In 1953, Watson and Crick proposed the double helix DNA model based on X-ray diffraction studies of DNA photographs by Wilkins and Franklin. DNA has two polynucleotide strands that wind together to form a long, slender helical molecule. The strands run in opposite directions and are intertwined in a clockwise direction. The structure is stabilized by hydrogen bonds between the nitrogenous base pairs and the negative charge of the phosphate groups.

1. DNA structure
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.
If you can do, you shall do; If you can’t do, you must do

2. • DNA is a macromolecular substance with double
stranded polynucleotide.
• 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”.

3. There are two grooves found in DNA molecule
namely
 Major groove and
 Minor groove.
The backbone of the helix is formed of sugar and
phosphate molecule. The nitrogenous bases are
attached to sugar molecules.
The two nucleotide strands are held together by
unstable hydrogen bonds

4. DNA Double Helix
DNA has two polynucleotide strands wound together to form a
long, slender, helical molecule, the DNA double helix.
B-DNA

6. • The two strand run antiparallely in opposite directions
ie. they run in opposite direction 5’ to 3’ end and 3’ to 5’
end.
• The two strands are interwined in clockwise direction.
• The width of DNA molecule is 20 Å. The strand
completes a turn every 34 Å along its length.
• There are ten nucleotides per turn. The internucleotide
distance is 3.4 Å.
• Watson and Crick model of DNA is called B-form DNA.
The chains in B-form DNA are in right handed
orientation.

7. Stability of double helix structure
 Internal and external hydrogen bonds
 Negative charge of phosphate groups
 Base pair stacking
Major and Minor Grooves

8. • Each nucleotide is made up of pentose sugar, a
phosphate group and a nitrogenous base.
Deoxyribose is the constituent sugar 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

9. • There are two hydrogen bonds
between adenine and thymine (A=T)
and there are three hydrogen bonds
between guanine and cytosine (GC)
pairing.
• The sub-unit containing only sugar and
nitrogenous base is known as
nucleoside. A nucleoside combines
with phosphate to form a nucleotide.
• Nucleotides are building blocks of
DNA.

10. • Erwin Chargaff in 1949 showed that
• (i) The bases pair in specific manner. Adenine always
pairs with thymine and guanine pairs with cytosine.
• (ii) Total amount of purine nucleotides is always equal
to the total amount of pyrimidine nucleotides i.e.[A] +
[G] = [T] + [C].
• (iii) The proportion of adenine is equal to thymine and
so also of guanine is equal to cytosine. But the [A] + [T]
need not necessarily be equal to [G] + [C].
• These empirical rules regarding the composition of
bases in DNA is collectively known as Chargaff’s law or
Base pair rules.

11. Structure of DNA
Double Helix
• Right handed helix
• Rise = 0.33
nm/nucleotide
• Pitch = 3.4 nm /
turn
• 10.4 nucleotides
per turn
• Two groves – major
and minor
R
E
C
A
P

12. Reference:
Books:
Fundamentals of Genetics – B.D.Singh
Websites:
• www.biology200.gsu.edu
• www.biofo.com
• www.wikipedia.com

13. BY
R.ABARNA
BTH-12-001
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