate the process of gene expression. MicroRNAs (miRNA; 21-22 nt) for example are found in eukaryotes and act through RNA interference (RNAi). These miRNA and enzymes can break down mRNA which the miRNA is complementary to. This can block the mRNA from being translated, or accelerate its degradation.
On the other hand small interfering RNAs (siRNA; 20-25 nt) are often produced by breakdown of viral RNA, there are also endogenous sources of siRNAs. They act similar to miRNA. An mRNA may contain regulatory elements itself, such as riboswitches, in the 5' untranslated region or 3' untranslated region; these cis-regulatory elements regulate the activity of that mRNA.
RNAs in RNA tertiary structure processing
Many RNAs are involved in modifying other RNAs. For example, introns are spliced out of pre-mRNA by spliceosomes, which contain several small nuclear RNAs (snRNA).
RNA as genetic material or RNA genomes
Like DNA, RNA can carry genetic information. This is seen in many RNA viruses.
RNA in reverse transcription
Some viruses like HIV have RNA as their genetic material that copies into DNA in a reverse transcription manner. These DNA copies are then transcribed to new RNA.
These are called dsRNA where the RNA has two complementary strands, similar to the DNA found in all cells. dsRNA forms the genetic material of some viruses (double-stranded RNA viruses).
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)