STRUCTURE OF DNA

STRUCTURE OF DNA

• Most accepted model of DNA was given by ‘Watson & Crick’ on the X-ray diffraction.
• DNA is located in nucleus and is also present in mitochondria.
• Watson & Crick model features for DNA are-
• DNA is a double stranded molecule (right handed).
• Two strands are antiparallel.
• Alternating phosphate and deoxyribose sugar unit form the backbone structure of DNA strand.
• The sugar of DNA is deoxyribose.
• Base pairing rule is-
• Adenine pairs with Thymine.
• Guanine pairs with Cytosine.

Bonding in DNA–

1. Glycosidic bond (within nucleotides)
2. Hydrogen bond(between two strands of DNA)

• Adenine = Thymine by 2 hydrogen bonds
• Guanine = Cytosine by 3 hydrogen bond

        3. 5’-3’ phosphodiester bonds & 3’-5’ phosphodiester bonds (between two successive nucleotides)

Chargaff’s Rule (base pairing rule)– Number of Purines = Number of Pyrimidines (AC= TG)

• Width of double helix is 20A⁰
• Each turn of helix is 34 A⁰.
• The genetic information resides on one of the two strands called as template strand or sense strand.
• Opposite strand is antisense strand.
• Coding strand matches the sequence of the transcribed RNA.
• The information in template (antisense strand) is read in 3¹ —>5¹ directions and also called negative strand. Whereas it is read in 5^{1 —->}3¹ directions in coding (sense) strand and also called plus strand.
• The purine and pyrimidines are stacked on the inside of the helix and perpendicular to the helical axis. 
• Methods to separate DNA-  DNA can be separated by HPLC, thin layer chromatography, paper chromatography and Gel electrophoresis

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