Index Notes Labs Web Quests Assignments Quizzes Links Student Work
Genes are expressed during
interphase meaning that this is when they are transcribed and translated.
Cells must constantly turn genes on
and off in response to signals from their external and internal environments.
Gene expression must also be
controlled for cellular differentiation (the specificity of cells for different
Highly specialized cells only
express a tiny fraction f their genes.
The enzymes that transcribe DNA
must find the right genes at the right time or serious imbalances and diseases
Gene activity is regulated by
DNA-binding proteins that may interact with other proteins.
Usually, it is the transcription that is specifically controlled.
The Structural Organization
Eukaryotic chromatin consists of
DNA that is complexed with a large amount of protein.
During interphase, the chromatin
fibers are highly extended and tangled.
During mitosis, these strands coil
and fold up to form a number of short, thick discrete chromosomes.
Eukaryotic chromosomes contain a
great deal of DNA
Histones are responsible for the
first level of DNA packing in chromatin.
The amount of histones is equal to
the amount of DNA present.
Histones are highly positive and
are attracted to the highly negative DNA.
There are five types of histones.
Unfolded chromatin looks like beads
on a string. Each bead is a
nucleosome which is the basic unit of DNA packing.
The nucleosome consists of DNA
wound around a protein core composed of two molecules of each histone (H2A, H2B,
H3, and H4
H1 may be attached to the outside
of the bead.
Higher Levels of DNA Packing
With the aid of H1, the beaded
string can coil tightly to make a cylinder of 30nm in diameter.
This is known as the 30nm chromatin fiber.
This 30nm chromatin fiber the forms
loops called looped domains which are attached to a nonhistone protein scaffold.
The looped domains then coil and
fold on themselves.
Chromatin during interphase is in
the 30nm chromatin fiber.
Some think that the interphase
chromatin is attached to a nuclear scaffolding that helps organize areas of
Even during interphase portions of
certain chromosomes are dense and is referred to as heterochromatin.
Euchromatin is true uncondensed
Heterochromatin is not transcribed.
Noncoding Sequences and Gene
In eukaryotic genomes, most of the
DNA does not encode protein or RNA.
Certain sequences may be present in
multiple copies and coding sequences may be interrupted by long stretches of
noncoding DNA called introns.
Approximately 10% to 25% of the
total DNA is made up of short sequences repeated in series of thousands of
The nucleotide compositions of
highly repetitive sequences are often different enough to have a different
Satellite DNA is DNA that can be
separated by ultracentrifugation because it appears as a “satellite” band
separated from the rest of the DNA.
Most of the satellite DNA is
located at the tips and at the centromeres.
Satellite sequences are important
at the ends of chromosomes called the telemetries.
Telemetries are repeating sequences
at the ends of each chromosome to prevent the shortening of the chromosomes
Telemetries periodically restores
this sequence to the ends of the DNA molecule.
Many of the repeating sequences are
These transposons are generally
regarded as nonfunctional but have been associated with a number of diseases
like elephant man’s disease and a number of different forms of cancer.
Normally, these are repeating
sequences but mutations can take place which causes malfunctions.
These malfunctions include fragile X syndrome and Huntington’s disease.
DNA sequences that code for
proteins or RNA (genes) are usually present as unique sequences or single copies
in the genome.
some genes are represented by more than one copy and others resemble each other
in nucleotide sequence.
A collection of identical or very
similar genes I called a multigene family.
The members of a multigene family
may be clustered or dispersed in the genome.
Some exist as multiple identical
genes and are clustered together.
Multigene families usually consist
of genes for RNA products like those for ribosomal RNA.
They are repeated in series and
enable the cell to produce millions of ribosomes.
An example of nonidentical genes
are those the code for the alpha and beta polypeptide in hemoglobin.
One family is found on chromosome
16 and codes for the alpha globulin while another family on chromosome 11 codes
for beta globulin.
The different versions are
expressed at different times during development.
Families of genes arise from one
gene by tandem gene duplication and results from mistakes made in DNA
replication and recombination.
Families of nonidentical genes
arise from mutations that accumulate in duplicated genes over a long period of
The existence of DNA segments
called pseudogenes is evidence of this.
Pseudogenes have sequences very
similar to functional genes but lack the sites for gene expression.
Significant amounts of noncoding
DNA are found within genes as introns.
The Control of Gene
One function of packaging DNA is
putting a great deal of information into a small area.
This packaging is not random; the
physical state of DNA within and around a gene is important in helping control
which regions are available for transcription.
Thus, condensed heterochromatin is
not expressed and location relative to scaffold attachment sites and Nucleosomes
can determine whether a specific gene is expressed.
Being available does not ensure
that a gene will be expressed.
Cells must be able to alter gene
expression in response to changing environments, external signals or new demands
These genes must also be tightly
Organization of a Typical
Eukaryotic DNA that makes up a gene
is typically organized as enhancer sequences, promoters, exons, introns, and the
poly A addition site. This
information is the transcribed with the introns left out to form mRNA.
The mRNA is then translated into a polypeptide segment.
The difference between this and a
prokaryotic cell is the presence of introns. The introns are removed during RNA
processing. The processing of the
RNA also includes the poly a tale at the 3’ end and the GTP cap at the 5’
Another feature is the presence of
other non-coding control sequences. Some located in close proximity to the promoter.
These genes are called enhancers and have an influence on transcription
of the associated gene.
RNA synthesis begins with an RNA
polymerase enzyme acting at a promoter with numerous proteins called
Once the appropriate initiation
complex forms, the polymerase moves on the DNA template producing a strand of
Transcription factors and enhancer
sites play an important role in control of gene expression.
Transcription is probably enhanced
when a loop in the DNA brings the transcription factor attached to the enhancer
in contact with the transcription factor and polymerase at the promoter region.
The control of transcription
depends on regulatory proteins that bind selectively to DNA and to other
Hundreds have been discovered.
Each domain (structural region)
that binds to DNA is made of one of three types.
Transcription alone does not make
The expression is measured in types
and amounts of functional proteins.
Gene expression may be blocked or
stimulated at any posttranscriptional step.
RNA Processing and Export
A cell must process initial
transcripts before they may act as mRNA, tRNA or rRNA.
An mRNA transcript must receive a
5’ cap and a poly-A tail and the introns must be removed and the exons spliced
These mRNAs are transported out of
the nucleus through nuclear pores and usually have proteins attached.
In the cytoplasm, the mRNA
interacts with a number of proteins and may be associated with ribosomes to
Each step represents a possibility