So what you see on the screen
here are some antibody
stained chromosomes and their antibodies are
designed to bind specifically to the telomeres.
And when you see the telomeres, not surprisingly based on
what I just told you, they are always at the ends of a DNA.
Now telomeres are interesting sequences. They are
short pieces of DNA that are replicated
over and over and over and over, okay?
Now the sequences vary between about 3,000 and
20,000 base pairs at the end of a chromosome.
And when they were first discovered, people described
them as junk DNA; because, they didn't code for protein
and they had this really odd feature that they
were just repeats of the same sequence.
In humans the sequences are repeats of TTAGGG.
And so that was just sort of a mindless sequences
and that's why they originally called junk DNA.
But it later it become apparent
why these sequences were important.
If you loose this repeated sequence and this
repeated sequence does not code for protein,
then shortening the sequence
has very little effect
until you lose all the repeats.
Now some people think that the cellular life
span is actually linked to telomere length.
The longer the telomeres, the longer the lifespan; because,
once you shorten past the point where those repeats are
you gonna start losing critical DNA.
How do the telomers get there? The telomeres in
eukaryotic cells have to be built in each generation.
And they are built by germ cells
and embryonic cells, as we shall see.
Now the building of the telomers requires
action of an enzyme known as telomerase.
Telomerase works as follows.
So telomerase is also a reverse transcriptase
and it's a very unusual reverse transcriptase. Now I'd
remind you that reverse transcriptase is an enzyme
that makes DNA by copying RNA.
So, how does it work?
The telomerase enzyme, the unusual
thing it does, is it carries
a template strand with it,
that is something to copy.
And the telomerase is present primarily
in embryonic cells and in cancer cells.
Now let's look at how
the telomerase functions
and then we will have a better idea about
how the telomerase themselves get made.
You can see on the image here that
there is a eukaryotic chromosome
the left portion of which is in yellow and then
there is a little gap and at that gap you see the
blue strand at the bottom and
the outline of the telomerase.
So the telomerase has brought this blue strand
to the end of this linear chromosome.
And if it's formed base pairs between the
A and the T, the A and the T and the G and the C.
So you see those three base pairs at the joint
between the blue and the yellow strands above.
So the blue again is the RNA template
that the telomerase has brought.
What the telomerase does after making these
base pairs is if you look at the top strand it's
pointed in the 5 prime to 3 prime direction.
What does it do? It takes that as a primer
and it extends it. So it extends the primer
and copies the RNA that it is carrying.
And then what does it do? It slides down.
And so it keeps copying the same sequence,
over and over and over and over.
In doing that, the repeat
of the telomere is made.
Now this can happen thousands of time lengthening
and lengthening and lengthening and lengthening
the eukaryotic chromosome.
So after that top strand has been replicated
now the bottom strand can also be made. But again
remember this is gonna be like the lagging strand
that's gonna be made in pieces.
So the pieces are going to involve
primase and then moving inwards.
Now the very last primer at the very
far end is not going to be replaced.
But the telomere has grown by
thousands of bases in the process
and enabled that chromosome to
be ready for the next generation.
So there we have the newly added repeat. There
is the primase that has made the RNA primer
and now we see on the other strand that
is the bottom strand is replicated.
And in doing so the eukaryotic
chromosome has been replicated.