It is summarised as follows:
- The DNA is a right handed double helix comparable to a twisted ladder. It consists of two polydeoxyribonucleotide
chains twisted around each other on a common axis. - Two strands are antiparallel, i.e., one strand runs in the 5′ to 3′ direction while the other in 3′ to 5′ direction.
- The width (or diameter) of a double helix is 20 A° (2 nm). Each turn (pitch) of the helix is 34 A° (3.4 nm) with 10 pairs
of nucleotides, each pair placed at a distance of about 3.4 A° (0.34 nm). - Each strand of DNA has a hydrophilic deoxyribose phosphate backbone (3′ − 5 ′ phosphodiester bonds) on the
outside (periphery) of the molecule while the hydrophobic bases are stacked inside (core). - The two polynucleotide chains are complementary to each other due to base pairing.
- The two strands are held together by hydrogen bonds formed by complementary base pairs. The A-T pair has 2
hydrogen bonds while G-C pair has 3 hydrogen bonds. The hydrogen bonds are formed between a purine and a
pyrimidine only. - The genetic information resides on one of the two strands known as template strand or anti-sense strand. The
complementary strand is sense strand. The double helix has major grooves and minor grooves along the phosphodiester
backbone.
Denaturation of DNA- Denaturation or melting is breaking of hydrogen bonds between nitrogen bases of complementary
DNA strands by heating at high temperature. DNA rich in A-T base pair have low melting point and undergoes denaturation
easily, as A-T base pairs have two hydrogen bonds. G-C rich region are more stable due to presence of three hydrogen bonds
and requires high temperature for melting.
Renaturation is the phenomenon by which DNA strands separated by melting can reassociate and form duplex on cooling
at low temperature.
Functions of DNA
(i) DNA is the genetic material which carries all the hereditary information coded in the arrangement of its nitrogen bases.
(ii) It has the property of replication (autocatalytic function) essential for passing genetic information from one cell to its daughters
or from one generation to the next.
(iii) Crossing over produces recombinations.
(iv) Changes in sequence and number of nucleotides produce mutations. Mutations are the fountain head of all variations and
formation of new species.
(v) DNA controls the metabolic reactions of cells through RNAs and RNA-directed synthesis of proteins, enzymes and other
biochemicals.
RNA
RNA is a polymer of ribonucleotides held together by 3 ′, 5′- phosphodiester bridges. The sugar in RNA is ribose in contrast
to deoxyribose in DNA. RNA contains the pyrimidine uracil in place of thymine found in DNA.
Types of RNA
Ribsomal RNA (rRNA) Transfer RNA (tRNA) Messenger RNA (mRNA)It is most abundant RNA, constitutes
about 70-80% of total RNA.It is also called soluble RNA and constitutes
about 15% of total RNA.It constitutes 2-5% of total RNA.Depending on sedimentation coefficient,
eukaryotic rRNA is of 4 types - 28S, 18S,
5.8S and 5S. Prokaryotic rRNA is of three
types-23S, 16S and 5S.It is smallest RNA and have sedimentation
coefficient of 4S. It contains AA binding
site, DHU loop, T Ψ C loop, anticodon
loop and extra arm.It contains 5’methylated cap, initiation
codon (AUG) and termination codon
(UAA, UAG, UGA)It provides structural framework for
ribosomes.It transfers amino acids to ribosomes for
polypeptide synthesis.It carries coded information for translation
into polypeptide formation.