1. MUTATIONS
• An error is the copying of a sequence of
bases is called a mutation.
• Can also occur during replication.
Base error can also occur during
transcription in protein synthesis.
2. These errors may have widely
varying consequences:
• the code for valine in mRNA can be GUA, GUG,
GUC, or GUU.
• In DNA, these codons correspond to
GTA, GTG, GTC, and GTT.
3. •Ionizing radiation (X rays, ultraviolet light,
gamma rays) can cause mutations.
Furthermore, a large number of organic
compounds can induce mutations by
reacting with DNA. Such compounds are
called mutagens.
•Many com- pounds (both synthetic and
natural) are mutagens, and some can
cause cancer when introduced into the
body. These substances are called
carcinogens
4. Mutations and Biochemical Evolution
We can trace the genetic relationship of different species through the
variability of thee amino acid sequences in different proteins.
The genes of closely related species, such as humans and apes,
have very similar primary structures, premsumably because these
two species diverged on the evolutionary tree only recently.
5. An oncogene is a gene that in one way or another participates in the
development of cancer. Cancer cells differ from normal cells in a variety
of structural and metabolic ways.
ONCOGENES
The uncontrolled proliferation allows cells to spread, invade other
tissues, and colonize them, in a process called metastasis
The normal EGF gene in the normal cell was tumed into an
oncogene, we call it a proto-oncogene.
Such conversions of a proto-onicogene to an oncogene can occur not
just by viral invasion, but also by a mutation of the gene.
6. A Central Tumor Suppressor Protein
Not all cancer-causing gene mutations have their origin is in
oncorgene. There are some 36 known tumor suppressor genes, the
products of which are proteins controlling cell growth. None of
them is more important than the protein with a molar mass of
53,000, simply named p53.
In about 40% of all cance cases, the tumor contains p53 that
underwent mutation. Mutated p53 protein can be found in 55% of
lung cancers, about half of all colon and rectal cancers, and some 40%
of lymphomas, stomach cancer, and pancreatic cancers. In addition, in
one third of all soft tissue sarcomas, p53 is inactive, eventhough it did
not undergo mutation.
7. Recombinant DNA techniques At this time, these DNA
techniques are being used mostly in bacteria, plants, and
test animals (such as mice), but they are slowly being
applied to humans as well.
One example of recombinant DNA techniques begins with
certain circular DNA molecules found in the cells of the
bacterium Escherichia coli. These molecules, called
plasmids.
8. The enzyme is so programmed that
whenever it finds this specific
sequence of bases in a DNA molecule,
it cleaves it as shown. Because a
plasmid is circular, cleaving it in this
way produces a double stranded chain
with two ends (as shown on the
figure). These are called "sticky ends"
because on one strand each has
several free bases that are ready to
pair up with a complementary section
if the can find one.
9. Gene Therapy
While viruses have traditionally been seen as
problems for humans, there is one field where
they are now being used for good. Viruses can be
used to alter somatic cells, where a genetic
disease in treated by the introduction of a gene
for a missing protein. This process is called gene
therapy.
10. The most successful form of gene therapy to date involves
the gene for adenosine deaminase (ADA), an enzyme
involved in purine catabolism. If this enzyme is missing,
dATP builde up in tissues, inhibiting the action of the
enzyme ribonucleotide reductase.
The result a deficienty of the other three
deoxyribonucleoside triphosphates (dNTPs). The dATP (in
excess) and the other three dNTPs (deficient) are
precursors for DNA synthesis. This imbalance particularly
affects DNA synthesis in lymphocytes, on which much of
the immune response depends.
11. Individuals who are homozygous for adenosine
deaminase deficiency develop severe combined immune
deficiency (SCID), the "bubble boy" syndrome. They are
prone to infection because of their highly compromised
immune systems. The ultimate goal of the planned gene
therapy is to take bone marrow cells from affected
individuals, introduce the gene for adenosine deaminase
into the cells using a virus as a vector, and then
reintroduce the bone marrow cells in the body, where
they will produce the desired enzyme.
12. Two type of delivery methods in human gene
therapy:
The first, called ex vivo, is the type used to
combat SCID. Ex vivo means that somatic
cells are removed from the patient, altered
with the gene therapy, and then returned to
the patient. The most common vector for
this approach is Maloney murine leukemia
virus (MMLV).
13. shows how the virus is used for
gene therapy. Some of the MMLV
is altered to remove the gag, pol,
and env genes, rendering the
virus unable to replicate. These
genes are replaced with an
expression cassette, which
contains the gene being
administered, such as the ADA
gene, along with a suitable
promoter. This mutated virus is
used to infect the packaging cell
line.
14. In the second delivery method, called in vivo, the
virus is used to directly infect the patient's
tissues. The most common vector for this delivery
is the DNA virus, adenovirus. A particular vector
can be chosen based on specific receptors on the
target tissue. Adenovirus has receptors in lung
and liver cells, and it has been used in clinical
trials for gene therapy of cystic fibrosis and
ornithine transcarbamoylase deficiency.
