Re: Why do bacteria and somatic animal cells replicate differently?

That's an excellent question.  
You are right that animal cells are more complex than bacterial cells.  Animal cells 
have more DNA.  This DNA is present in multiple linear pieces in animal cells, 
whereas bacterial cells have a single circular piece of DNA.  In addition, animal 
cells have structures called organelles, responsible for unique tasks required for 
cell survival.  The amount of DNA and the division of labor by organelles  
contributes to the complexity of animal cells.

It is precisely the increased complexity that gave rise to the different replication 
processes used by animal cells and bacterial cells.  While the biochemical process 
of replicating the DNA is very similar in animal cells and bacterial cells, the 
mechanical processes of segregating the DNA and intracellular components are 
different.  For the bacterial cell, as the DNA is replicated, the original DNA and the 
new copy are attached to the plasma membrane.  As the bacterial cell grows the 
two DNAs are separated because the plasma membrane between them expands.  
The septum (a new piece of plasma membrane) grows between the two 
chromosomes and eventually divides the cell in two.

The simple replication and division (called binary fission) of bacterial cells would 
not work for animal cells.  Why not?  First, the DNA material in animal cells is 
separated from the rest of the cell by a double membrane, so the DNA cannot  
readily become attached to the plasma membrane.  Second, in an animal cell there 
are many chromosomes.  In a human for example, there are 46 chromosomes.  
There would not be sufficient plasma membrane to which to attach in an organized 
fashion all of the chromosomes and their copies.  Third, because the animal cell 
has so many chromosomes, it must have a mechanism for keeping track of each 
chromosome and its copy to make sure each daughter cell gets one each of the 
chromosomes.  During mitosis (animal cell division) a spindle apparatus is formed.  
The spindle apparatus looks like two bicycle wheels right next to each other with 
the spokes of the two wheels connected in the center of the cell.  These spokes 
protrude into the center of the cell and become associated with the chromosomes.  
{Each bicycle wheel eventually corresponds to a daughter cell.}  The association of 
the spokes with the chromosomes allows one copy of each chromosome to be 
moved to each daughter cell.  Fourth, each daughter cell must not only inherit a 
copy of each chromosome, but also receive appropriate amounts of organelles.  
During the process of animal cell replication and division, the DNA and organelles 
are replicated then apportioned to the daughter cells.

The process of animal cell division (mitosis) is quite well understood on a 
molecular level and well-documented using fluorescence microscopy.  To get more 
details about mitosis, check out a basic cell biology textbook like Essential Cell 
Biology by Alberts and others (Garland Science).  Happy reading!