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What is Ribonucleic Acid (RNA)?

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RNA is generally single-stranded, contains ribose sugar (in place of deoxyribose found in DNA), and

the nitrogenous bases adenine, guanine, cytosine and uracil (U; in place of thymine found in DNA).

RNA chain is formed by phosphodiester linkages between ribonucleotides. A comparison between

DNA and RNA is presented in Table 3.5.

RNA may be classified, on the basis of its function, as (i) genetic, and (ii) non-genetic RNA.

• (i) Genetic RNA:

• It is found in several viruses where it acts as the genetic material; it may be either single-stranded or

double-stranded.

• (ii) Non-genetic RNA:

• These are found in all the eukaryotes and prokaryotes, except viruses.

• Non-genetic RNAs are of five types, namely:

(1) Messenger RNA (mRNA),

(2) Ribosomal RNA (rRNA),

(3) Transfer RNA (tRNA)

(4) Chromosomal RNA and

(5) Primer RNA.• Messenger RNA (mRNA):

• It functions as the carrier of genetic information (messenger) stored in DNA ; it supports

translation of this information into proteins. The proportion of mRNA in the total cellular RNA

varies from 8 to 10%. The length of mRNA chain depends on the length of the polypeptide it codes

for; and its life span is quite variable.

• Ribosomal RNA (rRNA):

• It is synthesized in nucleolus and is the major component (40- 60% by weight) of ribosome.

Ribosomal RNA is quite stable and constitutes about 80% of the total cellular RNA.

• Transfer RNA (tRNA):

• It is also called soluble RNA (sRNA) and it is quite stable and makes up about 10-15% of the cellular

RNA. tRNA is about 80 nucleotides long and acts as the amino acid adopter during translation. This

RNA contains some unusual bases and shows considerable internal pairing to form the widely

accepted clover-leaf structure.

• Chromosomal RNA:

• This class of RNA is found associated with chromosomes, and is essential for their organization. It

constitutes about 5% of the chromosome by weight, and has 40-60 bases per RNA molecule.

• Primer RNA:

• This RNA is synthesized at the initiation of DNA replication to provide the free 3′-OH necessary for

DNA polymerase to function. It has 10 to 60 bases, and is degraded toward the end of DNA

replication.

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