Biology Reference
In-Depth Information
5.1. Early genome projects: Chromosome maps
Chromosomes, when carefully stained, show a banding pattern. This pattern
is usually fairly coarse, yet extremely rich in the giant chromosomes that are
found in the salivary glands of the larvae of diptera (e.g., flies). Their banding
pattern provided a coordinate system to localize genes on a chromosome long
before it was found that a chromosome consists of a long strain of DNA (coiled
on histones) or, in the case of the giant chromosomes, of a bundle of parallel
strains.
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Morgan and his coworkers, mainly Sturtevant and C. B. Bridges, man-
aged to assign genes to certain
loci
on the chromosomes by analyzing partial
linkage between different Mendelian genes. Linkage of an eye color mutation
of
Drosophila melanogaster
to sex gave the starting point to relate genes to
chromosomes, since the sex chromosome could be easily identified under the
microscope. A large amount of data was collected during more than 20 years.
Eventually, fairly detailed chromosome maps were obtained (C. B. Bridges,
1935, 1938; P. N. Bridges, 1942). The compilation of the chromosome maps of
D. melanogaster
can be regarded as an early genome project to the extent that it
was an attempt to catalogue the complete inventory of structural features of the
chromosome and relate all known genes to these structures. Similar maps were
compiled later on for species lacking giant chromosomes, including
H. sapiens
(McKusick, 1998).
After the discovery of the DNA structure, gene mapping could move on to the
molecular level. The circular chromosomemolecule of
E. coli
was the first genome
that was tackled. Again a coordinate system was needed. A measure was found in
the duration of bacterial conjugation required for the transfer of a particular gene. A
100-min scale allows addressing any place on the chromosome. Thus, the genes -
as identified by phenotypic effects - can be mapped on the chromosome (Jacob
& Wollman, 1958). Again, the aim of the project was to “relate information on
[
E. coli
] K-12 genetic material gained by genetic and by physical methods” (p. 92).
5.2. Molecular genome projects
The refinement of the gene map of
E. coli
by analyzing the sequence of many of
its genes may count as the first genome project with molecular resolution. Later
on, with the development of quicker and partially automated DNA sequencing
techniques, scientists began to sequence not only single genes but also the whole
genome.
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This was a serious shift in perspective. The older genome projects
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In addition to the bands, there are larger morphological landmarks, dubbed, e.g., dog-collar, onion, and ballet
skirt, that help identify individual (parts of) chromosomes even when these are strongly altered (Bridges,
1935).
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As the perspective had shifted to gathering complete sequence information,
Haemophilus influenzae
,a
bacterium with a much smaller genome than
E. coli
, was a more promising object to finish such a task. Its
