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0:00
Welcome to the Henry
Stewart Talks series
on the biology and
significance of plasmids.
My name is Don Clewell,
and I'll be giving
the first lecture,
An Introduction to
Plasmid Biology.
0:15
What is a plasmid?
This is a microbial cell,
in this case, a bacterial cell,
we see it has two DNA molecules.
On the left, we have a
rather large molecule
corresponding to the
chromosome; and on the right,
we have a plasmid
being, a small,
physically separate
molecule capable of
maintaining itself autonomously
in this particular state.
0:39
Plasmids can be found in
all three domains of life.
They're extremely
common in prokaryotes,
which correspond
to the eubacteria
or true bacteria and the Archea,
which used to be lumped into
the bacterial world,
but now maintain
an independent position
corresponding to
an independent domain
of life called Archea.
Plasmids are also common in
eukaryotes, such as yeasts.
These correspond to
the domain Eukarya,
which not only includes
microorganisms,
but also much higher
systems, including humans.
1:13
The majority of the
organisms that we'll be
talking about in this
series are prokaryotes,
so it's worth
noting a little bit
about the prokaryotic world.
First of all, there are millions
of species on the planet,
most of which have
never been cultivated.
For example, a gram
of soil can contain
as many as 5000 to 10000
species of prokaryotes.
The combination of species can
vary tremendously from
one location to another.
Similarly, in seawater,
there are as many as
20000 species of prokaryotes
per liter, and again,
these can vary significantly
from one location to another.
You could even
have these numbers
down to great depths,
down to two miles
down in the ocean.
A human being carries about
a thousand different
species of bacteria.
For example, it's been
estimated that there are
maybe as many as 100
trillion bacterial cells
in and on a human being.
This compares to only about
10 trillion human cells.
So in a sense, of all
the cells that are
present in and on
the human body,
only about 10% of
them are human.
There are huge amounts of
interactions that occur
between these microorganisms
living together in animals,
soil, and the ocean.
They're all exchanging
DNA quite efficiently,
and this DNA
transfer is known as
horizontal gene transfer.
