Chapter 4

Nucleic Acids

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Base Structure and Properties

NAME ABRREV. TYPE.	STRUCTURE
Adenine A purine
Thymine T pyrimidine
Guanine G purine
Cytosine C pyrimidine

The Double Helix

There are three major types of double helix structures in nucleic acids, each structure being markedly different from the others.

B Helix

• it is right-handed with 10 nucleotide residues per turn.
• the plane of the bases is almost perpendicular to the helix axis
• there are prominent major and minor grooves
• the B helix is stabilized in the presence of water which fits perfectly into the minor groove

The space-filled representation of B DNA above clearly displays the major and minor grooves.

allows for an interactive display of the double helical B DNA form.

A Helix

In conditions of low humidity DNA can adopt an entirely different structure called the A form of DNA. The A helix has the following structural characteristics:

• it is right-handed with 11 nucleotide residues per turn.
• the plane of the bases are now tilted with respect to the helix axis
• the major and minor grooves are of comparable size
• due to the different goove size the A form can no longer accomdate water molecules and thus is the preferred structure in conditions of low humidity.

allows for an interactive display of a single turn of the A-DNA helix.

Click here to view a comparison of a turn in the B-DNA form, the structure on the left, with that of a turn in the A-form, the structure to the right. Note the marked difference in the orientations of the base-pairs with respect to the helix axis.

Hybrid structures of DNA-RNA also adopt the A-helix form. This is due to the fact that the additional hydroxy group present on the ribose sugar would result in steric interactions if the B-form were adopted.

allows for an interactive display of a DNA-RNA complex.

Z Helix

Recently an entirely new form of DNA, the Z form, was discovered. Z-DNA has following structural characteristics:

• it is left-handed with 12 nucleotide residues per turn.
• the plane of the bases is substantially tilted with respect to the helix axis
• the grooves are less pronounced than those of either the A or B form

allows for an interactive display of the Z DNA form.

The dominant feature of this Z-helix is that it is left handed. Click here to view an image that shows the opposite senses for B DNA, the left image , and Z DNA, the right image.

The Biological Function of Nucleic Acids

The basic functions of nucleic acids involve information storage, transmission and expression. DNA serves primarily as a storehouse of genetic information, while RNA functions in readout and translation. The expressed information is used to regulate and control biochemical processes and the formation of macromolecular structures.

A movie detailing the translation of RNA to Protein be viewed.