BOTANY Higher Secondary Second Year - Textbooks Online

cells was injected into the mouse, the mouse was dead. The living rough
strain of Diplococcus had been transformed into S-type cells. That is the
hereditary material of heat killed S-type cells had transformed R-type
cells into virulent smooth strains. Thus the phenomenon of changing the
character of one strain by transferring the DNA of another strain into the
former is called transformation.
Structure of DNA
DNA and RNA are identified in the nucleus. They are complex macro
molecules and made up of millions of smaller units called nucleotides.
Hence, DNA is a macromolecular substance with double stranded
G
T
G
A
C
C
A
G C
A
T
A – Adenine
G – Guanine
C – Cytosine
T – Thymine
Fig. 3.8 Watson and Crick
model of DNA
T
G
T
T
C
G
A
C
A
133
polynucleotide. Each nucleotide is
made up of pentose sugar, a
phosphate group and a nitrogenous
base. Ribose is the constituent sugar
in RNA and Deoxyribose in DNA.
The nitrogenous bases are of two
kinds – purines and pyrimidines.
Adenine and guanine are the purines
and thymine and cytosine are
pyrimidines. The nitrogenous bases
found in DNA are adenine, guanine,
cytosine and thymine, whereas in
RNA thymine is replaced by uracil.
The sub-unit containing only sugar
and nitrogenous base is known as
nucleoside. A nucleoside combines
with phosphate to form a nucleotide.
Thus, four kinds of nucleotides are
seen in DNA molecule. They are
adenine nucleotide, guanine nucleotide, thymine nucleotide and cytosine
nucleotide. Hence, nucleotides are building blocks of DNA.
In 1953, James Watson and Francis Crick proposed double helix DNA
model on the basis of x-ray diffraction studies with photographs of DNA
taken by Wilkins and Franklin. DNA is a double stranded structure in which
the two strands are coiled around each other forming a double helix. The
DNA duplex is “coil of life”. There are two grooves found in DNA molecule