Page 399 - Carrahers_Polymer_Chemistry,_Eighth_Edition

362 Carraher’s Polymer Chemistry

Duchenne muscular Retinitis pigmentosa
dystrophy
Anemia
Cleft palate

Hemophilia B

Color blindness

FIGURE 10.16 Selected mutations found in the X chromosome.

chromosome 21 can be present with more than one copy with the occupant having a healthy body, but
unfortunately those with an extra chromosome 21 are not normal but rather have Down syndrome.
The chromosomes are named in general order of size starting from the largest as chromosome 1
through 22 though it has recently been found that chromosome 21 is smaller than chromosome 22.
We also have as the remaining chromosome the twined sex chromosomes with women having two
large X chromosomes and men having one large X and a small Y. The X chromosome, in size, falls
between chromosomes 7 and 8, while the Y chromosome is indeed the smallest. Note that even
through the XY chromosome combinations are together one chromosome, researchers often refer to
each part in terms of a chromosome though in truth they are only a part of the total twined sex chro-
mosome. The variation from the smallest to the largest of the chromosomes is about 25 fold.
While 23 chromosomes is a relatively large number, it is by no means the largest number of
chromosomes within a species. Normal chromosome numbers for selected organisms appears in
Table 10.4.
All eukaryotic cells in our bodies contain the same 23 chromosomes with the same DNA base
sequences. The lone differences are the mitochondria. The mitochondria in typical somatic cells
contains less than 0.1% of the cell’s DNA but in fertilized and dividing egg cells this number is
greater. mtDNA is much smaller, often containing less than 20,000 base pairs. The value for humans
is 16,569 base pairs. The mtDNA is a circular duplex. mDNA codes for the mitochondrial tRNAs
and rRNAs but only a fraction of the mitochondrial proteins. More than 95% of the mitochondrial
proteins are encoded by nuclear DNA. The mitochondria divides when the cell divides.
The association of a particular disease with a particular gene or group of genes is rapidly increas-
ing. A spot check of www.ncbi.nlm.nih.gov/omim, the online version of Mendelian Inheritance
in Man, OMIN, gives an ongoing updated progress report of this activity. Currently, about 1,500
disease-mutations have been entered. With the advent of the mapping comes a number of shifts in
thinking and activity. Thus, we will move from so-called map-based gene discovery to looking at
the particular activity of gene sequences; from association of particular gene-associated diseases to
looking at tendency and susceptibility for given conditions and the variation with tendency/suscep-
tibility between individuals; from looking at the activity of a single gene or gene location to inves-
tigating combined activities of several genes from varying locations; from so-called genomics or
studying of genes themselves to proteomics and studying of the interaction between the genes and

proteins; from gene action to gene regulation; and from specific mutations to the mechanisms and
causes of such mutations. Much of this is a guessing game—hopefully an educated and educating
guessing game and it is currently very costly. As new techniques and strategies are developed the
cost should decrease.

9/14/2010 3:41:25 PM
K10478.indb 362 9/14/2010 3:41:25 PM
K10478.indb 362