Definition of Gene
Proof that DNA DNA Structure
is Genetic Material
DNA Structure
Watson and Crick Model of DNA
The following are the features of the DNA molecule as described by Watson and Crick in 1953.- 2 chains
- purine opposite a pyrimidine
- chains held together by H-bonds
- Guanine is paired with cytosine by three H-bonds
- Adenine is paired with thymine by two H-bonds
- anti-parallel orientation of the two chains
5'--------------->3' 3'<---------------5'
- the molecule is stabilized by:
- large # of H-bonds
- hydrophobic bonding between the stacked bases
Components of DNA
DNA is composed of two chains of repeating nucleotides. Each nucleotide consists of three components. These components are:- Phosphate Group
- 2-deoxyribose sugar
- A nitrogen containing base
- cytosine
- adenine
- guanine
- thymine
Types of DNA
The DNA molecule that Watson and Crick described was in the B form. It is now known that DNA can exist in several other forms. The primary difference between the forms is the direction that the helix spirals.A, B, C = right-handed helix
Z = left-handed helix (found in vitro
under high salt)
B is the major form that is found in the cell. Z-DNA was initially found only under high salt conditions, but the cellular environment is actually a low-salt environment. The question then is whether type Z exist under cellular conditions. Several features have been discovered that can stablize Z-DNA under in a low salt environment.
- Alternating purine/pyrimidine tracts
- poly GC or poly AT stretches
- 5-methyl-cytosine
In addition to the direction the molecule turns, several other differences exists between the various forms of DNA. The following table summarizes the features of the different forms of DNA.
| Form | Direction | Bases/ 360o Turn |
Helix Diameter |
|---|---|---|---|
| A | Right | 11.0 | 23A |
| B | Right | 10.0 | 19A |
| C | Right | 9.3 | 19A |
| Z | Left | 12.0 | 18A |
Copyright © 1998. Phillip McClean