RNA Structure, Types [mRNA, tRNA, rRNA], and Functions

What is RNA?

Unlike double-stranded helical structure of DNA, the RNAs are single-stranded. RNA is an unbranched linear polymer of ribonucleotides joined by 3′, 5′ phosphodiester bonds. The phosphodiester bonds join with the 3′- OH group of ribose of one nucleotide group to the 5′- OH group of ribose sugar of the next nucleotide. Connected to each sugar is one of four bases– adenine (A), uracil (U), cytosine (C), or guanine (G).

Types of RNA

The cell consists of three significant types of RNA:

  1. Messenger RNA (mRNA)
  2. Transfer RNA (tRNA)
  3. Ribosomal RNA (rRNA)

All of these are associated with the process of protein biosynthesis. Each varies from the others by size and function.

Messenger RNA (mRNA)

The mRNA makes up just about 5-10% of overall cellular RNA.mRNA is manufactured in the nucleus as heterogenous RNA (hnRNA), which are processed into functional mRNA.

The mRNA carries the hereditary information in the form of codons. Codons are a group of three surrounding nucleotides that code for the amino acids of the protein. In eukaryotes mRNAs have some distinct characteristics, e.g. the 5′ end of mRNA is “capped” by a 7- methyl-guanosine triphosphate. The cap is associated with the recognition of mRNA in protein biosynthesis and it helps to stabilize the mRNA by avoiding the attack of 5′- exonucleases.

A poly (A) “tail” is connected to the other 3′- end of mRNA. This tail includes a series of adenylate residues, 20-250 nucleotides in length joined by 3′ to 5′ phosphodiester bonds. The function of poly A tail is not completely understood, but it seems that it assists to support mRNA by avoiding the attack of 3′- exonuclease.

Structure of mRNA


Function of mRNA

mRNAs work as a design template for protein biosynthesis and transfer hereditary information from DNA to protein manufacturing machinery. If the mRNA codes for only one peptide, the mRNA is monocistronic. If it codes for two or more different polypeptides, the mRNA is polycistronic. In eukaryotes most mRNA is monocistronic.

Transfer RNA (tRNA)

Transfer RNA molecules differ in length from 74 to 95 nucleotides. In eukaryotic cells, 10-20% of the nucleotides of tRNA may be modified and known as unusual nucleotides. Some of them are:

  • Dihydrouridine (D), in which one of the double bonds of the base is reduced.
  • Ribothymidine (T), in which the methyl group is added to uracil to form thymine.
  • Pseudouridine (ψ), in which uracil is connected to ribose by a carbon-carbon bond rather than a nitrogen bond.


Structure of tRNA

All single-stranded transfer RNA molecules get folded into a structure that resembles a cloverleaf. All tRNAs include 4 main arms:

  1. The acceptor arm
  2. The D arm
  3. The anticodon arm
  4. The TψC arm

The arms have base-paired stems and unpaired loops. The structure of tRNA molecules is preserved by the base-pairing in these arms or stem regions.

  1. The acceptor arm

The acceptor arm consists of a base paired stem that ends in the sequence CCA at the 3′ end. This is the attachment site for the amino acid.

  1. The D arm

The D arm is named for the existence of the base dihydrouridine (D).

  1. The anticodon arm

The anticodon arm consists of the anticodon that base pairs with the codon on mRNA. Anticodon has a nucleotide sequence complementary to the codon of mRNA and is accountable for the uniqueness of the tRNA.

  1. The TψC arm

The TψC arm contains both ribothymidine (T) and pseudouridine (ψ, psi).

The additional arm is likewise referred to as a variable arm because it varies in size, is found between the anticodon and TψC arms.


Function of tRNA

Transfer RNA carries amino acids in an activated type to the ribosome for protein synthesis.

Ribosomal RNA (rRNA)

The RNA of the ribosomes is called the rRNA. A ribosome is a cytoplasmic nucleoprotein that functions as a machinery for the synthesis of proteins. The ribosome is a spheroidal particle and is composed of a big and a small nucleoprotein subunit. The eukaryotic ribosomes are composed of 60S and 40S subunits. Each subunit is made up of several strands of rRNA and many protein molecules. The 60S subunit contains 28S rRNA, 5S rRNA, and 5.8 S rRNA, while the 40S subunit contains 18S rRNA.


Functions of ribosomal RNA

The function of the ribosomal RNA molecules in the ribosomal particle are not totally comprehended, but they are:

  • Required to keep the ribosomal structure and also take part in protein synthesis by binding of mRNA to ribosome.
  • Recent studies reveal that ribosomal RNAs may likewise supply a few of the catalytic activities and hence is an enzyme “a ribozyme”.
Other Nuclear and Cytoplasmic RNAs

Besides mRNA, tRNA, and rRNA, eukaryotes have some other RNAs. These are:

– Heterogenous RNAs (hnRNAs)

– Small cytoplasmic RNAs (scRNAs)

– Small nuclear RNAs (snRNAs)

Functions of RNA

Overall functions of RNAs are:

  1. Carries the hereditary information from DNA to the cytosol, where it is utilized for protein synthesis.
  2. Acts as an “adaptor” molecule that brings particular amino acid to the site of protein synthesis.
  3. In association with protein functions as the sites for protein synthesis. Provides catalytic activities (peptidyl transferase activity).
  4. Works as a precursor for mRNA.
  5. Associated with recognition of signal sequence in protein synthesis on membrane-bound ribosomes.
  6. Associated with excising introns and splicing exons.