Page 393 - Carrahers_Polymer_Chemistry,_Eighth_Edition

356 Carraher’s Polymer Chemistry

profiling has also been used in the identification of 9/11 victims, and a number of mass graves

throughout the world.
In 1998, the Combined DNA Index System (CODIS) was begun by the FBI. It is an automated

forensic data bank and contains DNA profile data related to most of the recent major crimes. It is
also connected with state systems as well as similar worldwide data bases.
10.10 HUMAN GENOME—GENERAL

The unraveling of much of the human genome is one of the most important advances made since our
dawning. An online tour of the human genome is found at a number of web sites allowing access to
some of this valuable information.
As noted before, there are two general kinds of cells, those having a membrane-bound nucleus called
eukaryotic cells, and those without a nuclear envelope called prokaryotic cells. Humans have eukary-
otic cells. Other than blood cells, eukaryotic cells contain a nucleus that contains the genome, the com-
plete set of genes. Unless noted otherwise our discussion will be restricted to eukaryotic cells.
The human genome and other mammalian cells contain about 600 times as much DNA as E. coli.
But many plants and amphibians contain an even greater amount. While eukaryotic cells contains
more DNA than do bacterial cells (prokaryotic cells), the gene density of bacterial cells is greater.
For instance, human DNA contains about 50 genes mm while E. coli contains in excess of 2,500
genes mm. As will be noted, the human genome, while it contains a lot of nongene material, this
nongene material appears to be active in the organization of the chromosome structure playing a
number of roles, including supplying hyperfine contours to assist in the replication, protection, and

selectivity of the sites. The contour length, the stretched out helical length, of the human genome
material in one cell is about 2 m in comparison with about 1.7 m for E. coli. An average human body
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has about 10 cells giving a total length that is equivalent in length to traveling to and from the earth
and sun about 500 times or 1,000 one way trips.

Replication occurs with a remarkably high degree of fidelity such that errors occurs only once per
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about 1,000 to 10,000 replications or an average single missed base for every 10 –10 bases added.
This highly accurate reproduction occurs because of a number of reasons, including probably some
that are as yet unknown. As noted before, the GC group has three hydrogen bonds while the AT has
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two. In vitro studies have found that DNA polymerases inserts one incorrect base for every 10 –10
correct ones. Thus, other features are in place that assist in this process. Some mistakes are iden-
tifi ed and then corrected. One mechanism intrinsic to virtually all DNA polymerases is a separate
3′–5′ exonuclease activity that double-checks each nucleotide after it has been added. This process
is very precise. If a wrong base has been added this enzyme prevents addition of the next nucleotide
removing the mispaired nucleotide and then allowing the polymerization to continue. This activity
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is called proofreading and it is believed to increase the accuracy another 10 –10 fold. Combining
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the accuracy factors results in one net error for every 10 –10 base pairs, still short of what is found.
Thus, other factors are at work.
In general, replication occurs simultaneously as both strands are unwound. It is bidirectional
with both ends of the loop having preferentially active starting points or sites. A new strand is
synthesized in the 5′ to 3′ direction with the free 3′ hydroxyl being the point at which the DNA is
elongated. Because the two DNA strands are antiparallel, the strand serving as the template is read
from its 3′ end toward its 5′ end. If DNA replication always occurs in the 5′ to 3′ direction then how
can it occur simultaneously? The answer is that one of the strands is synthesized in relatively short
segments. The leading strand, or the strand that “naturally” is going in the correct direction, repli-
cates somewhat faster than the so-called lagging strand that is synthesized in a discontinuous matter
with the required direction occurs at the opposite end of the particular segment, consistent with the
observation that all new strands, and here strand segments, are synthesized in a 5′ to 3′ manner.
As a scientist, along with knowledge comes questions that in turn point to gathering more informa-
tion, and so forth. We have already noted that the precise folding and compaction for chromosomal

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