What is RNA?

Ribonucleic acid (RNA) is a nucleic acid present in all living cells that has structural similarities to DNA. Unlike DNA, however, RNA is most often single-stranded. An RNA molecule has a backbone made of alternating phosphate groups and sugar ribose, rather than the deoxyribose found in DNA. Attached to each sugar is one of four bases: adenine (A), uracil (U), cytosine (C), or guanine (G). There are different types of RNA that exist in cells: messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). In addition, some RNAs are involved in regulating gene expression. Certain viruses use RNA as their genomic material.

The types of RNA.

mRNA – transcribed from DNA and contains the genetic blueprint to make proteins. Prokaryotic mRNA does not need to be processed and can proceed to synthesize proteins immediately. In eukaryotes, a freshly transcribed RNA transcript is considered a pre-mRNA and needs to undergo the process of maturation to form mRNA. A pre-mRNA contains non-coding and coding regions known as introns and exons. During pre-mRNA processing, the introns are spliced, and the exons are joined together. A 5’ cap known as 7-methylguanosine is added to the 5’ end of the RNA transcript and the 3’ end is polyadenylated. Polyadenylation refers to the process where a poly(A) tail, which is a sequence of adenine nucleotides, is added to the transcript. The 5’ cap protects the mRNA from degradation, and the 3’ poly(A) tail contributes to the stability of mRNA and aids it in transport. Researchers are also studying mRNA as an anti-cancer treatment due to its ability to modify cells.

tRNA – RNA molecules that translate mRNA into proteins. They have a cloverleaf structure that consists of a 3’ acceptor site, 5’ terminal phosphate, D arm, T arm, and anticodon arm. The primary function of a tRNA is to carry amino acids on its 3’ acceptor site to a ribosome complex with the help of aminoacyl-tRNA synthetase. Aminoacyl-tRNA synthetases are enzymes that load the required amino acid onto a free tRNA to synthesize proteins. Once an amino acid is bound to tRNA, the tRNA is considered an aminoacyl-tRNA. The type of amino acid on a tRNA is dependent on the mRNA codon, which is a sequence of three nucleotides that codes for an amino acid. The anticodon arm of the tRNA is the site of the anticodon, which is complementary to an mRNA codon and dictates which amino acid to carry. tRNAs also regulate apoptosis by acting as a cytochrome c scavenger.

rRNA – a type of RNA that forms ribosomes, which are an essential element in protein synthesis. A ribosome contains a large and small ribosomal subunit. In prokaryotes, a small 30S and large 50S ribosomal subunit make up a 70S ribosome. In eukaryotes, the 40S and 60S subunits form an 80S ribosome. The ribosomes contain an exit (E), peptidyl (P), and acceptor (A) site to bind aminoacyl-tRNAs and link amino acids together to create polypeptides.