RNA Structure


Ribonucleic acid (RNA) is a nucleic acid. It is present in all living cells. It has structural similarities to DNA. However, RNA is most often single-stranded. An RNA molecule has a backbone consisting of alternating phosphate groups and the sugar ribose, in contrast to the deoxyribose found in DNA. One of the following four bases is attached to each sugar: adenine (A), cytosine (C), guanine (G), or uracil (U). Adjacent ribose nucleotide bases are chemically attached to one another via chemical bonds called phosphodiester bonds in the form of a chain. RNA is synthesized from DNA by RNA polymerase during transcription. The newly formed RNA sequences are complementary to their DNA counterparts. RNA is then translated into proteins by ribosomes. There are different types of RNA existing in cells: transfer RNA (tRNA), ribosomal RNA (rRNA), and messenger RNA (mRNA). Some of the RNAs are involved in the regulation of gene expression. Certain viruses use RNA as their genetic material.

RNA and its types
Schematic of RNA and its types


Messenger RNA (mRNA) was first described in 1956 by scientists Elliot Volkin and Lazarus Astrachan. It is a type of single-stranded RNA involved in protein synthesis. mRNA is obtained from a DNA template during transcription. mRNA carries protein information from the DNA in a cell’s nucleus to the cell’s cytoplasm. The mRNA sequence is read and each three-base codon is translated into its corresponding amino acid in a growing protein chain. Each mRNA molecule encodes the information for a protein, with the mRNA containing a sequence of three nitrogen-containing bases. The mRNA molecules are transported into the cytoplasm through the nuclear envelope, where they are translated by the rRNA of ribosomes. In prokaryotes,  mRNAs contain an exact transcribed copy of the original DNA sequence with a 3′-hydroxyl residue and a terminal 5′-triphosphate group. In eukaryotes, the mRNA molecules are more elaborate. The 5′-triphosphate residue is further esterified, thereby forming a cap. At the 3′ ends, eukaryotic mRNAs contain adenosine residues (polyA) that are added enzymatically after transcription. Eukaryotic mRNA molecules usually contain small segments of the original gene and are generated by cleavage and rejoining from an original precursor RNA  molecule. In general, prokaryotic mRNAs are degraded very rapidly. The cap structure and the polyA tail of eukaryotic mRNAs greatly increase their stability.

mRNA structure
Schematic of mRNA structure


Ribosomal RNA (rRNA) is a molecule in cells that forms part of the protein-synthesizing organelle known as a ribosome. The ribosome is exported to the cytoplasm, which helps in translating the information in mRNA into protein. In the cell nucleus called the nucleolus, which contains the genes that encode rRNA, rRNA molecules are synthesized. Each ribosome contains at least one small rRNA and one large rRNA. In the nucleolus, rRNAs combine with ribosomal proteins to form subunits of the ribosome. In eukaryotes, in a single cell, 10 million ribosomes and 50–5,000 sets of rRNA genes may be present. In prokaryotes, there are fewer sets of rRNA genes and ribosomes per cell.


tRNA structure
Schematic of tRNA structure

Transfer RNA (tRNA) is a small RNA molecule, and it plays a key role in protein synthesis. tRNA serves as a link between the mRNA molecule and the growing chain of amino acids that make up a protein. When an amino acid is added to the chain, a specific tRNA pairs with a complementary sequence of the mRNA molecules.  tRNA is a type of RNA molecule that helps decode an mRNA sequence into a protein. During translation, tRNA functions at particular sites in the ribosome. In this process, a protein is synthesized from an mRNA molecule. Proteins contain smaller units called amino acids. These are specified by three-nucleotide mRNA sequences called codons. Each codon represents a particular amino acid. Each codon is recognized by a specific tRNA. The tRNA molecule has a folded structure comprising three hairpin loops. These loops form a three-leafed clover shape. One of these loops contains an anticodon. The anticodon can recognize and decode an mRNA codon. To the end of each tRNA, a corresponding amino acid is attached. To the end of the growing amino acid chain, the tRNA transfers the appropriate amino acid, when in the ribosome, it recognizes and binds to its corresponding codon. The ribosome and tRNAs continue the decoding of the mRNA molecule, until the translation of the entire sequence into a protein.

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