Glenn raises a good point. There is also very interesting data to suggest
that the organization of the bacterial genome is more "advanced" than
eukaryotic genomes.
Eukaryotic genes are uniquely characterized as being broken, or
discontinuous. That is, any DNA sequence that encodes a specific protein
consists of a series of partial coding sequences, called exons, that are
interrupted by noncoding intervening sequences, called introns. The entire
sequence, including exons and introns, is transcribed into RNA, and enzymes
in the nucleus then "process" the RNA by removing the noncoding introns that
separate the coding exons. This involves the recognition of specific
splicing sequences that flank each coding exon. As the introns are cut out,
the adjacent exons are spliced together which results in the production of
messenger RNA which carries uninterrupted linear sequence information that
will be translated into protein. In the sequence of the spliced messenger
RNA, one can dectect the residual splice recognition sequences.
When the sequences of bacterial genes are compared to the sequence of
homologous eukaryotic genes, these residual splice sequences are found in
bacterial genes at the same sites where adjacent exons in eukaryotic genes
are spliced together. This is taken as evidence that bacterial genomes once
carried these noncoding introns.
So, does this mean that bacteria are more "advanced" than mammals which
still carry this noncoding "junk" DNA?
Steve
____________________________________________________________
Steven S. Clark, Ph.D . Phone: 608/263-9137
Associate Professor FAX: 608/263-4226
Dept. of Human Oncology and Email: ssclark@facstaff.wisc.edu
UW Comprehensive Cancer Center
CSC K4-432
600 Highland Ave.
Madison, WI 53792
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