Central Dogma

Definition of Gene

Proof that DNA
is Genetic Material

DNA Structure

Restriction Modification

Nucleic Acid

Course Topics

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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
  • 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
Because both of these conditions can exists in the cell, it is suggested that stretches of Z-DNA may actually exists in the cell along with other stretches of B-DNA.

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
A Right 11.0 23A
B Right 10.0 19A
C Right 9.3 19A
Z Left 12.0 18A

Copyright © 1998. Phillip McClean