Notes On Nucleic Acids - CBSE Class 12 Chemistry
Nucleic acids are biomolecules present in all living cells, and are responsible for transmitting genetic information from one generation to the next.   Chemical composition and structure of nucleic acids:   The basic components of nucleic acids include a pentose sugar, a phosphate group and nitrogen-containing heterocyclic compounds called “nitrogen bases.” Depending on the type of the pentose sugar present, nucleic acids can be broadly classified into DNA -deoxyribonucleic acid and RNA - ribonucleic acid.                               There are four nitrogenous bases present in DNA:  adenine, guanine, cytosine and thymine. There are also four bases in RNA. In RNA, the base uracil is substituted for thymine. Nitrogen bases are broadly categorised into 2 classes. 1. Purines: A, G 2. Pyramidines: C, T, U A nitrogenous base attached to the sugar is called a “nucleoside”.                A pentose sugar with a nitrogen base and a phosphate group is called a Nucleotide- “a base sugar- phosphate unit.”                    Nucleotides are linked together by phosphodiester linkages.    Primary structure of DNA: The sequence in which the four nitrogen bases are attached to the sugar phosphate backbone of a DNA strand is called the primary structure of DNA.     Secondary structure of DNA: James Watson and Francis Crick determined the double helix nature of DNA. DNA forms a secondary structure in the form of a double-stranded helix, like a twisted ladder. The two strands of DNA are held together by hydrogen bonds between specific pairs of bases. Adenosine always pairs with thymine. Cytosine always pairs with guanine. The strands are said to be complementary to each other. The complementary strands run in opposite directions. 2hydrogen bonds are formed when adenine pairs up with thymine. When a guanine base pairs with cytosine, 3 hydrogen bonds form between the bases.     Therefore, in RNA, uracil forms a complementary base pair with adenine.  Two hydrogen bonds are present between the bases.   There are three important types of RNA in living systems: Messenger RNA carries genetic information from DNA to ribosomes   for the synthesis of proteins. Transfer RNA brings amino acids to the messenger RNA. Ribosomal RNA is the RNA component of the ribosome where proteins are synthesised in the cell.  Nucleic acids play a number of important biological roles.  They are the chemical basis of heredity DNA copies itself, or self-replicates, during cell division, so that identical copies of the genetic code are transferred to the daughter cells. Nucleic acids are vital for protein synthesis.

#### Summary

Nucleic acids are biomolecules present in all living cells, and are responsible for transmitting genetic information from one generation to the next.   Chemical composition and structure of nucleic acids:   The basic components of nucleic acids include a pentose sugar, a phosphate group and nitrogen-containing heterocyclic compounds called “nitrogen bases.” Depending on the type of the pentose sugar present, nucleic acids can be broadly classified into DNA -deoxyribonucleic acid and RNA - ribonucleic acid.                               There are four nitrogenous bases present in DNA:  adenine, guanine, cytosine and thymine. There are also four bases in RNA. In RNA, the base uracil is substituted for thymine. Nitrogen bases are broadly categorised into 2 classes. 1. Purines: A, G 2. Pyramidines: C, T, U A nitrogenous base attached to the sugar is called a “nucleoside”.