Biology Reference
In-Depth Information
Figure 1.14
The cell cycle of a eukaryotic cell with a generation time of
≈
24 hours. DNA synthesis
occurs during the S phase. During G
1
and G
2
, no DNA synthesis occurs. Mitosis (M) occurs after G
2
.
when it reaches the RNA primer at the 5
′
end of the next Okazaki fragment.
Then, DNA polymerase I of
E. coli
removes the ribonucleotides from the Okazaki
fragment and replaces them with deoxyribonucleotides. When all the ribonu-
cleotides have been replaced, DNA polymerase I replaces nucleotides on a short
distance into the DNA region, before it dissociates from the new double-helix
molecule. The Okazaki fragments are then joined up by
DNA ligase
that catalyzes
the formation of a phosphodiester bond between the neighboring nucleotides.
DNA replication also requires that the double helix be unwound, as well as
unzipped. There are
≈
400,000 turns in 400kb of DNA. This unwinding is accom-
plished with the aid of enzymes called
DNA topoisomerases
. DNA topoisomerases
unwind a DNA molecule without rotating the helix by causing short-term breaks in
the polynucleotide backbone just in front of the replication fork. The reverse reac-
tion is performed by DNA topoisomerases so that DNA molecules can be coiled.
1.19 DNA Replication during Mitosis in Eukaryotes
The goal is to replicate the genome once during mitosis and provide equiva-
lent chromosomes to the daughter cells (
Sclafani and Holzen 2007
). The replica-
tion of prokaryotic and eukaryotic DNA is similar, but differs in several aspects,
the details of which are not fully resolved (
Gavin et al. 1995, Huberman 1995,
Baker and Bell 1998, Leipe et al. 1999, Sutton and Walker 2001
). DNA replication
takes place during the eukaryotic cell cycle before the condensed metaphase
