In article <1994Apr24.131637.23432 at dal1>, arlin at ac.dal.ca writes:
>> Certainly its a cool question, but saying (as I did) that bacteria
> *ancestrally* have circular chromosomes is not the same as saying
> "bacteria only have circular chromosomes" (Tim's words, not mine).
> Linear bacterial chromosomes are, as far as I know, restricted to
> members of the spirochete genus _Borrelia_, and the inference that
> circular chromosomes are ancestral is therefore a sound one.
>> Arlin
Actually, two streptomycetes also have linear chromosomes,
but Arlin's point still stands as it is clear that the ancestral
state for both of these groups of bacteria is a circular chromosome.
In addition both Archaebacteria and Eubacteria insert membrane proteins
directly into the plasma membrane. And before someone says
"of course, because neither has an ER".....this shows the useful
ness of the term prokaryote to describe a grade of cellular organisation.
No one denies the many unique molecular features of archaebacteria,
and no one denies the characters they share with eukaryotes. It
is simply a lie, however, to state that the term prokaryote is
defined purely negatively and is not useful. It is also misleading
to argue that the prokaryote/eukaryote division is not "natural"
or is "phylogenetically invalid". Prokaryotes may be paraphyletic,
but then so are protozoa and reptiles....both natural groups with
synapomorphies.
Andrew Roger
aroger at ac.dal.ca
