Molecular Genetics

Facts

Bacteriophage T4 DNA and the replicative form of bacteriophage FX174 DNA were treated briefly with DNaseI, digested with an exonuclease, or treated with a restriction enzyme for which there is a single recognition site in the DNA. The treated and untreated DNAs were analyzed by electrophoresis with the result shown at left. In the original experiments, characterization was by sedimentation analysis using the ultracentrifuge.

Interpretations

• The T4 DNA is a linear duplex. It has free ends available for exonuclease digestion; a single restriction cut produces two fragments.
• The FX174 DNA is a circular duplex. It is resistant to exonuclease digestion; and a single restriction cut produces only one DNA fragment.
• The native form of FX174 DNA is more compact (moves faster in gel electrophoresis) than the nicked circular form produced by brief DNase I treatment. This property is due to an underwinding of the DNA double helix, inducing negative supercoils.

Further information

• The degree of supercoiling is quantitatively described using:

This is page 11225 of Molecular Genetics by Ulrich Melcher, © 1997, 1998