RNA: definition, types, structure &function
In this article we will discuss about the RNA (Ribonucleic acid) :- (1) definition,(2) types, (3) structure, (4) function.
RNA (Ribonucleic Acid)
Ribonucleic acid is a nucleic acid containing ribose sugar. It is found large amount in cytoplasm and a lesser amount in a nucleus. In the cytoplasm it is mainly found association with the ribosome and in the Nucleus . It is mainly found in the nucleolous. RNA is formed of single strand. It consists of several units called Ribonucleotides. Hence each RNA molecule is formed of several nucleotide.
Each Nucleotide is formed of 3different molecules. Which are :-
I. Ribose sugar
II. Nitrogenous bases
III. Phosphoric acid
The Nitrogen bases are of 2types purine and Pyrimidine. The purine present in the RNA are Adenine and Guanine. The pyrimidine present in the RNA are Cytosin & Urasine.
The RNA molecules is normally single stranded sometimes the strand may be folded back upon itself and these double strand may be coiled to form a helical structure like that of DNA. Very rarely it’s double stranded in case of reo virus in animals and wound Tumour virus in plant.
Types of RNA:-
Depending upon the heridity information RNA may be of following 2types:-
I. Genetic RNA/Genomic RNA
II. Non genetic RNA/nongenomic RNA
I. Genetic RNA:-
The RNA act as a genetic material is called genetic RNA. Present in Ribovirus & viroids .
Genetic RNA is single stranded in most cases TMV, HIV, Influenza virus etc.
It is double stranded in wound, tumour, virus , reo virus & Rice dwarf virus.
These RNA molecule do not act as genetic material rather they are associated with protein synthesis. They are synthesize from DNA by the process of Transcription all the non-genetic RNA are single stranded & they are :-
1.messenger RNA (mRNA)
1.Messenger RNA :-
Messenger RNA is a ribonuclic acid, which carries genetic information for protein synthesis, from the DNA to the cytoplasm.
The term mRNA was coiled by Jackob. The mRNA forms 3-5% of the total cellular RNA. The mRNA is synthesised as a complimentary strand upon the chromosome DNA.
The mRNA present inside the Nucleus is called Heterogeneous nucleus RNA. In the cytoplasm mRNA act as templet for protein synthesis.
In lifespan of mRNA in bacteria is about 2minutes. In eukaryotes it lives for few hours to a few days.
Structure of mRNA:-
The mRNA is a single stranded polynucleotide chain each polynucleotide chain made up of Ribonucleotide. Each nucleotide contain a phosphoric acid a Ribose sugar and an nitrogenous base. In nitrogenous bases are Adenine, guanine, cytosine & uracil.
One and of the mRNA is called 5′ end and the other end is called 3′ end. In the 5′ end a cap is found in most eukaryotes & Animal viruses. The cap help the mRNA to bind with Ribosomes.
The cap also protein the mRNA from Hexonuclease. The cap is followed by non-coding region 1(NC-1).
It is also called 5′ untranslated region (UTR).
It does not contain code for protein and hence it can translate protein. It is formed of 10-100 nucleotide & rich A-U residues. The non-coding region is followed by initiation codon. It is made up of AUG. The initiation calon is followed by the coding region which contain the code for protein. It is an Average of 1,500 Nucleotides.
The coding region is followed by Termination codon. It complete the translation. It is made up of UAA Or UAG Or UGA in Eukaryotes.
The termination region is followed by non-coding region-2(NC-2).
At the 3′ of mRNA, there is a Polyadenylate sequence (poly-A) it consists of 200-250.Aldehylate Nucleotides (AAA). But as the Age increase the poly-A shorten . mRNA synthesis from DNA strand through the action of an enzymes called RNA polymerase is called Transcription.
Among RNA, mRNA is the longest one, most of the mRNA contain 900-15000 Nucleotides.
Transfer RNA(t-RNA) :-
The t-RNA is a ribonuclic acid which transfer the activated amino acid to the ribosome to synthesize proteins. It’s so small that it remain in the supernant during centrifugation hence it is also called soluble RNA Or supernatnt RNA. It serves as an adapter molecule to attach amino acid hence t-RNA is also called adaptor RNA. It Constitution about 10-15% of RNA of total weight of RNA of cells.
Structure of t-RNA:-
The t-RNA is made up of 73 to 95 Nucleotides units called Ribonuclotide units. Each Ribonuclotide units is made up of three components namely phosphoric acid a ribose sugar and Nitrogen base. The nitrogenous bases are Adenine,cytosine, uracil, guanine.
The t-RNA is the form of single polynucleotide chain having 3′ & 5′ end. The polynucleotide or the chain folded on it self and attain the shape of cloverleaf. The 3′ and 5′ end of t-RNA lie side by side as a result of folding the 3′ end always ends in CCA base sequence. This is the site for the attachment of activated AA. The 5′ end terminates in G&C.
The t-RNA has 5 arms they are:-
a) Amino acid acceptor arm
c) Anticodon Arm
d) Variable arm
e) Thymine Ψ cytosipearm ( τ Ψ c arm)
The t-RNA pickup a specific activated amino acid full in the cytoplasm. Then the AA is transfered to the Ribosome in the cytoplasm where protein are synthesized finally. It transmit amino acid in to new polypeptide chain.
iii) Ribosomal RNA r-RNA:-
Ribosomal RNA is the Ribonucleic acid present in the Ribosome and hence it is called ribosomal RNA. It is also called Insoluble RNA. It constituent about 80% of total cellular RNA.
The ribosomal RNA is formed of single strand. It is a polynucleotide chain. Each strand is formed of many Nucleotide chain. Each nucleotide chain contain a phosphoric acid a ribose sugar & a nitrogenous sugar. The nitrogenous bases are A, G, C & V.
Each strand has 5′ end 3′ . In some region the single strand is twisted upon itself to form double helix. The helical region are connected by intervening still single stranded regions. In a helical region most of the base pairs are complimentary. They are joint by hydrogen bond.
In the unfolded single strand region the base pair are not complimentary.
The r-RNA are classify into following 7types according to their sedimentation co-efficient.
a) 28s r-RNA
b) 18s r-RNA
c) 5.8s r-RNA
d) 5s r-RNA
e) 20s r-RNA
f) 16s r-RNA
g) 55s r-RNA
All these 28s, 18s, 5.8s, 5s r-RNA are existing in Eukaryotes cells. 23s, 16s, 5s r-RNA are exist in prokaryotes.
In Eukaryotes ribosomes, the large 60s ribosomal subunit contain 28s r-RNA, 5.8s , 5s r-RNA.
The small 40s ribosomal subunits contain 18s r-RNA. In prokaryotic ribosomes, the large 50s subunits contain contain 23s r-RNA and 5s r-RNA.
The small 30s ribosomal subunits contain 16s r-RNA.
The r-RNA synthesize by the DNA by process of transcription. The r-RNA play the measure role in. Protein synthesis in Ribosome.