In this talk, I'd like to discuss with you
the issue concerning histone dynamics,
their heritability, and their existence in the form of variants.
DNA in the nucleus is present in the form of chromatin,
and that consists of a complex together with proteins.
The main proteins found in chromatin are called histones.
They are small, basic proteins
called H3, H4, H2A, and H2B
for the core histones and H1 for the linker histone.
They ensure the compaction of DNA in the nucleus.
In the human cells about 2 meters of DNA
will fit in a volume of about a few micrometers
for the diameter of common somatic cells.
And this is represented here with a tennis ball on the right-hand panel.
So that will impose a problem of accessibility to the DNA
for all DNA transactions,
which thus necessitates dynamic properties
in order for all the metabolic pathways
of the DNA level to operate.
The additional aspect that histones can bring
in the chromatin organization
is a source of information.
Indeed, this organization can contribute to a differential use
of genetic information provided by the DNA
and thus, ensure a functional diversity
in the different cells.
Then the question that ensues is, how is it established,
and how can it be inherited?
If we now come to compaction,
which is the first aspect I wanted to present,
in the chromatin organization as depicted
by this watercolor, which was drawn by Nicolas Bouvier
from the ?"????
you can see that there are different levels
that ensure this compaction in the nucleus.
First, you have the DNA in the form of a helix,
which is wrapped around the core histones
to form the nucleosome—the sort of tennis ball that was represented before—
which then folds up further to give chromatin fibers,
which further fold up and then organize in the nucleus
to form different domains.
So this high level of compaction is
dynamic, and the enzymes that have to get access to DNA
have to deal with this kind of organization.
So if we get back to the DNA helix