Double helix structure of DNA

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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