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DNA replication, part 2
Single-strand binding proteins
Table 20.1
Figure 20.19
Stimulation of DNA
synthesis by T4
phage SSB, gp32
amB17(gene23)
Effect of
the SSB
gene 32
amE727 (gene 49), tsP7 (gene 32),
tsP7 (gene 32), amB17(gene23)
Topoisomerases
Rubber band
model of
supercoiling in
DNA
Nicking one strand - relaxation of supercoiling
Problem of Strain due to Unwinding of DNA by
Helicase is solved by the Swivel Concept
DNA gyrase is a a DNA topisomerase
relaxed
supercoiled
•Topoisomerase II forms a covalent DNA-enzyme
intermediate with its Tyr residues
Cyrstal structure of yeast DNA topoisomerase
Model in segment passing in topoisomerase II
DNA polymerases
Proofreading in DNA synthesis
DNA polI
Crystal structure of the Klenow fragment
Purification of DNA polymerases by DEAE sephacel
polA1 strain
Wild type with &
w/o Nethylmaleimide
(blue line)
Separation of polIII
core and alpha
subunit
Exonuclease activity of the epsilon subunit
Perfectly paired DNA substrate
Imperfectly paired DNA substrate
Termination of DNA replication
Replication Leaves Catenated Daughter Chromosomes
Two-step Model for Decatenation in Prokaryotes
Cozarelli and Colleagues: Topo IV is
Responsible for Decatenation in Bacteria
•Studied decatenation of intertwined pBR322 plasmids
•Used ts mutations S. typhimurium; Mutations were in gryA
(encodes gyrase), parC and parE (encode two subunits
of topo IV) and parF (unrelated gene linked to parC and E)
•Found mutations only in parC and parE (and not gyrA)
prevented decatenation. Therefore, topo IV acts in
decatenation.
In eukaryotes topo II is the probably the
decatenating enzyme.
The problem with replication of
Linear DNA
Telomerase activity
on four repeats of
TTGGGG
Tetrahymena
Telomeres in the
protozooan
Tetrahymena
Telomere binding
proteins in ciliates
and yeast