Tazswana's Story: How Alternative
Splicing Leads to Genetic Disease
Handout 1—“Junk DNA”
Junk DNA- are the
introns of a pre mRNA.
They are junk because
of human evolution.
In prokaryotes splicing
does not occur.
Splicesome is the
machinery in charge of
cutting and pasting
mRNA so a person
cannot have a disease
caused by having
noncoding regions in
Handout 4—Tazswana’s Blood
Miss X’s Sample
The subject has severe
symptoms since the
function of hemoglobin is
to transport oxygen to the
body for aerobic
respiration, and her red
blood cells, are small and
weak due to the color and
distortion of shape.
Compared to the mother
and a normal person, the
altered shape causes a
protein to function
improperly or not at all
making her symptoms
Alternative Gene Splicing
Post-transcriptional modification in which a
single gene can code for multiple proteins.
Done in eukaryotes, prior to mRNA translation,
by the differential inclusion or exclusion of
regions of pre-mRNA.
Enables a single gene to increase its coding
capacity, allowing the synthesis of protein
isoforms that are structurally and functionally
Modes of Gene Splicing
Alternative selection of promoters: this is the only method of splicing which
can produce an alternative N-terminus domain in proteins. In this case, different
sets of promoters can be spliced with certain sets of other exons.
Alternative selection of cleavage/polyadenylation sites: this is the only
method of splicing which can produce an alternative C-terminus domain in
proteins. In this case, different sets of polyadenylation sites can be spliced with
the other exons.
Intron retaining mode: in this case, instead of splicing out an intron, the intron is
retained in the mRNA transcript. However, the intron must be properly encoding
for amino acids. The intron's code must be properly expressible, otherwise a stop
codon or a shift in the reading frame will cause the protein to be non-functional.
Exon cassette mode: in this case, certain exons are spliced out to alter the
sequence of amino acids in the expressed protein.
New proteins could be allowed to evolve much faster
than in prokaryotes.
Invalidates the old theory of one DNA sequence
coding for one polypeptide (the "one-gene-one-
For eukaryotes it was a very important step towards
higher efficiency, because information can be stored
much more economically.
Gene therapy is a technique for correcting defective genes
responsible for disease development.
A normal gene may be inserted into a nonspecific location within
the genome to replace a nonfunctional gene. This approach is
A normal gene is inserted into the genome to replace an
abnormal, disease-causing gene.
A carrier molecule called a vector must be used to deliver the
therapeutic gene to the patient's target cells. The most
common vector is a virus that has been genetically altered to
carry normal human DNA (retrovirus).
Target cells such as the patient's are infected with the viral
The vector then unloads its genetic material containing the
therapeutic human gene into the target cell. The generation of
a functional protein product from the therapeutic gene restores
the target cell to a normal state.
The commonly used virus in gene therapy is called a retrovirus.
It is a class of viruses that can create double-stranded DNA
copies of their RNA genomes.
Human immunodeficiency virus (HIV) is a retrovirus.
The simplest method is the direct introduction of therapeutic DNA
into target cells. This approach is limited in its application
because it can be used only with certain tissues and requires
large amounts of DNA.
Current gene therapy is experimental and has not proven very
successful in clinical trials.
Factors that have kept the therapy from becoming an effective
Short-lived nature of gene therapy
Problems with viral vectors
In the case of
thalassemia, it is
required the transfer of
healthy genes into the
stem cells of the bone
However, an important
problem with the
retroviruses methods is
ensuring that the β-
chains produced as a
result of the healthy
gene introduced are of
sufficiently high quantity
• Premier Biosoft International, (2008). Gene Splicing
Overview and Techniques. Retrieved April 27,
2008, from Premier Biosoft International Web
• Genetics Home Reference, (2008, April 25). What is
gene therapy?. Retrieved April 28, 2008,
from Genetics Home Reference Web site: