CBSE Class 12 Biology
Revision Notes
CHAPTER- 06
MOLECULAR BASIS OF INHERITANCE
DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are two types of nucleic acid
found in living organisms. DNA acts as genetic material in most of the organisms. RNA
also acts as genetic material in some organisms as in some viruses and acts as
messenger. It functions as adapter, structural, and in some cases as a catalytic
molecuThe DNA - it is a long polymer of deoxyribonucleotides. A pair of nucleotide is also
known as base pairs. Length of DNA is usually defined as number of nucleotides present
in it. Escherichia coli have 4.6 x 106 bp and haploid content of human DNA is 3.3 × 109
bp.
Structure of Polynucleotide Chain
•A nucleotide has three components – a nitrogenous base, a pentose sugar
(ribose incase of RNA, and deoxyribose for DNA), and a phosphate group.
There are two types of nitrogenous bases – Purines (Adenine and Guanine),
and Pyrimidines (Cytosine, Uracil and Thymine).
Cytosine is common for both DNA and RNA and Thymine is present in DNA.
Uracil is present in RNA at the place of Thymine.
Fig.: 6.1 A Polynucleotide Chain
A polynucleotide chain
A nitrogenous base is linked to pentose sugar with N-glycosidic linkage to
form to form a nucleoside. When phosphate group is linked 5’-OH of a
, nucleoside through phosphoester linkage nucleotide is formed. Two
nucleotides are linked through 3’-5’ phosphodiester linkage to form
dinucleotide. More nucleotide joins together to form polynucleotide.
In RNA, nucleotide residue has additional –OH group present at 2’-position in
ribose and uracil is found at the place of Thymine.
Structure Differences
DNA RNA
(a) The sugar present in DNA is (a) The sugar present in RNA is
2-deoxy-D-(-)-ribose. D-(-)-ribose.
(b) DNA contains cytosine and (b) RNA contains cytosine and uracil
thymine as pyrimidine bases are pyrimidine bones and guanine and
guanine and adenine is purine bases. adenine as purine bases.
(c) DNA has double strand alpha-helix (c) RNA has a single stranded α-helix
structure. structure.
(d) DNA molecules are very large their (d) RNA molecules are comparatively
molecular mass may vary from much smaller with molecular mass
6×10^6- 16×10^6u. ranging from 20,000 – 40,000.
Functional Differences
(a)DNA has uniqueproperty of (a) RNA usually does not replicate.
replication.
(b) DNA controls the transmission of (b) RNA controls the synthesis of
hereditary effects. proteins.
Double Helix Model for Structure of DNA-James Watson and Francis Crick,
based on X-ray diffraction data produced by Wilkin and Rosalind proposed
this model of DNA.
The silent features of this model are-
a) DNA is made of two polynucleotide chains in which backbone is made up
of sugar-phosphate and bases projected inside it.
b) Two chains have anti-parallel polarity. One 5’à3’ and with 3’à5’.
Revision Notes
CHAPTER- 06
MOLECULAR BASIS OF INHERITANCE
DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are two types of nucleic acid
found in living organisms. DNA acts as genetic material in most of the organisms. RNA
also acts as genetic material in some organisms as in some viruses and acts as
messenger. It functions as adapter, structural, and in some cases as a catalytic
molecuThe DNA - it is a long polymer of deoxyribonucleotides. A pair of nucleotide is also
known as base pairs. Length of DNA is usually defined as number of nucleotides present
in it. Escherichia coli have 4.6 x 106 bp and haploid content of human DNA is 3.3 × 109
bp.
Structure of Polynucleotide Chain
•A nucleotide has three components – a nitrogenous base, a pentose sugar
(ribose incase of RNA, and deoxyribose for DNA), and a phosphate group.
There are two types of nitrogenous bases – Purines (Adenine and Guanine),
and Pyrimidines (Cytosine, Uracil and Thymine).
Cytosine is common for both DNA and RNA and Thymine is present in DNA.
Uracil is present in RNA at the place of Thymine.
Fig.: 6.1 A Polynucleotide Chain
A polynucleotide chain
A nitrogenous base is linked to pentose sugar with N-glycosidic linkage to
form to form a nucleoside. When phosphate group is linked 5’-OH of a
, nucleoside through phosphoester linkage nucleotide is formed. Two
nucleotides are linked through 3’-5’ phosphodiester linkage to form
dinucleotide. More nucleotide joins together to form polynucleotide.
In RNA, nucleotide residue has additional –OH group present at 2’-position in
ribose and uracil is found at the place of Thymine.
Structure Differences
DNA RNA
(a) The sugar present in DNA is (a) The sugar present in RNA is
2-deoxy-D-(-)-ribose. D-(-)-ribose.
(b) DNA contains cytosine and (b) RNA contains cytosine and uracil
thymine as pyrimidine bases are pyrimidine bones and guanine and
guanine and adenine is purine bases. adenine as purine bases.
(c) DNA has double strand alpha-helix (c) RNA has a single stranded α-helix
structure. structure.
(d) DNA molecules are very large their (d) RNA molecules are comparatively
molecular mass may vary from much smaller with molecular mass
6×10^6- 16×10^6u. ranging from 20,000 – 40,000.
Functional Differences
(a)DNA has uniqueproperty of (a) RNA usually does not replicate.
replication.
(b) DNA controls the transmission of (b) RNA controls the synthesis of
hereditary effects. proteins.
Double Helix Model for Structure of DNA-James Watson and Francis Crick,
based on X-ray diffraction data produced by Wilkin and Rosalind proposed
this model of DNA.
The silent features of this model are-
a) DNA is made of two polynucleotide chains in which backbone is made up
of sugar-phosphate and bases projected inside it.
b) Two chains have anti-parallel polarity. One 5’à3’ and with 3’à5’.
