Richard P. Grant
r_grant at see.sig.for.address
Mon Aug 23 05:34:27 EST 1999
In article <8wNv3.485$LT3.49092 at cac1.rdr.news.psi.ca>, "John Allen"
<john at klamsystems.com> wrote:
>I am trying to help my daughter with her HS biology/
>I understand from the text that bacterial chromosome consist of DNA in a
>double wheel (circular) structure, unlike higher animals where it is in the
>from of a double helix.
Ah, no - DNA is always (when static) in the double helical form. In other
words, you have two strands of DNA that wrap around each other. In
bacteria this double helix is circular, i.e. it doesn't have any ends. If
you could take two elastic bands, cut one and wrap it around the other,
then rejoin the cut you would have a representation of the bacterial
chromosome. In 'higher' animals and plants (eukaryotic organisms) the two
strands of DNA are wrapped around rach other in a helical fashion, but
they are not joined at the ends - your two elastic bands are both cut, as
it were. So the ends are 'free', but probably 'anchored' to some nuclear
(er, 'of the nucleus', NOT 'atomic' (-: ) protein or scaffold.
> When a bacterial cell divides a catalyst/enzyme
>causes the circular structure to separate, rather like a zipper parting,
>creating two circular structures and a second catalyst/enzyme acts to
>reconstruct the (now) missing half of both structures.
More or less, except you pull apart your elastic bands and create new
elastic alongside each strand as you go. The helix (one half new, one
half old) reforms as soon as possible, but I don't know whether this is
too 'true' for your daughter :-/
>second where on the ring does the process start in bacteria? I understand
>that it starts at the ends in the helix, is this true?
Remember, all DNA is helical. The circle is a different level of structure.
In bacteria the process starts at a special sequence of DNA called the
origin. In eukaryotes the process starts at many origins along the
>what is the catalyst, is it the same in both helical and circular DNA?
Functionally, yes. :)
Look for a book called Genes VI by Benjamin Lewin for a detailed,
undergraduate level discussion.
Richard P. Grant MA DPhil | rgrant at cmtech.co.uk
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home: www.avnet.co.uk/adastra | work: www.cmtech.co.uk
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