EukaryGeneRegulation211S - Eukaryotic Gene Expression and...

Eukaryotic Gene Expression and Regulation - 1
We have recently discussed some examples of how viruses regulate gene expression
and the operon of prokaryotic gene expression. The eukaryotic genome is more
complex than the prokaryotic genome. Not only do eukaryotes have more DNA and
more genes than prokaryotes, in multicellular organisms different genes are activated
or repressed in different tissues from the time of early development, so we have
differential or selective gene expression depending on the cell and tissue type.
There are also more variations in the DNA of eukaryotes. As we shall study in our
section on genomics, only about 2 – 5% (or less) of genes code for proteins.
Another portion codes for RNA, but most of our DNA is not transcribed. Being able to
sequence genomes of many species, including the human genome, has been of
tremendous benefit to the growing understanding of eukaryotic DNA and its gene
expression. We will look at genomics and the characteristics of eukaryotic DNA in our
next section.
To begin our discussion of gene regulation in eukaryotes, let's briefly review the
structure of the eukaryotic DNA molecule.
We have learned that a chromosome is composed of DNA that is 2 nm in diameter
wrapped around a set of histone proteins to form a bead-like set of nucleosomes that
are about 10nm in diameter.
We know, too, that during mitosis, chromosomes condense. (There are also
intermediate levels of DNA organization that are important to our understanding of
gene function and regulation.) Nucleosomes coil to form a chromatin fiber of about
30nm in diameter.
The chromatin fibers continue to fold to form 300nm looped domains along the
chromosome during prophase. The loops are anchored to a non-histone protein
scaffold.

Eukaryotic Gene Expression and Regulation - 2
Ultimately, at metaphase, chromosomes achieve their densest packing of about
700nm per chromatid and are readily visible with a microscope.
However, packing of chromosomes is not restricted to condensation for mitosis.
During Interphase, many regions of a chromosome remain as 10nm and 30nm
chromatin fibers, anchored to the nuclear matrix and some regions, including
telomeres and centromeres, remain tightly condensed and clumped. Highly condensed
chromatin regions are called heterochromatin. The heterochromatin is too
compacted to be transcribed. The less compacted regions of chromatin are known as
euchromatin. Only the more stretched out or dispersed euchromatin can be
transcribed. Cells also have chemical means of blocking DNA access for gene
expression, as we shall discuss.
Another function of the nuclear matrix is to organize the chromosomes during
Interphase. Each chromosome has a fixed, non-overlapping territory within the nuclear
matrix.