2. Overview
■ DNA is a thread-like chain of nucleotides carrying the genetic instructions used in the
growth, development, functioning and reproduction of all known living organisms and
many viruses.
■ DNA is nucleic acid, which consist of two bipolar strands coiled around each other to
form a double helix.
■ The two DNA strands are called polynucleotides since they are composed of simpler
monomer units called nucleotides.
12-03-2018 Nishank Mehta - 16BCB0125 2
4. Nucleotides
■ They are the building blocks of nucleic acids.
■ They are composed of:
– Nitrogenous base
– Sugar (deoxyribose)
– Phosphate
■ A nucleoside is a nitrogenous base and a 5-carbon sugar.Thus a nucleoside plus a
phosphate group yields a nucleotide.
12-03-2018 Nishank Mehta - 16BCB0125 4
6. Nitrogenous Bases
■ Divided into two groups:
– Pyrimidines (thymine and cytosine)
– Purines (adenine and guanine)
■ They are bound together, according to base pairing rules, with hydrogen bonds to
make double-stranded DNA.
■ ‘A’ pairs with ‘T’ and ‘C’ pairs with ‘G’.
12-03-2018 Nishank Mehta - 16BCB0125 6
8. Deoxyribose
■ It is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H.
■ The nitrogenous base is attached to the 1′ ribose carbon.The 5′ hydroxyl of each
deoxyribose unit is replaced by a phosphate that is attached to the 3′ carbon of the
deoxyribose in the preceding unit.
■ Deoxyribose is generated from ribose 5-phosphate by enzymes called ribonucleotide
reductases.
12-03-2018 Nishank Mehta - 16BCB0125 8
10. Phosphate
■ The sugar-phosphate backbone forms the structural framework of nucleic acids.
■ This backbone is composed of alternating sugar and phosphate groups, and defines
directionality of the molecule.
■ The sugar-phosphate backbone is negatively charged and hydrophilic, which allows
the DNA backbone to form bonds with water.
12-03-2018 Nishank Mehta - 16BCB0125 10
12. Chromosomes
■ Within eukaryotic cells DNA is organized into long structures called chromosomes.
■ Most eukaryotic chromosomes include packaging proteins which, aided by chaperone
proteins, bind to and condense the DNA molecule to prevent it from becoming an
unmanageable tangle.
■ Within the eukaryotic chromosomes, chromatin proteins such as histones compact
and organize DNA.
12-03-2018 Nishank Mehta - 16BCB0125 12
14. Properties
■ The structure of DNA is dynamic along its length, being capable of coiling into tight
loops, and other shapes.
■ In all species it is composed of two helical chains, bound to each other by hydrogen
bonds.
■ Both chains are coiled round the same axis, and have the same pitch of 3.4
nanometres.
■ The pair of chains has a radius of 1 nanometre.
12-03-2018 Nishank Mehta - 16BCB0125 14
