This document discusses the roles of topoisomerase I and II in DNA topology. Topoisomerases catalyze changes in DNA linking number by increasing or decreasing underwinding. Topoisomerase I breaks one DNA strand and changes linking number in increments of 1, while topoisomerase II breaks both strands and changes linking number in increments of 2. Without topoisomerases, supercoiling tension would increase during DNA replication, leading DNA to fragment and replication to stop, causing cell death.

1. Role ofTopoisomerase I and
II in DNATopology
Presented By: NageenTalib

2. What will we discuss?
▪ Topoisomerases catalyze changes in the linking number of DNA
▪ Topoisomerase I
▪ Topoisomerase II
▪ What happens if there is no topoisomerase?

3. Topoisomerases Catalyze Changes
in the Linking Number of DNA
▪ All cells, from bacteria to eukaryotes, have enzymes with the sole
function of underwinding and relaxing DNA.
▪ Topoisomerases increase or decrease the extent of DNA
underwinding by changing the linking number.
▪ They play an especially important role in the complex changes in
DNA topology during replication and DNA packaging.

4. Topoisomerase I
▪ Type I enzymes breaks one of the two DNA strands
▪ The type I enzymes are called topoisomerases I and III
▪ They change Lk in increments of 1
▪ Remove negative supercoils (increasing Lk)
▪ Type I enzymes do not require any high-energy cofactor for activity;
the reaction is driven by the energy stored in the supercoiled DNA as
torsional strain

6. Topoisomerase II
▪ Type II topoisomerases break both DNA strands
▪ The type II enzymes are topoisomerases Ila and IIb
▪ They change Lk in increments of 2
▪ Bacterial topoisomerase II, also known as DNA gyrase, can introduce
negative supercoils (decrease Lk)
▪ Type II enzymes utilize ATP hydrolysis to drive conformational
changes in the protein during the reaction cycle

8. EukaryoticTopoisomerase II
▪ Eukaryotic type II topoisomerases cannot underwind DNA (introduce
negative supercoils), but they can relax both positive and
negative supercoils.

10. What happens if there is no topoisomerase?
▪ Topoisomerase alleviates supercoiling downstream of the origin of
replication. In the absence of topoisomerase, supercoiling tension
would increase to the point where DNA could fragment.
▪ DNA replication will stop and leads to cell death