Gene Cloning

slower rate than junk DNA. (To be more accurate, the rate of change at the
DNA level is probably roughly constant for all bases, but changes in control
and coding regions are much more likely to be deleterious to the organism
than changes in the junk DNA and so these changes will be selected
against.) The net effect of this will be that changes in the control and coding
regions will be less common than those in the junk DNA. Therefore
when you compare genomes from different organisms the sequences of
interest are highlighted as regions of sequence similarity against the junk
DNA background. An example of the power of comparative biology comes
from studies on the pufferfish, Fugu rubripes. Fugu contains the same
number of genes as humans but has a relatively compact genome of 365
million base pairs when compared to other vertebrates, i.e. it is one eighth
the size of the human genome. The compact size is due to the presence of
very little repetitive DNA and the small size of its introns. Fugu contains
one gene for every 10 kb of genomic sequence. Comparison of the Fugu
and human sequences has shown that even in these two vertebrates that
are separated by 450 million years of evolution, there is some synteny.
More importantly, comparative analysis has led to the identification of 900
novel putative human genes.
By comparing the human genome sequence with the genome sequence
of more distantly related animals including other mammals, we can catalog
important sequences, including genes and regulatory regions, because
they have been conserved throughout evolution. In contrast, by comparing
the human genome with the recently sequenced chimpanzee genome it is
hoped that we will be able to identify the important differences that make
us human. The human and chimpanzee genomes that have been
sequenced only differ by 1.23%. However, some of the differences are
within SNPs that are present in both genomes so at these positions some
human and chimpanzee individuals will have the same sequence; the
“real” difference between the human and chimpanzee genomes may be
less than 1.06%. The fact that most human genes have homologues in all
other mammals means that we can use animal models for most human
genetic diseases to develop treatments that can then be used in human
medicine and, in many cases, veterinary medicine.
As with animals, the size of plant genomes varies considerably: from
around 50 megabases to over 100 gigabases. Plants thus exemplify the C-
value paradox particularly well. For example, the wheat genome is over five
times the size of the human genome. The main reason for the differences
in genome size is related to the number of copies of repeat sequences
within the genome: organisms such as wheat that have large genome sizes
tend to have genomes littered with genome-wide repeats. For example,
retrotransposons make up over 70% of the maize genome. The regular
occurrence of transposons within plant genomes has been exploited in
mapping studies. Another aspect of plant genetics is the phenomenon of
Genome Organization 23