For modern biology, people often want to generate multiple copies of
certain DNA sequences in the lab, so they can analyze these sequences
in detail or use them to construct other pieces of DNA or express the
DNA in some organism in order to make RNA or protein. The handiest
way to generate extra DNA is by 'cloning' it, which in the molecular
biology world means 'making extra copies of DNA'. The easiest way to
do this is to put the DNA into bacteria. As the bacteria multiply
(which they do every few minutes, which is why you should go clean
your toilet seat NOW), they also copy the DNA, which means you end up
with more DNA. For a bacteria to be fooled into thinking the DNA is
its own, and therefore copy (we actually say 'replicate') it, the DNA
needs to be in a certain form: a circular piece of DNA with sites
where replication can start and some other stuff. This form of the
DNA is called a 'plasmid'. If the plasmid has certain required
sequences that fool the bacteria into not only copying the DNA, but
transcribing parts of it into RNA and translating those into protein,
then the plasmid is suitable for 'expression' of the DNA. So if you
have some DNA encoding mitochondrial cytochrome C (an enzyme important
for energy production in cells), and you want to (for example) study a
bunch of the protein, you could stick the cytochrome C gene into an
expression plasmid and grow the bacteria in a big flask of nutrient
stuff that bacteria like to live in ('broth') and they would be
happily producing cytochrome C protein for you. Cool, eh? Of course,
to do this, you need to be able to stick your cytochrome C gene into
the right place in the plasmid, which requires that you know where
origin of replication and promoters and restriction enzyme sites and
all the other information you'll need to know is. And the best way to
know how all that stuff is organized is if you simply have a drawing
of the plasmid. Only we scientists don't sound so cool when we refer
to 'drawings' of 'circular bits of DNA like those found in bacteria',
so instead we refer to those drawings as 'physical maps' of 'plasmids'
You can type 'plasmid maps' into Google and get hits like this:
http://www.stanford.edu/group/nolan/plasmid_maps/pmaps.html
http://www.premierbiosoft.com/plasmid_maps/glossary/plasmid_maps.html
That shows you what they look like.
And here is a decent description of the stuff I talked about, from a
Google search of 'DNA cloning bacterial expression':
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/RecombinantDNA.html
Hope this answers your question. I just couldn't stand to see
Pafalafa collect any more money from simple bogus Google searches that
most questioners could have done themselves. Since I am not a
registered 'answerer', you can't pay me (which is fine). But don't
pay anyone else unless he/she really comes up with something useful --
more useful than rephrasing a freaking Google search. There is lots
more to learn about the topic you ask about, and someone could spend a
lot more time explaining the mechanics of cloning. You could try
searching some textbooks. The greatest online repository of
biomedical texts online can be found at:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=Books
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