The forms and functions of
RNA are incredibly diverse,
as we shall see in this presentation.
In the first part I will talk about the processing of
RNA and how RNA can actually help process other RNAs.
Then I will talk about
an important process that exists
in eukaryotic cells whereby
RNAs again help to control
the synthesis of other RNAs.
And finally I will talk about genetic
code considerations which will include
how transfer RNAs are best prepared
for the process of translation.
Most RNAs that exists in cells
end up in a form different from what they started
and this process is called RNA processing.
The processing of RNAs can involve
a couple of things. It can involve
for example chemical alteration of the bases
that happens in some RNAs like transfer RNA.
It can also involve removal, deletion,
and alteration of the bases
within an RNA sequence as we
shall see. Now enzymes called RNAses
participate in the processing of
ribosomal and transfer RNAs in most cells.
The ribosomal RNAs are usually made as
larger molecules called pre-ribosomal RNAs
and these are processed in
the nucleolus of eukaryotes.
There are small RNAs called snoRNAs that help
in this processing, as I will describe.
Now transfer RNAs are also processed
but interestingly transfer RNAs
are made on the pre-ribosomal RNA, meaning that
in some cases they are made with the ribosomal RNA.
And the individual transfer RNAs are chopped out and
then modified as appropriate for their function.
The processing of transfer RNAs
involves quite a few enzymes.
But one of the enzymes
is actually not an enzyme
meaning that one of the things that catalyzes a
reaction is actually another RNA called a ribozyme.
This ribozyme is known as RNAase P and,
as we will see, it helps to form
the proper 5 prime end of a transfer RNA.
Now transfer RNAs, of course, are
the RNAs molecules that
carry the amino acid to the
ribosome for translation.
They are single strand of RNA that you can
see on the figure in the upper right,
having a 5 prime end
and a 3 prime end.
The 3 prime end is that part that sticks up
a little bit further on right on that image.
That 3 prime end with the sticking up part is
actually a specific sequence that ends in CCA,
and that CCA is added by a polymerase
after the RNA has been processed.
Now chemical modifications of ribosomal RNAs
and transfer RNAs
occur in prokaryotes and also in eukaryotes
and these chemical modifications are
quite common. They will actually change
what the structure of an
individual nucleotide base
is within these individual molecules.
Now you can see for example in
this figure shown on the right,
the conversion of uridine which
of course is a U within a transfer RNA
to a pseudouridine, as shown on the right.
And this transformation has added another
nitrogen to the ring, as you can see.
Now ribosomal RNA modifications in addition to some of
the general ones involving clipping out of sequences
are also help to long by some small nuclear
RNAs that are given the name snoRNAs.
So these small RNAs actually help to
process ribosomal RNAs to their final form.
Base alterations meaning the actually editing
and changing of base sequence within an RNA
can also occur and this happens in
some eukaryotic messenger RNAs
particularly among the lower eukaryotes on the evolutionary
ladder. But it also occurs in the human as well.
Now messenger RNAs in eukaryotes are very
different than messenger RNAs in prokaryotes
in comparing the ways in
which they get processed.
Messenger RNAs in prokaryotes get
very little processing
and the reason that this happens is
because, translation in prokaryotes
is occurring almost at the same
time as the RNA is being made.
And this is possible in prokaryotes;
because, prokaryotes don't have a nucleus
and transcription and translation
are occurring in the same place.
In eukaryotes, transcription
is occurring in the nucleus
and translation is occurring in the cytoplasm.
So between the nucleus and the cytoplasm ,
there is a significant amount of
alteration that can and does happen.
Now I have drawn on the screen here a representation
of a eukaryotic mRNA and it includes
some of those modifications
that I talked about.
First of all at the 5 prime end we
can see that the eukaryotic mRNAs
have a structure that we call a cap.
We don't see that structure of
a cap in a prokaryotic messenger RNA.
At the 3 prime end, or the other end of the
messenger RNA, we see what's called a poly A tail.
The poly A tail is the
addition of a large number of
adenine residues or adenosine residues
to the 3 prime end of the eukaryotic messenger RNA.
This polyadenylation appears
to play a very important role
in the stability of the messenger RNA
and also improving the
efficiency of translation
of the messenger RNA as it interacts with the ribosome.
The chemical modification at the 5 prime end
to put the cap on is an interesting one.
It involves the addition of a 7-methyl
guanosine to the 5 prime end of the messenger RNA.
But that 7-methyl guanosine
is added in an unusual way.
It's joined in a 5 prime to 5 prime linkage,
as you can see in the image there.
That 5 prime to 5 prime linkage
is very unusual in biochemistry
and this is the only place it occurs.
And that cap also appears
to play two functions.
First, it protects the mRNA against
degradation by nucleases.
Second, the cap plays a very important role
in helping to start the process
of translation in the ribosome.
Now the long string of the
As at the end is as I said are
probably there to help stabilize
the messenger RNA and function in transport.
There is a sequence within
the messenger RNA as it's being made
that goes AAUAAA.
And when that sequence is
encountered it is a signal to the cell
to chop the RNA and begin adding
the poly-A tail to the end of that.
Now in the middle of the messenger RNA, we are
seeing the final messenger RNA right here.
But the pre-messenger RNA that existed
may well have a had other sequences
that are not present in this final version.
The removal of those other sequences
from the internal parts of an RNA
is called splicing and splicing produces a product
that has those internal parts removed and the
final product shown as you see on the screen.