In our first lecture
on gene expression,
I introduced you to the
processes of transcription
and translation in
prokaryotes and eukaryotes
as well as introducing
you to the genetic code.
Now, it’s time to dig a little deeper
into the process of transcription.
So without ado, let’s move forward.
By the end of this lecture,
you will be able to describe
transcription in prokaryotes
as well as differentiate
between the initiation,
elongation and termination
phases of transcription
as well as explain differences in
prokaryotic and eukaryotic transcription
and diagram aspects
of RNA processing.
So to start with, I’d like to ask you
a question from the previous lecture.
Can you recall any differences from
prokaryotic and eukaryotic gene expression?
I’ll give you a hint.
It has to do with the
structure of the cells.
But let’s move forward and see if we can
answer some of those questions one more time.
First of all, this was figure we used to
introduce the process of transcription,
looking at a
and the pink square represents
the entire RNA polymerase
and we are simply using
the template strand
to make a messenger RNA that
looks like the coding strand
and that messenger RNA can then go on
to be translated in a future lecture.
So in this lecture, I’d like to
introduce you to the idea that
RNA polymerase is not quite
as simple as a pink box.
We know that there are several
domains to the RNA polymerase
and there’s the core enzyme.
That core enzyme is really in charge
of doing the synthesis of RNA
and adding RNA nucleotides to
the growing messenger RNA.
However, there are some other subunits
associated with the core enzyme
that actually helped get the
core enzyme onto to the DNA
in the right location and read
it in the right direction.
All of these pieces together,
we call the holoenzyme.
So the holoenzyme is
the whole enzyme.
That whole enzyme is going
to associate itself
with the DNA strand in
the correct location
and the holoenzyme will recognize
the promoter sequence for the DNA,
and it will bind to that DNA and begin the
process of transcribing the genes.
So this is the phase of where
I’m at laptop in my office
and I am hitting print so
that I can print the recipe
and take a copy that I can get dirt all
over in my kitchen and make the recipe.
So right now, we’re taking the copy and let
me introduce you to the promoter sequence.
The promoter sequence is before
the start site for transcription
and the enzyme will
dock onto this.
The holoenzyme will dock
onto this promoter sequence.
These promoter sequences in prokaryotes
have some distinct regions.
There are two sites of sequences
that are very common and conserved
in all bacterial cells.
There is a -35 base sequence.
So this is 35 base pairs before the
start site that’s why we call it -35.
And it has a very conserved sequence.
And then we have the -10 sequence, so
10 base pairs before the start site.
So the holoenzyme docks
onto this promoter region
and unwinds a very small region of the
DNA, starting at the -10 sequence
and only begins transcription at the start
site and continues in that direction.
How does it know which way to go?
Well, because of the orientation of these
two pieces, the -35 and the -10 sequence,
we can get directionality, which
direction the polymerase needs to read,
and which strand is the template strand
and which strand is the coding strand.
So the asymmetry of
these two locations
allows the polymerase to
know which direction to go.
So here we have initiated transcription.