So now, let’s take a quick look at a
summary of all of this gene expression
before we move on or the
translational portion at least
because in our eukaryotic cell, we
have a little bit more complexity.
We have transcription where we are
making a copy with messenger RNA
using DNA polymerase II and we
make this copy of messenger RNA.
In a eukaryotic cell, we have
to do something with that
because we have to be ready
to go out into the cytosol.
So that is where we have messenger
RNA processing to make mature mRNA.
We chop out all the introns,
but we chop them out and we add a 5’ cap
and poly-A tail to our messenger RNA.
It is now mature and ready to go.
Once that messenger RNA
leaves the nucleus,
it is able to be translated.
The ribosome will assemble
the small subunit
and the messenger RNA
will assemble with that.
The 5’ cap will be the leading end.
Recall there is an unstranslated region,
so the start codon will be a little
distanced down the messenger RNA.
Once we get there, we
assemble the whole ribosome
and we bring in the tRNAs and we begin
the process of the elongation cycle.
As the polypeptide is elongated,
the ribosome is moving
along the messenger RNA,
sort of chomp, chomp, chomps
three base pairs at a time.
And when it is time for the end of the
sequence, we’ll run into a stop codon
and the release factor or
termination factor comes into play,
drops into the A-site,
blows the whole thing up
and we release the
and that will go on to do whatever a
polypeptide chain is supposed to do.
Depending on what it is,
it could be insulin,
we’ll have some posttranslational
It will be folded into its final form,
maybe using chaperone proteins.
And if it’s to go, it’s in
the endoplasmic reticulum,
it will make its way to
the outer cell membrane
and be released into
the blood stream.
So that is the fate
of our polypeptides.
We have now covered how genes
are transcribed and translated,
so we know what gene
expression is all about.
Now, you should be able to
explain how tRNA charging works
and diagram a ribosomal anatomy,
how they come together,
as well as describe what happened
during the initiation, elongation, and
termination phases of translation.
And finally, you’ll be able to distinguish
the differences in bound and free ribosomes
as they translate their proteins.
Thanks so much for listening. I
will see you in the next lecture.