Molecular Genetics

Superhelical tension produced by DNA replication must be released. Enzymes that relax DNA are topoisomerases.

Facts

• Topoisomerases relax DNA by changing the DNA linking number.
• Topoisomerases II change the linking number in steps of 2 by passing both strands of double-stranded DNA through a break.
• Eukaryotic topoisomerases isolated to date only relax supercoiled DNA, while prokaryotic topoisomerases can, given ATP, add supercoils.

Topoisomerases I change the linking number in steps of 1. They pass a single DNA strand through a nick.Topoisomerase I is a protein of the metaphase chromosome scaffold. In interphase, topoisomerase is bound to the nuclear matrix. The DNA replication machinery also appears bound to the matrix.

Interpretations

• Topoisomerase action can be divided into three steps: nicking (1), strand passage (2); resealing (3).
• Topoisomerase I probably functions to relieve tension caused by unwinding of the DNA helix at the replication fork.
• Topoisomerase II should resolve catenanes.
• Topoisomerase binding sites may delimit chromosome structural loops.

Further information

• The molecular action of topoisomerases I and II has been explored.
• Crystal structure information for topoisomerase I ** and II has been obtained.

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This is page 1328 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 1999