Ribonucleic Acid

2)Ribonucleic Acids(R.N.A):

R.N.A is a type of nucleic acid to appear first on earth during the origin of life. It is formed by the polymerization of few to several hundred ribonucleotides. It is a macromolecule which is smaller than the D.N.A having the molecular weight of about 20,000-2,000,000 daltons.

Location:

It is found in the nucleolus of nucleus, ribosomes, mitochondria, chloroplast and cytoplasm of eukaryotic cells and cytoplasm and ribosomes of prokaryotic cells. Moreover, it is also present in some viruses where it acts as a genetic material.

Structure:

The structural features of R.N.A :

1) R.N.A is a single stranded non-helical nucleic acid.

2)It is also known as polyribonucleotides as it is formed by the polymer of few to several hundred ribonucleotides by a phosphodiester bond.

3)Each ribonucleotide has got phosphate group, ribose as pentose sugar and nitrogen base like in a D.N.A.

4)The nitrogen bases such as Adenine(A) and Guanine(G) of purine and Cytosine(C) and Uracil(U) of pyrimidine are present. The Thymine of pyrimidine is altogether absent.

5)The strand or backbone of R.N.A is formed by ribose sugar and phosphate group by phosphodiester bonds.

6)The proportion of A+G ≠ C+U as they are unpaired to each other.

Functions-

The functions of R.N.A:

1)It helps in the synthesis of polypeptide chain or protein.

2)In some viruses, R.N.A acts as a heredity material.

Types of R.N.A

On the basis of molecular size and functions, R.N.As are of three types:

A)Ribosomal Ribonucleic Acid (rRNA)

B)Messenger Ribonucleic Acid(mRNA)

C)Transfer Ribonucleic Acid(tRNA)

A)Ribosomal Ribonucleic Acid (rRNA):

I) Ribosomal Ribonucleic Acid(rRNA) us the stable and abundant RNA consisting of 70-80% total cellular RNA in a cell.

II)It has 40,000-1,000,000 dalton molecular weight.

III)It is found in the ribosomes of the cells. It is synthesized from DNA in the nucleus by transcription in the presence of RNA polymerase-I enzyme. Then it is transferred to the cytoplasm through the nuclear pore of the nuclear membrane.

IV)In the cytoplasm, rRNA is associated with the ribosomes

• Functions:

a)It helps in the formation of ribosomes for protein synthesis.

B)Messenger Ribonucleic Acid(mRNA)

I) It is a long RNA that constitutes 3-10% of the total cellular RNA. It is short-lived and unstable.

II)Its molecular weight is 500,000-1,000,000 daltons.

III)It is formed within the nucleus from D.N.A in the presence of enzyme RNA polymerase-II.

IV)It carries the genetic information from D.N.Afor the synthesis of specific proteins.

V)It combines with ribosomes in the cytoplasm to form polyribosomes.

VI)It has methylated region at 5'end and functions as a cap for the attachment with ribosomes. Cap is followed by a non-coding region and then to the initiation codon AUG or GUG to initiate the protein synthesis.

VII)At the 3'end there is a poly-A region having one of the three termination codons UAA or UAG or UGA after a short non-coding region.

VII)In between the initiation and termination codons, there are many codons that represent the coding regions for the synthesis and elongation of proteins,

•Functions:

a)mRNA carries genetic information from DNA(genes) to the site of protein synthesis for the formation of polypeptide chain or proteins.

b)It can form compact genes through reverse transcription which is used in genetic engineering.

C)Transfer Ribonucleic Acid(tRNA)

I) It constitutes 10-20% of the total cellular R.N.A having molecular weight 25,000-40,000 daltons.

II)It is short-lived and unstable.

III)It is synthesized from D.N.A inside the nucleus in the presence of enzyme RNA polymerase III during transcription.

•Funtions:

a)It functions as the adaptor molecule that helps in transferring activated specific amino acid from the amino acid pull, of the cytoplasm to the ribosomes for protein synthesis.

•Structure:

The tRNA becomes twisted and folds itself due to the modification of some ribonucleotides and forms clover leaf shape, i:e two-dimensional structure. tRNA has 5 important sites, which are as follows:

1) The 3'end of tRNA opposite to anticodon site contains CCA-OH. It is the specific binding site for the attachment of specific amino acid. It is also called AA site or acceptor site.

2)The first loop consists of dihydrouridine or DHU loop and consists of 8-12 unpaired bases. This is the binding site for the enzymes, aminoacyl synthetase necessary for translation.

3) The second or central loop is called anticodon loop and has got 7 unpaired bases. Out of these bases, 3 are complementary with that of the codon of mRNA,

4) The fourth loop consists of 7 unpaired bases. It has got T, pseudouridine, and C so-called TΨC loop. It is the binding site for the ribosomes.

5) The third loop is located in between the second and fourth loop. The function of this loop is still unknown.

Reference

Keshari, Arvind K. and Kamal K. Adhikari. A Text Book of Higher Secondary Biology(Class XII). 1st. Kathmandu: Vidyarthi Pustak Bhandar, 2015.

Mehta, Krishna Ram. Principle of biology. 2nd edition. Kathmandu: Asmita, 2068,2069.

Jorden, S.L. principle of biology. 2nd edition . Kathmandu: Asmita book Publication, 2068.2069.