RNA (RIBONUCLEIC ACID)

A)  RNA is type of nucleic acid containing ribose sugar.

B) It is present in all organisms except some viruses. (DNA viruses).

C) It is found in cytoplasm as well as nucleus.

D)  Chemical composition of RNA:-

It is composed of: -

1) Sugar: The sugar of RNA is ribose (C5H10O5).

2) Phosphoric acid: It is an inorganic molecule (H3PO4).

3)    N – Base: These are purines and pyrimidines. Purines are Adenine and Guanines whereas pyrimidines are cytosine and uracil. 

E) Molecular Structure of RNA:-

1) Single stranded Nature.

1) RNA is single stranded molecule consisting of single polynucleotide chain.

2) Polynucleotide chain: It is formed by linking of many smaller subunits called as ribonucleotides (functional and structural unit)

3) Nucleotide: Nucleotide of RNA is called as ribonucleotide.

These are of four different types namely:-

Adenine, guanine, Cytosine and uracil.

4) Formation of Poly nucleotide chain:-

Ribonucleotides are linked together with the help of Phosphodiester linkages to form a polynucleotide Chain.

5) Heteropolymer: RNA shows repetitive sequence of nucleotide such as A, G, C, U hence, it is a heteropolymer. It has high molecular weight.

6) Base pairing: RNA is a single stranded molecule which may form loops. Usually base pairing is absent, if present, found in coiled regions only.

The base pairing in loop follows Watson & Crick pairing.

 7) Purine – Pyrimidine Ratio: Purine pyrimidine ratio never equals to one.

8) Polarity: RNA is polarized molecule with definite direction 5’- 3’.

F) Functions of RNA.

1) Different types of RNAs play major role in protein synthesis.

2) RNA through formation of RNA primer participates in DNA replication.

3) RNA acts as genetic material in certain viruses.

Types of RNA (Additional)

Basically there are 2 types of RNA

A) Genetic RNA.

1) RNA which forms genetic material is called as genetic RNA.

2) It carriers and stores genetic information.

3) It is found in some viruses only. Ex: - TMV – Tobacco mosaic   virus 

B) Non Genetic RNA:

1) It does not carry any genetic information.

2) It is present in all organisms except some viruses.

3) It actively participates in protein synthesis and also carries out certain vital functions.

4) Non-genetic RNA is off three types namely, mRNA, r RNA, and t RNA.

m RNA

1) It is present in nucleus as well as cytoplasm.

2) It is longest of all RNA types and short-lived. (Ephemeral).

3) It forms (3-5) % of total RNA contents of cell.

4) It is known as messenger RNA.

5) The number, nature and sequence of nucleotides present in mRNA strand is decided b4y the nature of protein molecule to be synthesized and is complimentary to DNA strand from which it is synthesized. (Cistrons or sense strand).

6) It shows presence of initiation codon i.e. AUG at 5´ end and at 3´ end it shows termination codon which is either UAA, UAG UGA.

7) It is linear, unfolded, single stranded structure without base pairing.

8) It is synthesized from DNA from the process known as transcription.

(The information present on mRNA is termed as cryptogram.)

7) Functions:

1) It carries messages for protein molecule to be synthesized from DNA to site of protein syntheses i.e. r RNA.

rRNA.

1) It is present in ribosomes as their structural component (in cytoplasm.)

2) It forms 75 – 85 % of total RNA content of cell.

3) It is single stranded and may show coiling or folding to form loops.

4) The loops show base pairing.

5) It is most stable type of RNA

6) It is synthesized from DNA.

7) Functions:

a) Provides site for protein synthesis.

b) Helps to arrange m-RNA properly.

c) Provides binding site for m-RNA strand.

t– RNA or s RNA

1) It is smallest type of RNA

2) It is present in cytoplasm in soluble form, hence the name s – RNA,

3) It is relatively stable and forms 12 – 18 % of total RNA contents of cell.

4) Two structural models were proposed to explain t – RNA Structure.

  a)   Hair pin Model, b) Clover leaf Model.

5) According to clover leaf model t – RNA shows presence of three major arms, one mini arm or variable arm, two free ends such as 3´ and 5´ and loop at the end of each major arm.

6) The nitrogen base in arm shows base pairing while that of loop remains unpaired.

7) The central loop shows presence of sequence of three consecutive nucleotides known as anticodon or NODOC.

8) At 3’ end it shows presence of CCA sequence and that at 5’ end a single unpaired guanine is present.

9) The important feature of t – RNA is that it shows presence of unusual nitrogen bases like hydroxyuracil and methylated purines.

10)  Functions:

a) It identifies codon on m – RNA strand.

b) It brings about transfer of amino acid from cellular pool to site of protein synthesis i.e. r – RNA