13 THE REPLICON (Full Edition)
5 Each eukaryotic chromosome contains many replicons
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S phase is the restricted part of the eukaryotic cell cycle during which synthesis of DNA occurs.
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Eukaryotic replicons are 40-100 kb in length.
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A chromosome is divided into many replicons.
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Individual replicons are activated at characteristic times during S phase.
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Regional activation patterns suggest that replicons near one another are activated at the same time.
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In eukaryotic cells, the replication of DNA is confined to part
of the cell cycle. S phase
usually lasts a few hours in a higher eukaryotic cell. Replication of
the large amount of DNA contained in a eukaryotic chromosome is
accomplished by dividing it into many individual replicons. Only some
of these replicons are engaged in replication at any point in S phase.
Presumably each replicon is activated at a specific time during S
phase, although the evidence on this issue is not decisive (for review
see 114).
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The start of S phase is signaled by the
activation of the first replicons. Over the next few hours, initiation
events occur at other replicons in an ordered manner. Much of our
knowledge about the properties of the individual replicons is derived
from autoradiographic studies, generally using the types of protocols
illustrated in Figure 13.5 and Figure 13.6.
Chromosomal replicons usually display bidirectional replication.
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Figure 13.8
Measuring the size of the replicon requires a stretch of DNA in which adjacent replicons are active.
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How large is the average replicon, and
how many are there in the genome? A difficulty in characterizing the
individual unit is that adjacent replicons may fuse to give large
replicated eyes, as illustrated in Figure 13.8.
The approach usually used to distinguish individual replicons from
fused eyes is to rely on stretches of DNA in which several replicons
can be seen to be active, presumably captured at a stage when all have
initiated around the same time, but before the forks of adjacent units
have met.
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In groups of active replicons, the
average size of the unit is measured by the distance between the
origins (that is, between the midpoints of adjacent replicons). The
rate at which the replication fork moves can be estimated from the
maximum distance that the autoradiographic tracks travel during a given
time.
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Individual replicons in eukaryotic
genomes are relatively small, typically ~40 kb in yeast or fly, ~100 kb
in animals cells. However, they can vary >10-fold in length within a
genome. The rate of replication is ~2000 bp/min, which is much slower
than the 50,000 bp/min of bacterial replication fork movement.
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From the speed of replication, it is
evident that a mammalian genome could be replicated in ~1 hour if all
replicons functioned simultaneously. But S phase actually lasts for
>6 hours in a typical somatic cell, which implies that no more than
15% of the replicons are likely to be active at any given moment. There
are some exceptional cases, such as the early embryonic divisions of
Drosophila embryos, where the duration of S phase is compressed
by the simultaneous functioning of a large number of replicons (558).
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How are origins selected for initiation at different times during S phase?
In S. cerevisiae, the default appears to be for origins to replicate early,
but cis-acting sequences can cause origins linked to them to replicate at late times.
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Available evidence suggests that
chromosomal replicons do not have termini at which the replication
forks cease movement and (presumably) dissociate from the DNA. It seems
more likely that a replication fork continues from its origin until it
meets a fork proceeding toward it from the adjacent replicon. We have
already mentioned the potential topological problem of joining the
newly synthesized DNA at the junction of the replication forks.
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Figure 13.9
Replication forks are organized into foci in the
nucleus. Cells were labeled with BrdU. The leftmost panel was stained
with propidium iodide to identify bulk DNA. The right panel was stained
using an antibody to BrdU to identify replicating DNA. Photographs
kindly provided by A. D. Mills and Ron Laskey.
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The propensity of replicons located in
the same vicinity to be active at the same time could be explained by
"regional" controls, in which groups of replicons are initiated more or
less coordinately, as opposed to a mechanism in which individual
replicons are activated one by one in dispersed areas of the genome.
Two structural features suggest the possibility of large-scale
organization. Quite large regions of the chromosome can be
characterized as "early replicating" or "late replicating," implying
that there is little interspersion of replicons that fire at early or
late times. And visualization of replicating forks by labeling with DNA
precursors identifies 100-300 "foci" instead of uniform staining; each
focus shown in Figure 13.9 probably contains >300 replication forks.
The foci could represent fixed structures through which replicating DNA must move.
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114 Fangman, W. L. and Brewer, B. J.
(1991).
Activation of replication origins within yeast chromosomes.
Annu. Rev. Cell Biol. 7, 375-402.
PubMed Journal
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558 Blumenthal, A. B., Kriegstein, H. J., and Hogness, D. S.
(1974).
The units of DNA replication inD. melanogaster chromosomes.
Cold Spring Harbor Symp. Quant. Biol. 38, 205-223.
PubMed
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©Jones and Bartlett Publishers (2007)
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