DNA STRUCTURE

1° structure

Primary Structure : the sequence of bases along the pentose-phosphodiester backbone of a DNA molecule.

• Base sequence is read from the 5’ end to 3’ end

• System of notation single letter ( A, G, C and T )

2° Structure

Secondary Structure : the ordered arrangement of nucleic acid strand

• The double helix model of DNA 2° structure

• Double helix : two antiparallel poly nucleotide strands are coiled in a right-handed manner about the same axis.

Base Pairing

• Base pairing is complimentary

• A major factor stabilizing the double helix is base pairing by hydrogen bonding between T-A and between C-G

T-A base pair comprised of 2 hydrogen bonds

G-C base pair comprised of 3 hydrogen bonds

3° Structure

Tertiary Structure : the three-dimensional arrangement of all atoms of nucleic acid (supercoiling)

• If the DNA is twisted in the direction of the helix, it is said that the positive supercoiling, and the bases are held together more closely.

• If the DNA is twisted in the opposite direction is called negative supercoiling, and away basis. In nature, most DNA has slight negative supercoiling that is produced by enzymes called topoisomerases.

4° Structure

Quaternary Structure : The structure of chromatin

• Each ‘bead’ is a nucleosome. Nucleosome consist of DNA wrapped around histone core.

• Histone : a protein, found associated with eukaryotic DNA

• Chromatin : DNA molecules wound around particles of histones in a beadlike structure.