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

• 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”.

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

DNA Double Helix

DNA has two polynucleotide strands wound together to form a long, slender, helical molecule, the DNA double helix.

B-DNA

• 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.

Stability of double helix structure

Internal and external hydrogen bonds

Negative charge of phosphate groups

Base pair stacking

Major and Minor Grooves

• 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

• There are two hydrogen bonds between adenine and thymine (A=T) and there are three hydrogen bonds between guanine and cytosine (GºC) 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.

• 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.

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

RECAP

Reference:

Books:Fundamentals of Genetics – B.D.Singh

Websites:• www.biology200.gsu.edu• www.biofo.com• www.wikipedia.com

BYR.ABARNABTH-12-001

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