Gene
Expression
Control
Points
Ideally, each cell in your body is genetically
identical. This means that you have the same
two alleles for each of your 30,000 genes in
each of your somatic cells. Your cells develop
different structures and functions mainly due
to the fact that they vary in the combinations
of active genes.
Recall that the most common purpose of a
gene is to build a polypeptide via
transcription and translation. A gene is being
expressed when a cell works toward this
purpose. A gene is being repressed when
something interferes with polypeptide
production. At any given time, only 3-5% of
the DNA in a typical eukaryotic cell is actually
expressed.
The fact that genes may be expressed (turned
on) and repressed (turned off) further
complicates genetics. An organism may
possess a certain genotype, but if the gene is
repressed, it will not exhibit the predicted phenotype. In addition, an organism may be
genetically normal, but the inability to express necessary genes may still lead to abnormalities.
This packet summarizes how several factors are able to contribute to the expression and
repression of eukaryotic genes. Each typed description and most diagrams are taken from your
text.
In order to improve your chances on your eventual quiz, you’ll want to put the descriptions into
your own words in the space provided. Be sure to focus on how the genetic process is
expressed or repressed.
Chromatin Modification
(heterochromatin vs. euchromatin)
Euchromatin
Nucleosome (cluster of histones)
Heterochromatin
During interphase, portions of certain chromosomes in some cells exist in the highly condensed
state represented in the figure above. This type of interphase chromatin, which is visible with a
light microscope, is called heterochromatin, to distinguish it from the less compacted
euchromatin ("true chromatin"). What is the function of this selective condensation in
interphase cells? The formation of heterochromatin may be a sort of coarse adjustment in the
control of gene expression, for heterochromatin DNA is not transcribed.
DNA Methylation
DNA methylation is the attachment of methyl groups (-CH3)
to DNA bases after DNA is synthesized. The DNA of most
plants and animals has methylated bases, usually cytosine.
About 5% of the cytosine bases in methylated eukaryotic
DNA have methyl groups. Inactive DNA, such as that of
inactivated mammalian X chromosomes, is generally highly
methylated compared to DNA that is actively transcribed,
although there are exceptions. Comparison of the same
genes in different types of tissues shows that the genes are
usually more heavily methylated in cells where they are not
expressed. In addition, demethylating certain inactive genes
(removing their extra methyl groups) turns them on.
Histone Acetylation
There is mounting evidence that histone
acetylation and deacetylation play a direct
role in the regulation of gene transcription.
Histone acetylation is the attachment of
acetyl groups (-COCH3) to certain amino acids of histone proteins; deacetylation is the removal
of acetyl groups. When the histones of a nucleosome are acetylated, they change shape so that
they grip the DNA less tightly. As a result, RNA polymerase has easier access to genes in the
acetylated region. In other words, histone acetylation and the initiation of gene transcription
seem to be coupled structurally as well as functionally.
Alternative Splicing
The RNA transcripts of some
genes can be spliced in more
than one way, generating
different mRNA molecules.
Notice in this example that one
mRNA molecule has ended up
with the “C” exon and the other
with the “D” exon. With
alternative splicing, an organism
can get more than one type of
polypeptide from a single gene.
A
B
C
D
E
A
B
C
D
E
A
B
C
E
A
B
D
E
Protein Degradation (proteasomes)
A proteasome is an enormous protein complex that chops up unneeded proteins in the cell. In
most cases, the proteins attacked by a proteasome have been tagged with short chains of
ubiquitin, a small protein . A proteasome recognizes the ubiquitinated protein, unfolds it,
and sequesters it in its central cavity . Enzymatic components of the proteasome cut the
protein into small peptides , which can be further degraded by cytosolic enzymes.
Repressible Operon
Circle the correct boldfaced terms.
By default…
 Repressor active/inactive
 Operon is on/off
Operon may be turned on/off when reactant/product is present/absent.
Inducible Operon
Circle the correct boldfaced terms.
By default…
 Repressor active/inactive
 Operon is on/off
Operon may be turned on/off when reactant/product is present/absent.