Page 409 - Carrahers_Polymer_Chemistry,_Eighth_Edition

372 Carraher’s Polymer Chemistry

tumor-suppressor genes. Interestingly, some oncogenes, such as the MYC gene, also hold in
their code cell death. The death code is held in check by chemical signals know as survival
signals. Thus, if the MYC cell begins to operate in a cancer mode, the cell is signaled to kill
itself. There are in fact three different oncogenes, MYC, RAS, and BCL-2 that appear to hold
one another in balance. Normal cells can exist only if all three of these oncogenes are operating
“correctly.”
Mutations often occur but most have no long-term consequence. Some mutations are responsible
for specific diseases and may cause tumor growth and cancer. Mutations are responsible for some

of our diseases including genetic-related diseases. Figure 10.16 shows the general location on the X
chromosome of the genes where selected disease-related mutations have been found to occur. There
are a number of kinds of mutations. Many of them can be divided into two groups. Substitution
mutations occur when one of the bases in a condon is changed from the intended base creating a

flaw in the gene. Deletion mutations occur when a base is deleted often causing the other bases to
shift altering the remaining codons. Knowing where such mutations occur allows scientists to better
understand and treat such mutations.
When we take the sum total of humanity, it is not our human genomes, but rather a complex
group of events, that include our genome that actually make us what we are. In some cases, the
genome casts boundaries, but more likely, it tells us probabilities though some outcomes are so
sure as to be nearly certainties. We are humans with moral, ethical, and gene-driven tendencies and
freedoms. We have just begun another trip where moral, ethical, and gene-driven aspects all play a
role. Good fortune and wisdom to us in this new adventure.
The secrets of the human genome are just beginning to be uncovered, discovered, understood,
and finally utilized. It is an eventful, important trip to which we are both witness to and have an

essential stake in, and some of us may take an active part in. New genes are continually being
discovered. So while we have decoded the human genome, we have not unlocked most of its
secrets.
New information is being found daily. Some of this information is maybe intuitive and con-
sistent with information gathered from other sources. For instance, it is found that certain genes
behave differently in men and women. While men and women have the same genes, the information
expressed by the genes varies based on gender. The differences are particularly apparent in the liver
where the number of copies of a particular gene varied by gender meaning that there is a general
difference in how we metabolize drugs so that different doses of a drug is more suitable for one sex
than the other. The difference is greater than previously suspected and is leading for different treat-
ment strategies to be developed based on sex.
(Some ideas for the chromosome section were taken from the timely, lively, interesting, and easy
to read book—Ridley, M. [1999]: Genome, HarperCollins, NY.)

10.12 SPLICEOSOMES
As noted before, the number of genes in the human genome is actually relatively small, in the range
of 20,000–25,000 genes. Yet, these genes produce about 150,000 unique proteins so the notion that
each gene is responsible for coding for a single protein is not true. The missing link is called the spli-
ceosome, a large protein-RNA hybrid complex that is present in the nucleus of each of our cells. It
splices and recombines the RNA transcribed from our DNA into different forms before translation
into protein formation by the ribosome. The spliceosome cuts out introns and joins the remaining
exons into unique combinations of mRNA. These combinations are responsible for the synthesis of
the large number of proteins. The number of splice sites on our mRNA varies but is at least three.
Some human mRNAs get spliced into thousands of different combinations.
The spliceosome is truly immense being about 3 megadaltons in size involving five RNAs and

more than 150 proteins. It is present in only small amounts, less than one percentage of the dry weight
of a cell. By comparison, ribosomes compose about 25% of a cell’s dry weight. The spliceosome

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