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Naturally Occurring Polymers—Animals 357

DNA is not yet fully know (Section 10.10). Further, as noted above we are not fully aware of how
replication occurs in such a precise manner.
A third major area of evolving knowledge involves how so much information is packed into the
relatively small number of genes we believe we now have. Shortly ago we believed that the number
of human genes was on the order of 100,000, a number that appeared appropriate even after the fi rst
two animal genomes were deciphered. The roundworm, sequenced in 1998 has 19,098 genes, and in
2000 the fruit fly was found to have 13,601 genes. Currently the number of human genes is believed

by many, but not all, to be on the order of 20,000–40,000, less than half of the original number.
This means that the genes are probably more complex than originally believed. It is now believed
that the reason why we are able to function with so few genes is that our genes carry out a variety
of activities. This ability is a consequence of several features. One involves the coordinated interac-
tions between genes, proteins, and groups of proteins with variations of the interactions changing
with time and on different levels. There is then a complex network with its own dynamics. It is a
network that is probably largely absent in lower species such as the roundworm. The roundworm
is a little tubed animal with a body composed of only 959 cells of which 302 are neurons in what
passes for a brain. Humans, by comparison have 100 trillion cells including 100 billion brain cells.
While protein domains exist in primitive animals such as the roundworm, they are not as “creative”
as those found in more advanced animals. These domains that we have allow the “creation” of more
complex proteins.
It appears that another way to gain complexity is the division of genes into different segments
and by using them in different combinations increasing the possible complexity. These protein cod-
ing sequences are known as exons and the DNA in between them as introns. The initial transcript of
a gene is processed by a spliceosome that strips out the introns and joins the exons together into dif-
ferent groupings governed by other active agents in the overall process. This ability to make differ-
ent proteins from the same gene is called alternative splicing. Alternative splicing is more common
with the higher species. Related to this is the ability of our immune system to cut and paste together
varying genetic segments that allow the immune system to be effective against unwanted invaders.
In eukaryotic cells transcription and translation occur in two distinct temporal and spacial events
whereas in prokaryotic cells it occurs in one step. Humans have eukaryotic cells so we will look at
this process. Transcription occurs on DNA in the nucleus and translation occurs on ribosomes in
the cytoplasm.
Our genes are split into coding or exon and noncoding or intron regions. The introns are removed
from the primary transcript when it is made into a so-called mature or completed RNA—namely
mRNA, tRNA, rRNA, and so on.
Such split genes occur in a wide variety of sizes and interruptions. Even so, the transcription
must be precise. Several features are worth noting about this process. First, the order of the exons
is fixed as is the size and order of the introns. Also, the order of the exons on the mature RNA is

the same as in the original DNA. Second, each gene has the same pattern and size of exons and
introns in all tissues and cells of the organism and, with the exception of the immune response

and the major histocompatibility complex, no cell-specific arrangements exist. Third, many introns
have nonsense codons in all three reading frames so nuclear introns are nontranslatable. Introns are
found in the genes of mitochondria and chromoplasts as well as in nuclear genes.
We must remember that each of these steps consist of simple, thought complex when considered
as a whole, chemical reactions.
Another source of increased complexity involves the fact that human proteins often have sugars
and other chemical groups attached to them allowing subtle, and possibly not so subtle, changes in
behavior to occur.
Also, it has been found that at least some, about 75%, of the DNA sequences in our genome, is
apparently nonactive material. The coding regions may occupy only about 1%–1.5% of the genome.
(It must be remembered that while only a small amount contain coding regions, that the structure
about these regions is also important and that these structures are also important to the overall

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