RNA (Ribonucleic Acid) : Definition, types, Structure

RNA (Ribonucleic Acid) 

RNA is found chiefly in the cytoplasm and in the nucleolus. Inside the cytoplasm it occurs freely as well as in the ribosomes.

 ➤  RNA is a single stranded structure consisting of an unbranched polynucleotide chain, folded back on itself forming helix. DNA is a double stranded structure and its two polynucleotide chains are would spirally around a main axis. 

1.  RNA, like DNA, is made up of many hundreds or thousands of nucleotides that are arranged in a linear sequence and linked together by 3' - 5' phosphodiester bonds. 

2.  The sugar found in the nucleotides of RNA is ribose, where as it is deoxyribose in DNA. The nucleotides of RNA are ribonucleotides. The four nitrogenous bases found in RNA are adenine, cytosine, guanine and thymine. 

3.  Therefore in RNA thymine of DNA is substituted by uracil. The base composition of RNA does not agree to the AU = GC = 1 as it is found in DNA. 

4.  Intermolecular pairing between nucleotides of a single strand of RNA provides stability to RNA. In DNA nucleotides of two strands of DNA pair through hydrogen bonds. 

5.  DNA is the hereditary material, whereas RNA is of different types performing different functions during protein synthesis. In most of the plant viruses and some animal viruses RNA acts as genetic material. 

6.  When DNA is the hereditary material, different types of RNA are non-genetic. They are synthesized from DNA template during protein synthesis.

Read more - Satellite DNA

Types of RNA
RNA may be of two types:

1. Genetic RNA
2. Non genetic RNA

1. Genetic RNA or Vital RNA - In several plant or animal, viruses, and in bacteriophages the DNA is not found and RNA acts as hereditary material. This type of RNA may be single or double stranded. For example - In Tobacco mosaic virus, Influenza virus, F₂ bacteriophage the RNA is single stranded and in wound tumour virus, Reo virus the RNA may be double stranded.

2. Non genetic RNA - In those plants and animal where DNA is the hereditary material the RNA is of non genetic type. These non genetic RNA may be of the following types:

(i) Messenger RNA or nuclear RNA (m RNA)
(ii) Ribosomal RNA (r RNA)
(iii) Transfer RNA (t RNA)

(i) Messenger RNA 

Messenger RNA (mRNA) is synthesized as a complementary strand and carries genetic information from chromosomal DNA to the cytoplasm for the synthesis of proteins. This species of RNA is rather short-lived and therefore has rapid turnover. The molecular weight of this RNA varies, typically ranging from 500,000 to 2,000,000.

It has following characteristics -

1.  It constitutes about 5-10% of the total RNA of the cell. m-RNA is synthesized from the DNA, which contains message for the formation of proteins.

2.  It is synthesized as a complementary strand of the DNA. This process of RNA formation is called Transcription.

3.  As RNA is copied from DNA, it carries the same genetic information as contained by the DNA.

4.  Eukaryotic mRNA is more stable and compact than prokaryotic mRNAs.

5.  However, in eukaryotes, mRNA initially is highly unstable and heterogeneous (having coding and non coding regions). It is thus called Hn-RNA (heterogeneous RNA).

6.  Hn-RNA, then undergoes a process called RNA processing to become highly stable and compact. In RNA processing, the non coding regions (introns) are selectively removed off from the mRNA. Both ends also are stabilized. After RNA processing eukaryotic mRNA becomes stable and functional.

7.  m-RNA in cytoplasm acts as template for the synthesis of proteins.

8.  Eukaryotic m-RNA are noncistronic (i.e., having only one protein synthesis initiating site or a single mRNA can be used for the synthesis of only one protein) while the prokaryotic mRNAs are polycistronic (having more than one such sites, i.e. a single mRNA can be used for the synthesis of more than one protein)

(ii) Ribosomal RNA (rRNA) 

Ribosomal RNA is the most stable kind of RNA and is associated with the ribosomes of which this RNA species makes 40%-60% by weight. This species of RNA also makes about 80% of the total RNA in the cell.

It has following characteristics-

1.  5S rRNA is common to both eukaryotes and prokaryotes. In eukaryotes all rRNAs are synthesizes inside the nucleolus except the 5 S rRNA, which is synthesized outside the nucleolus (but inside the nucleus) 5S rRNA therefore has an extra-nucleolar origin.

2.  In prokaryotes, all rRNAs are synthesized in cytoplasm from the DNA (as there is no nucleus).

3.  rRNA molecules are highly methylated and are packaged with the specific ribosomal proteins.

4.  rRNA has a high degree of secondary structure. About 70% of it, becomes double stranded and helical due to intra-molecular base pairing.

5.  These double stranded regions from hair pin loops between the complementary regions of the same linear r-RNA molecules.

6.  rRNA provides a 3-D matrix to which various enzymes of the protein synthesizing machinery bind, in an orderly fashion.

(iii) Transfer RNA (t RNA) 

tRNA also known as soluble RNA (sRNA), makes another small fraction (10-15%) or RNA. These are the smallest molecules of RNA and work as adapter smallest molecules of RNA and work as adapter molecules for carrying amino acid molecules to the site of protein synthesis. Being the smallest in size, these molecules have been intensively investigated.

It has following characteristics -

1. tRNA molecules are smallest containing 75 - 80 nucleotides.

2. Its polynucleotide chain is bent in the middle and folded back on itself (clover leaf model) and the two arms are coiled over one another.

3. Some bases of the two arms of tRNA molecule exhibit intra-molecular base pairing.

4. The 3' end of the polynucleotide chain ends in the CCA base sequence. It represents the site for the attachment of active amino acids. The 5' end of the chain ends with the guanine base.

5. The bend in the chain of each t RNA molecule forms anticodon loop. It contains a definite sequence of three nitrogenous bases, which constitute the anticodon. It recognizes codons on mRNA.

6. tRNA molecules are synthesized at particular regions of the DNA.

7. The tRNA plays key role in protein synthesis.

In conclusion, RNA is a vital molecule in the field of molecular biology, playing a central role in the transfer of genetic information and controlling cellular processes. The study of RNA has led to the development of new therapeutic approaches and has opened up exciting opportunities for advancing medical research.