Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

252 Chapter 5: DNA Replication, Repair, and Recombination
3′
5′
one end of the DNA
double helix cannot
rotate relative to the
other end
5′
3′
Figure 5–21 The reversible DNA nicking
reaction catalyzed by a eukaryotic DNA
topoisomerase I enzyme. As indicated,
these enzymes transiently form a single
covalent bond with DNA; this allows free
rotation of the DNA around the covalent
backbone bonds linked to the blue
phosphate.
CH 2
HO
CH 2
type I DNA
topoisomerase
with tyrosine at
the active site
DNA topoisomerase covalently
attaches to a DNA phosphate,
thereby breaking a phosphodiester
linkage in one DNA strand
OH
CH 2
the two ends of the DNA double
helix can now rotate relative to
each other, relieving accumulated
strain
OH
CH 2
the original phosphodiester bond
energy is stored in the phosphotyrosine
linkage, making the reaction reversible
OH
HO
CH 2
spontaneous re-formation
of the phosphodiester bond
regenerates both the DNA
helix and the DNA
topoisomerase
the energy of the cleaved phosphodiester bond, resealing is rapid and does not
require additional energy input. In this respect, the rejoining mechanism differs
from that catalyzed by the enzyme DNA ligase, discussed previously (see Figure
5–12).
A second type of DNA topoisomerase, topoisomerase II, forms a covalent
MBoC6 m5.22/5.21
linkage to both strands of the DNA helix at the same time, making a transient