Ribonucleic Acid: Definition, Types, & Examples

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Ribonucleic Acid Definition

Ribonucleic acid is a type of nucleic acid that is comprised of a ribose sugar, phosphate group, and nitrogenous base. The structure of RNA is single-stranded and folded onto itself.

What is Ribonucleic Acid?

A group of complex compounds made up of linear chains of monomeric nucleotides is referred to as nucleic acid. A nucleotide is composed of phosphoric acid, sugar, and a nitrogenous base.

The major functions of nucleic acid are preservation, replication, and the expression of hereditary information. Nucleic acid is classified into two types: deoxyribonucleic acid and ribonucleic acid.

History of Ribonucleic Acid

Nucleic acid was first discovered by Friedrich Miescher in 1868. He was the first person who isolated a biomolecule from nuclei of white blood cells which is neither a protein, nor a lipid, nor a carbohydrate. The compound is named nuclein by Friedrich Mischer. Later, a German chemist Albrecht Kossel discovered the acidic properties of the compound. The term nucleic acid was first coined by a German pathologist, Richard Altman in 1889.

Earlier, DNA and RNA were not distinguished. Both are termed nucleic acids based on their location inside the cell. Later, it was found that RNA is different from DNA in some cases. For example, DNA comprises deoxyribose sugar whereas RNA is made of the ribose sugar.

In RNA the thymine base is also replaced by uracil. The double-helical structure of DNA was first proposed by Watson and Crick. They also credited or central dogma of molecular biology which describes that the DNA forms the RNA, which leads to the formation of proteins.

Ribonucleic Acid Structure

An RNA is made up of a long linear chain of nucleotides. The nucleotide is the monomeric unit that is comprised of a phosphate group, ribose sugar, and a nitrogenous base. The carbons of ribose sugar are numbered 1’ through 5’, which is a pentose sugar. The 1’ carbon of ribose sugar has connected to the nitrogenous base and the 3’ carbon of ribose has connected to the phosphate group. The nitrogenous bases of RNA are cytosine, guanine, adenine, and uracil.

Types of Ribonucleic Acid

There are various criteria for the classification of RNA. For example, based on the length of the chain, RNA is divided into small RNA and long RNA. RNA chains comprised of 200 or fewer chains are termed small RNA whereas RNA consisting of more than 200 chains is termed as long RNA. The tRNA, 5S rRNA, microRNA, siRNA, srRNA, piRNA are some examples of small RNA. Examples of long RNAs are m RNA and long non-coding RNA.

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RNA is further classified as coding RNA and non-coding RNA. An example of coding RNA is mRNA that translated into a protein. The non-coding proteins include rRNA, microRNA, siRNA, piRNA, snoRNA, snRNA, exRNA, and scaRNA, etc. These RNAs cannot be translated into proteins.

i. mRNA

The prokaryotes produce mRNA by splicing a large primary transcript from a DNA sequence. In eukaryotes, the production of mRNA occurs in the nucleus during transcription. To become mature, the newly formed mRNA is processed extensively, which includes the following steps: (1) addition of 5’ cap at the 5’ end and a sequence of adenylate groups at the 3’ end (2) removal of introns (3) splicing together of exons. The completely processed RNA is called mature mRNA. The mature mRNA is transported into the cytoplasm through a nuclear pore.

ii. tRNA

Transfer RNA or tRNA plays an important role in the transportation of specific amino acids to the ribosome. These amino acids are then added to the growing chain of amino acids. The tRNA molecule is comprised of two major regions: (1) trinucleotide region containing the anticodon (2) attachment side for a specific amino acid.

iii. rRNA

Ribosomal RNA or rRNA involves in the formation of ribosomes together with proteins. A ribosome is a cytoplasmic organelle that is also known as protein factories of the cell. It has a major function in protein synthesis.

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The ribosome consists of three binding sites including A, P, and E sites. The tRNA binds at the A site of the ribosome. The P site of the ribosome carries the peptidyl tRNA. The E site is the site for deacylated tRNA on transit out from the ribosome. The ribosomal RNA is found in the form of two subunits: large subunits and small subunits that collectively form the ribosome. Ribozyme is the larger subunit that also acts as an enzyme.

Ribonucleic Acid Synthesis

The ENA is synthesized by the process of transcription. Transcription can be defined as a process of copying a segment of DNA by the enzyme RNA polymerase. The DNA unwinds by enzyme helicase then, enzyme RNA polymerase works along the direction of 3’to 5’. After formation, the newly formed RNA is processed by removing introns, and by adding a poly (A) tail.

Ribonucleic Acid Degradation

The nucleic acids such as RNA produce purines, pyrimidines, phosphoric acid, and pentose sugar, either D-ribose or D-deoxyribose after degradation.

Protein Synthesis

The proteins are synthesized by the process of transcription and translation. Transcription is the process of producing mRNA. It is followed by the translation that completes in the cytoplasm of the cell.


The process of producing mRNA from a DNA template is called transcription. The process use enzyme RNA polymerase that only transverse the DNA template strand from 3’ to 5’. There are several differences in DNA replication and transcription.

For example, instead of DNA replication, the process does not require a primer and it also replaces thymine with uracil. The transcription occurs in the cytoplasm of prokaryotes whereas, in eukaryotes, it completes in the nucleus.


The process of protein biosynthesis from mRNA molecules is called translation. The translation is followed by the process of transcription and completes in the cytoplasm of the cell. It consists of four phases:

(1) Bioactivation, include binding of correct tRNA with correct amino acid.

(2) Initiation, consists of binding of the small subunit of the ribosome to 5’end of mRNA.

(3) Elongation, include binding of next aminoacyl-tRNA to the ribosome along with GTP.

(4) Termination, A site of ribosome faces a stop codon.

Biological Importance of Ribonucleic Acid

RNA or ribonucleic acid has various important roles in a living being. The major roles of RNA include (1) protein synthesis, (2) post-translational modification or DNA replication, and (3) gene regulation. RNA also acts as genetic material in some organisms like RNA viruses. Examples of RNA viruses are Ebolaviruses, SARS coronavirus, rhinovirus, measles virus, influenza virus, etc.

The ribosomal RNA is used as a base in taxonomy and evolution. They are used in the identification of the taxonomic group of an organism and also used to estimate the rate of species divergence.

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