Spectrum biology

Most Primitive Hereditary Vectors-RNA

Ex per i ments in the 1960s, showed that mes sen ger RNA has the abil ity to store ge netic

in for ma tion, while trans fer and ri bo somal RNA have the abil ity to trans late ge netic

in for ma tion to pro teins. Ex per i ments per formed two de cades later showed that some RNAs

can even act as en zyme to self edit their own ge netic code. These re sults raised ques tions

like why RNA play so many roles in the flow of ge netic in for ma tion? and why ge netic

in for ma tion is stored in DNA, if RNA alone could do the job? RNA has great ca pa bil ity as

ge netic mol e cule, if one had to carry on hereditary processes on its own.

It now seems certain that RNA was the first molecule of heredity, so it evolved all the

essential methods for storing and expressing genetic information before DNA came into the

scene. However, single stranded RNA is rather unstable and is easily damaged by enzyme.

By essentially double the existing RNA molecule and using deoxyribose sugar instead of

ribose, DNA evolved as a much more stable form to pass genetic information with accuracy.

Mes sen ger RNA or mRNA or Tem plate RNA

It makes 3 5- % of total cellular RNA. The sedimentation coefficient of mRNA
is 8 S. The name messenger RNA was proposed by Jacob and Monod (1961).

The structural components of mRNA includes
(i)CAP (at 5 ′ end) (ii) Non-coding region-1
(iii)Initiation codon (AUG) (iv) Coding region
(v)Termination codon (vi) Non-coding region-2
(vii)Poly A sequence (at 3 ′ end)

The mRNA formed in nucleus, comes out with proteins into cytoplasm and
normally swim as spherical balls, known as informosomes.

Ribosomal RNA or rRNA

It makes about 80% or more of total cellular RNA. It is the basic constituent of
ribosomes and developed from the Nucleolar Organiser Region (NOR) of
chromosomes in eukaryotes. In prokaryotes, it is developed from rDNA.

There are three types of rRNA present

(i)High molecular weight rRNA (mol. wt > 1

million)

e.g. 21 29S – S rRNA.

(ii)High molecular weight rRNA (mol. wt < 1

million)

e.g. 12 S– 18 S rRNA.

(iii)Low molecular weight rRNA (mol. wt ~

40000)

e.g. 5 S rRNA.

Transfer RNA or tRNA or

Soluble RNA

It makes about 10 20- % of total cellular RNA

with sedimentation coefficient of 3. 8 S. It

contain 73 - 93 nucleotides.

tRNA is synthesised in nucleus on DNA

templates. About 0.25% of DNA codes for

tRNA. The chief function of tRNA is to carry

amino acids to ribosomes for protein

synthesis.

5 ′

Cap NC 1

(10-100)

nucleotides

Coding

region

1600 nucleotides

Poly (A)

sequences

(200-250

nucleotides)

UAA or UAG or UGA

(Termination codon)

AUG

(Initiation codon)

NC 2

(50-150)

nucleotides

3 ′

Structure of mRNA

Unpaired

bases

Paired bases

Coiled

regions

Uncoiled region

Structure of ribosomal RNA (schematic)

Amino

acid

Amino acid

binding site

ACC

Synthetase site

Ribosome

recognition site

Anticodon site

tRNA (a) The binding sites

DHU loop

Anticodon loop

CCA terminus

3 ′

TψC loop 5 ′

(b) The tertiary structure