Ribonucleic acid, known by its acronym RNA, is a sequence of chemical code complementary to the more famous DNA. Similar to DNA, four nucleotides make up the RNA sequence – adenine (A), uracil (U, the counterpart to thymine, T, in DNA), cytosine (C), and guanine (G). However, unlike DNA, RNA exists in multiple forms based on its structure, function, and size. Though traditionally taught to occur as three types that function in gene expression, recent years have seen the discovery of other types of RNA whose roles in the cell are still being studied. RNA can be separated by size based on sedimentation rate, denoted as S.
Messenger RNA (mRNA)
The function of mRNA is clear from its name – it acts as a messenger within the cell. This form of RNA is single-stranded and created in the cell nucleus during transcription. The DNA sequence is scanned by the transcriptional machinery (proteins) to create a complementary RNA sequence, so its size is dependent on the length of the gene being encoded, as well as post-transcriptional processing that removes noncoding regions, such as introns.
This relatively small (compared to DNA) nucleic acid is able to travel to the cytosol for protein synthesis. In humans, each mRNA contains the code for a specific gene, which allows a specific protein to be made by assembling amino acids according to the codon sequence. A codon consists of three nucleotides, with each potential variation of A, U, C, and G representing start, stop, or an amino acid (which is shown in tabular form via Elmhurst College).
Transfer RNA (tRNA)
The function of tRNA is to transport and transfer amino acids to the growing protein made from the mRNA sequence. The molecule is relatively small (~4S) and double-stranded to itself, appearing cloverleaf-shaped in two dimensions but L-shaped in three dimensions as shown in this comparison schematic. A different tRNA exists for each amino acid, allowing specificity in protein translation, with approximately 32 different tRNA being found in a typical eukaryotic cell. Translation is the process of mRNA being translated into a protein – ultimately creating the gene product originally coded for by DNA.
At the middle loop of the tRNA molecule, represented as the bottom in visualizations, is an anticodon – a nucleotide triplet that binds a complementary codon on the mRNA. At the “top” of the molecule, specifically its 3’ (three prime) end, the tRNA binds a specific amino acid. A protein-RNA complex called a ribosome directs the placement of the amino acid when the tRNA and mRNA bind.
Ribosomal RNA (rRNA)
There are four kinds of rRNA, which act as subunits of ribosomes. The 18S rRNA binds to approximately 30 different proteins to make up the small ribosomal subunit (40S in eukaryotes). The 28S, 5.8S, and 5S rRNA bind to even more proteins to make the large ribosomal subunit (60S in eukaryotes). Ribosomes are found in the cytoplasm and provide the means for mRNA to be read by tRNA and for amino acids to bind to create proteins.
Small nuclear RNA (snRNA) and small nucleolar RNA (snoRNA)
The various machinery involved in mRNA processing, such as spliceosome removal of introns and alternative splicing, are created by snRNA or snoRNA binding to proteins. These small (<300 nucleotides) sequences are present in the nucleus or nucleolus to assist in a number regulatory activities. One form of snRNA is used as the template for telomere synthesis during DNA replication. In fact, these small RNA are so important that a mutation in a gene for one snRNA has been shown to have severe physiological consequences due to disrupted RNA splicing (Science, 2011).
Micro RNA (miRNA)
Micro RNAs are short (22 nucleotide), single-stranded sequences identified in C. elegans several years ago. Humans have been found to generate about a thousand such small sequences of 19 to 25 nucleotides, with some cell and tissue specific expression. Their exact functions are not yet known, but therapeutic uses have been pursued.
Other small RNAs used in lab settings are short hairpin RNA (shRNA) and the double-stranded version of miRNA called small interfering RNA (siRNA).
