Bond Formation in Nucleotides
The pentoses are bound to nitrogenous bases by β - N - glycosidic bonds. The N-9 of a purine ring binds with (1′C) of a
pentose sugar to form a covalent bond in the purine nucleoside. In case of pyrimidine nucleosides, the glycosidic linkage
is between N-1 of a pyrimidine and 1′C of a pentose. The compound formed in the above manner is called a nucleoside.
The nucleoside combines with a phosphate group at 5′ position by an ester bond to form a nucleotide, or nucleoside
monophosphate. A nucleotide having ribose sugar is called ribonucleotide and a nucleotide containing deoxyribose sugar
is termed deoxyribonucleotide.
Adenosine triphosphate : High energy molecule
Adenosine triphosphate (ATP) was discovered by Karl Lohmann in 1929
from muscle cell. But its role in a cell was found by Fritz Lipmann in
1941.
ATP is mobile and transports the chemical energy to the energy-consuming
process anywhere within the cell. It acts as a storehouse as well as a
source of energy in a cell. Thus, it is called energy currency of cell.
The ATP provides energy by dissociating into ADP and Pi. The ‘ATP ADP + Pi + Energy’ reaction can take place in a cyclic
manner, involving building up and breakdown processes. Thus, ATP remains continuously available in cell.Functions of Nucleotides
(i) They are building blocks of nucleic acids, ribonucleotides for RNAs
while deoxyribonucleotides produce DNA.
(ii) Cyclic AMP (cAMP) functions as second chemical messenger in many
hormone controlled chemical reactions.
(iii) Cyclic GMP (cGMP) is functional in Ca2+ or calmodulin mediated
chemical reaction.
(iv) Higher nucleotides behave as energy carriers. ATP is known as energy
currency of the cell since it contains high energy phosphate bonds which
are built up when energy is available and broken down when energy is
to be utilised.
(v) Nucleotides produced by nicotinamide and riboflavin function
as coenzymes (NAD+, NADP+, FMN and FAD) of dehydrogenases or
oxidases.
NUCLEIC ACIDS
Nucleic acids are long chain macromolecules which are formed by end to
end polymerisation of large number of repeated units called nucleotide.
Nucleic acids show a wide variety of secondary structures. There are two
types of nucleic acids-deoxyribonucleic acid or DNA and ribonucleic acid
or RNA.
DNA
DNA is a polymer of deoxyribonucleotides. It is composed of
monomeric units namely deoxyadenylate (dAMP), deoxyguanylate
(dGMP), deoxycytidylate (dCMP) and deoxythymidylate (dTMP). DNA
does not contain uracil. The monomeric deoxynucleotides in DNA are
held together by 3 ′, 5′-phosphodiester bridges.
Double Helix Structure of DNA
The double helical structure of DNA was proposed by James Watson and Francis Crick in 1953 (Nobel Prize, 1962).