DNA Replication Rates lower Eukaryotes and Prokaryotes... why a difference...

Graham Dellaire dellaire at ATLAS.ODYSSEE.NET
Tue Jan 16 01:05:18 EST 1996


>for a biology paper,=20
>i need to solve the follwing question:

>WHY IS DNA REPLICATION IN PROKARYOTES NEARLY 100 TIMES FASTER THAN
>REPLICATION IN EUKARYOTES ?

The numbers I have quote:
E.coli at 500 nt/s and Human Fibroblast at 50 nt/s

that would be 10X faster

>i have wondered if it is because of (a) life span of pro vs eu
>karyotes, (b) because of existence ofa nuclear membrane in eukaryotes,
>where one is not found in prokaryotes...

>if anyone can shed some light on this question, please reply by
>posting or send email to:

k.venkiteswaran at odyssey.on.ca

Okay=20

Here are a few possibilities I could come up with.

One, in E.coli you are not worrying about mutations that much. 
In fact =
a high mutation rate is desirable if the bacterium is to adapt 
to =
changing environments. The result is you can replicate your DNA 
faster =
without worrying about a mutation occurring. In higher 
eukaryotes such =
as mammals this is a big problem... mutations can kill the 
whole =
organism if well placed (i.e. cancer). For yeast I don't think 
this is =
as strong a point as they are unicellular and much like in 
bacteria =
--high mutation rates are adaptive.

The error rate in E.coli is consequently ~10-8 where as in 
human =
Fibroblasts it is ~10-10

Two in E.coli you have very little packing of DNA compared to 
higher =
eukaryotes. To start DNA replication you need to gain access to 
the DNA =
and expose the template for the polymerase. For instance you 
have the =
HU protein of bacteria compared to H1, H2A, H2B, H3 and H4 as 
well as =
the HMG proteins in mammals. The bacterial genome is much less 
complex =
so by simple logic to replicate DNA it is much easier to gain 
accesss to =
DNA in bacteria then in a mammal and hence it is faster in 
bacteria. =20

Further as a point of interest

In E.Coli you only have one origin of replication where as in 
mammalian =
cells it is estimated there are between 10 to the 4 --10 to the 
6 =
possible origins. This may reflects both the difference in size 
and =
complexity of the two genomes as well as the speed at which DNA 
is =
replicated. Multiple initiations of replication could help 
increase the =
time to replicate the genome if it is very large.

E.Coli has 3 X10 to the 6 bp and in the human genome there are 
~3X 10 to =
the 9 or=20

1000X more DNA to replicate. =20

1000 X 10 (the difference in speed of DNA replication)=3D 10 
000 or 10 =
to the 4

hmmmm by a rough calculation it makes sense why you have ~10 to 
the 4 =
origins in the human genome and only 1 in bacteria.

---------------------------------

Lastly,

To your suggestion of the nuclear membrane as a possible reason 
for =
higher replication rates I think the membrane provides a way of 
=
segregating transcription and translation first of all and may 
not =
really have much to do with replication on a surface level. =20

Graham Dellaire

dellaire at odyssee.net






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