`=>` Every generation of each and every species resembles its ancestors in many ways.

`=>` It has been observed that nucleus of a living cell is responsible for this transmission of inherent characters, also called heredity.

`=>` The particles in nucleus of the cell, responsible for heredity, are called chromosomes which are made up of proteins and another type of biomolecules called nucleic acids.

`=>` These are mainly of two types, the deoxyribonucleic acid (`DNA`) and ribonucleic acid (`RNA`).

`=>` Since nucleic acids are long chain polymers of nucleotides, so they are also called polynucleotides.

`=>` Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases).

`=>` In DNA molecules, the sugar moiety is `color{red}(β)`-D-2-deoxyribose whereas in RNA molecule, it is `color{red}(β)`-D-ribose.

`=>` DNA contains four bases viz. adenine (`color{red}(A)`), guanine (`color{red}(G)`), cytosine (`color{red}(C)`) and thymine (`color{red}(T)`).

`=>` RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (`color{red}(U)`).

`=>` A unit formed by the attachment of a base to `1′` position of sugar is known as nucleoside.

● In nucleosides, the sugar carbons are numbered as `1′`, `2′`, `3′` etc. in order to distinguish these from the bases (Fig. 14.5a).

● When nucleoside is linked to phosphoric acid at `5′`-position of sugar moiety, we get a nucleotide (Fig. 14.5).

`=>` Nucleotides are joined together by phosphodiester linkage between `5′` and `3′` carbon atoms of the pentose sugar.

● The formation of a typical dinucleotide is shown in Fig. 14.6.

A simplified version of nucleic acid chain is as shown in fig.

`=>` Information regarding the sequence of nucleotides in the chain of a nucleic acid is called its primary structure.

`=>` Nucleic acids have a secondary structure also.

`=>` James Watson and Francis Crick gave a double strand helix structure for DNA (Fig. 14.7).

`=>` Two nucleic acid chains are wound about each other and held together by hydrogen bonds between pairs of bases.

`=>` The two strands are complementary to each other because the hydrogen bonds are formed between specific pairs of bases. Adenine forms hydrogen bonds with thymine whereas cytosine forms hydrogen bonds with guanine.

`=>` In secondary structure of RNA, helices are present which are only single stranded.

● Sometimes they fold back on themselves to form a double helix structure.

● RNA molecules are of three types and they perform different functions.

● They are named as messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA).

`=>` It is known that every individual has unique fingerprints.

`=>` These occur at the tips of the fingers and have been used for identification for a long time but these can be altered by surgery.

`=>` A sequence of bases on DNA is also unique for a person and information regarding this is called DNA fingerprinting.

`=>` It is same for every cell and cannot be altered by any known treatment.

`=>` DNA fingerprinting is now used

(i) in forensic laboratories for identification of criminals.

(ii) to determine paternity of an individual.

(iii) to identify the dead bodies in any accident by comparing the DNA’s of parents or children.

(iv) to identify racial groups to rewrite biological evolution.

`=>` DNA is the chemical basis of heredity and may be regarded as the reserve of genetic information.

`=>` DNA is exclusively responsible for maintaining the identity of different species of organisms over millions of years.

`=>` A DNA molecule is capable of self duplication during cell division and identical DNA strands are transferred to daughter cells.

`=>` Another important function of nucleic acids is the protein synthesis in the cell.

`=>` Actually, the proteins are synthesised by various RNA molecules in the cell but the message for the synthesis of a particular protein is present in DNA.