Molecular Biology
• Study of Storage and Information Flow in
Biology
• DNA is information storage medium in
cells
• DNA Structure
• Flow of Biological Information
DNA Structure
• DNA is a polymer (deoxyribonucleic acid)
• Building Blocks are Nucleotides
• Nucleotides composes of
– A Sugar (deoxyribose)
– A Phosphate
– A Nitrogen Base
Nitrogen Bases
• In DNA, 4 types of Nitrogen Bases
–
–
–
–
Adenine (A): Purine
Cytosine (C): Pyrimidine
Guanine (G): Purine
Thymine (T): Pyrimidine
• 4 types of nucleotides distinguished by their
nitrogen bases
Backbone
• Nucleotides joined into chain by covalent
bonds between phosphate and sugar units
• Sugar-Phosphate-Sugar-Phosphate…
• Bases extend to side joined to sugar
• Chemical Direction to chain (I.e. strand)
– Phosphate at one end, Hydroxyl in sugar at
other
– 5’ to 3’ (Carbons) by convention
Base Pairing
• Watson-Crick Complements (Pyrimidine to
Purine)
– T-A
– C-G
• W-C Complements pair by hydrogen bonds
• Template Matching Reaction
(Hybridization)
• Nucleotide Bases Hydrophobic
INTER-STRAND HYDROGEN BONDING
(+) (-)
(-) (+)
to Sugar-Phosphate
Backbone
Adenine
Thymine
to Sugar-Phosphate
Backbone
(-) (+)
(+) (-)
to Sugar-Phosphate
Backbone
(+)(-)
to Sugar-Phosphate
Backbone
Guanine
Hydrogen Bond
Cytosine
Helical Structure
•
•
•
•
•
Flat molecules
Stack and Twist upon each other
DNA double-stranded molecule
W-C base pairing
Molecule has helical form
PHYSICAL STRUCTURE OF DNA
20 Å
5’ C
34 Å
3’ OH
Minor
Groove
5’
3’
3’
5’
Sugar-Phosphate
Backbone
Major
Groove
Nitrogenous
Base
C 5’
Central Axis
3’ 0H