Ribonucleic acid (RNA) is a polymer of ribonucleotides composed of ribose sugar and nitrogen bases Adenine (A) Guanine (G), Cytosine (C) and Uracil (U) which are joined  together by a phosphodiester bond.

Occurrence: RNA is synthesized in in nucleus but it is found in nucleolus,  mitochondria,  chloroplast, nissil granules and cytoplasm and on the membranes of ribosomes. RNA also acts as a material for some viruses.

Structure of RNA

• RNA is  also a macromolecule however, it is is comparatively smaller than DNA.


• RNA is single stranded and formed of polynucleotide chain


• It is formed of several number of nucleotides arranged in in linear sequence and connected by 3' to 5' phosphodiester bonds.


• Each nucleotide consists of a ribose sugar phosphate and one of the four nitrogen bases nucleotides of RNA are called ribonucleotides.

Types of RNA

On the basis of presence of genetic mechanism RNA are of two types:

• Genetic RNA


• Non- genetic RNA

Genetic RNA 

It acts as a genetic material and is found in most of the the plant viruses some animal viruses and certain bacteriophages. Genetic RNA is found in the absence of DNA and has self-replication property.

Non genetic RNA

Non genetic RNA is found in all organisms except some viruses and occurs in association with DNA. Such RNA has no role in hereditary mechanism but it helps in biosynthesis of proteins hence are non genetic.

On the basis of molecular size and functions they are three major types of non genetic RNA they are

• Ribosomal RNA (r-RNA)


• Messenger RNA (m- RNA)


• Transfer RNA (t- RNA)

Ribosomal RNA (r- RNA)

• It is the most abundant and stable type of RNA present in ribosome.


• It it comprises 70 to 80% of cellular RNA.


• It is synthesized from DNA in nucleolus by transcription in the presence of RNA polymerase-I 


• r-RNA associates with ribosomal proteins to form complete functional ribosome and plays a key role in in binding of m- RNA to ribosome during its translation.

Messenger RNA (m-RNA)

• It is is also called nuclear RNA and is most heterogeneous in size and stability.


• It constitutes 3 to 10% of total cellular RNA.


• m RNA is formed in in nucleus with the help of DNA during the process of transcription.


• It carries the the genetic information specifying the the amino acids in a protein to ribosomes. It is the only type of RNA that can be translated into protein.

Transfer (t- RNA)

• It is the second most stable type of RNA and comprises 10 to 15% of total cellular RNA.


• Smallest in molecular structure compared to other two RNA.


• It works as a a adaptor molecule for carrying amino acids to the site of protein synthesis.

Functions of RNA 

• RNA acts as  a genetic material for some viruses such as Tobacco Mosaic Virus (TMV) and Human Immune Virus (HIV).


• Non genetic RNA plays a great role in protein biosynthesis.
  

Differences between DNA and RNA