A HISTORY OF DNA
• Discovery of the DNA double helix
A. Frederick Griffith – Discovers that a
factor in diseased bacteria can transform
harmless bacteria into deadly bacteria
B. Rosalind Franklin - X-ray photo of
C. Watson and Crick - described the
DNA molecule from Franklin’s X-ray.
SEE p. 292-293
X-ray diffraction patterns produced by
DNA fibers – Rosalind Franklin and
The Watson-Crick Model:
DNA is a double helix
• 1951 – James Watson learns about x-ray
diffraction pattern projected by DNA
• Knowledge of the chemical structure of nucleotides
(deoxyribose sugar, phosphate, and nitrogenous
• Erwin Chargaff’s experiments demonstrate that
ratio of A and T are 1:1, and G and C are 1:1
• 1953 – James Watson and Francis Crick propose
their double helix model of DNA structure
Watson & Crick proposed…
•DNA had specific pairing between the
ADENINE – THYMINE
CYTOSINE - GUANINE
•DNA was made of 2 long stands of
nucleotides arranged in a specific
way called the “Complementary Rule”
Deoxyribonucleic Acid (DNA):
• Deoxyribonucleic acid, also abbreviated as DNA, is the
principal informational macromolecule of the cell,
which stores, translates and transfers the genetic
• In the prokaryotes, the DNA is found mostly in the
• In eukaryotes it is found in the nucleus, mitochondria
• The present understanding of the storage and
utilization of the cell’s genetic information is based
upon the discovery of the structure of DNA by Watson
and Crick in 1953.
Structure of DNA:
DNA is made of two helical chains coiled around the
same axis, to form a right-handed double helix.
The two chains in the helix are anti-parallel to each
other, i.e., the 5′-end of one polynucleotide chain and
the 3′-end of the other polynucleotide chain is on the
same side and close together.
The distance between each turn is 3.6 nm (formerly 3.4
There are 10.5 nucleotides per turn (formerly 10
• Adenine must pair with Thymine
• Guanine must pair with Cytosine
• Their amounts in a given DNA molecule will
be about the same.
G CT A
The spatial relationship between the two strands creates major
and minor grooves between the two strands. In these grooves
some proteins interact.
The hydrophilic backbones of alternating deoxyribose and
negatively charged phosphate groups are on the outside of the
The hydrophobic pyrimidine and purine bases are inside the
double helix, which stabilizes the double helix of the DNA.
The double helix is also stabilized by inter-chain hydrogen bond
formed between a purine and pyrimidine base.
A particular purine base, pairs by hydrogen bonds, only with a
particular pyrimidine base, i.e., Adenine (A) pairs with Thymine
(T) and Guanine (G) pairs with Cytosine (C) only.
Two hydrogen bonds pairs Adenine and Thymine (A = T),
whereas three hydrogen bonds pairs Guanine and Cytosine (G
The base pairs A = T and G ≡ C are known as complementary
Due to the presence of complementary base pairing, the two
chains of the DNA double helix are complementary to each
Hence the number of A’ bases are equal to the number of T’
bases (or ‘G’ is equal to ‘C) in a given double stranded DNA.
One of the strands in the double helix is known as sense strand,
i.e., which codes for RNA/proteins and the other strand is
known as antisense strand.
• Different arrangements
of NUCLEOTIDES in a
nucleic acid (DNA)
provides the key to
The Code of Life…
• The “code” of the chromosome is the
SPECIFIC ORDER that bases occur.
A T C G T A T G C G G…
DNA is wrapped tightly around
histones and coiled tightly to
See p. 297
• DNA must be copied
• The DNA molecule produces 2
IDENTICAL new complementary
strands following the rules of
•Each strand of the
original DNA serves
as a template for the
new strand See p.
1. Watson and Crick
showed: the two
strands of the parental
molecule separate, and
each functions as a
template for synthesis
of a new
Essential features of B-DNA
• Right twisting
• Double stranded
• Bases on the inside
• Uniform diameter
• Major and minor
• Structurally, purines (A and G pair best with
pyrimidines (T and C)
• Thus, A pairs with T and G pairs with C, also
explaining Chargaff’s ratios
RNA but not DNA
is prone to base-
evolved as the
but not RNA?
planes of base pairs
perpendicular to the
helix axis passes
through the base pairs
and hence B-DNA has
no internal spaces
B-DNA has a wide and
deep major groove
and a narrow and deep
– right-handed double helix with a wide and narrow
– major groove is very deep and the minor groove is quite
– consists of dinucleotides, each with different
4 stranded DNA
– Telomeric DNA
both form right-handed double
B-DNA helix has a larger pitch and
hence a smaller width than that of
In B-DNA, the helix axis passes
through the base pairs and hence
B-DNA has no internal spaces,
whereas that of A-DNA has a 6
Angstrom diameter hole along its
The planes of the base pairs in B-
DNA are nearly perpendicular to
the helix axis, whereas in A-DNA,
they are inclined from this.
Therefore, B-DNA has a wide and
deep major groove and a narrow
and deep minor groove, whereas A-
A DNA B DNA
B-DNA forms a right-handed
double helix in which the
repeating unit is a
whereas Z-DNA forms a left-
handed double helix in
which the repeating unit is a
The Z-DNA helix has a
larger pitch and is therefore
narrower than that of B-
B-DNA has a wide and deep
major groove and a narrow
and deep minor groove,
whereas Z-DNA has a
narrow and deep minor
B DNAZ DNA
Double stranded DNA
Genetic material may be DNA
Single stranded DNA
Double stranded RNA
Genetic material may be RNA
Single stranded RNA
Retroviruses like HIV
RNA / DNA hybrids
e.g. during retroviral
AMAZING DNA FACTS…
• DNA from a single human cell
extends in a single thread for
almost 2 meters long!!!
• It contains information equal
to some 600,000 printed
pages of 500 words each!!!
(a library of about 1,000 books)
Functions of DNA:
• The base sequence of the DNA constitutes the
informational signal called the genetic material.
This nucleotide base sequence enables the DNA to
function, store, express and transfer the genetic
• Hence it programs and controls all the activities
of an organism directly or indirectly throughout
its life cycle.
• (a) DNA stores the complete genetic information
required to specify (form) the structure of all the
proteins and RNA’s of each organism.
DNA is the source of information for the synthesis of
all cellular body proteins. Some of the proteins are
structural proteins and some are enzymes. These
enzymes arrange micro-molecules to form
macromolecules. It determines the activities of an
organism throughout its life cycle, i.e., the period of
gestation, birth, maturity, senescence and death.
It determines the individuality and identity of a given
It duplicates (replicates to form two daughter DNA)
itself and transfers one of the copy to the daughter cell
during cell division, thus maintaining the genetic
material from generation to generation.
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