Table 8.2 : Difference between DNA and RNA
	Characters	DNA	RNA
1.	Molecule	Double stranded, helical	Single stranded, straight or variously folded and twisted.
2.	Pentose sugar	Deoxyribose	Ribose
3.	Pyrimidine base	Thymine	Uracil
4.	Complementary base pairing	Always present and exists between A = T and G = C	Normally absent, but may be present in twisted segments of a molecule. If present, pairing is between A = U and G = C
5.	Ratio of Purines: Pyrimidines	Always 1 : 1	Not necessarily 1:1
6.	Replication	Can replicate	Cannot replicate

Types of non-genetic RNA and their functions

There are three types of non-genetic RNA.

(1) mRNA or messenger RNA, (2) rRNA or ribosomal RNA and (3) tRNA or transfer RNA.

(1) mRNA (Messenger RNA) : This is called messenger RNA because it carries information for protein synthesis from the DNA to the ribosomes in the cytoplasm (the site of protein synthesis). m-RNA constitutes about 3-5% of the total RNA. It is produced on the DNA strand. The process is called transcription. Hence, the base sequence of mRNA is complementary to that of the DNA strand. The bases on the mRNA strand are organized into triplets. Each triplet consists of a sequence of three consecutive bases (nucleotides) and is called a codon (code word). Each codon specifies one amino acid. The sequence of codons on the mRNA strand is called the mRNA language. It indicates the sequence of amino acids for the synthesis of a protein. It begins with the codon AUG (initiation codon or starting codon) and ends with either UAA, UAG or UGA (stop codons). The single-stranded mRNA molecule is always straight (Fig. 8.10) and therefore, base pairing is totally absent in mRNA.

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1. Represents the sequence of codons from the DNA strand (transcription).

2. Brings the sequence to the ribosomes (site of protein synthesis) in the cytoplasm.

3. Provides the sequence for the synthesis of specific protein from the amino acids with the help of t-RNA (translation).

(2) rRNA (Ribosomal RNA) : r RNA forms about 80% of the total RNA. It is present in the ribosomes in the cell cytoplasm (site of protein synthesis) and hence called rRNA. The single-stranded molecule of rRNA is variously folded and twisted upon itself in certain regions (Figure 8.11). In such folded regions, complementary bases form pairs and are joined by hydrogen bonds.

Role of rRNA in protein synthesis : The role of rRNA in protein synthesis is not yet very clearly known but it is known to complex with various protiens. The resulting structure is a ribosome, and this complex reads the coded sequence in mRNA to link amino acids together into particular protiens.

1. It provides proper binding sites for the mRNA of the ribosomes.

2. It orients the mRNA in such a way that its nitrogen base triplets or codons are properly read or translated.

3. It also releases tRNA after the transfer of activated amino acid.

4. It protects the mRNA strand from the action of enzymes (nucleases).

5. It protects the growing (nascent) polypeptide chain from proteolytic enzymes.

(3) tRNA (Transfer RNA) : It is the smallest of all the types of RNA. About 10 to 20% of the total RNA of the cell is of this type. tRNA strand is folded upon itself forming loops. It results in either a clover leaf pattern or hair pin pattern (Figure 8.12). One end of the strand has guanine, while the other end carries the CCA combination of nitrogen bases. A triplet of nitrogen bases called anticodon is present on one of the loops. The anticodon pairs with the complementary codon on the mRNA molecule. The tRNA molecules carry amino acids to the mRNA during the process of protein synthesis. Each type of the amino acid is carried by a specific tRNA molecule. tRNA is synthesized on the DNA template. It has complementary base pairs in folded regions.

Role of tRNA in protein synthesis

1. tRNA carries the required specific amino acids from cell cytoplasm to the ribosome (site of protein synthesis).

2. Each type of amino acid is carried by a specific type of tRNA.

3. In the ribosome, tRNA helps to arrange the amino acids in their proper sequence for the synthesis of a protein. This is done with the help of the codons on the mRNA and the matching (complementary) anticodons on the tRNA (translation).

Table of Contents

8.0 Introduction
8.1 Packaging of Hereditary Material
8.2 The Structure of DNA
8.3 Replication Of DNA In Eukaryotes
8.4 Replicatin of Pokaryotic Chromosome
8.5 Plasmids
8.6 RNA: Structure and Types
8.7 The Genetic Code
8.8 The Central Theme of Protein Synthesis

Chapter 9

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