48
4 Life Evolves
Fig. 4.3 Left: A scheme of Mendelian inheritance. Center: Illustration of crossing over by Morgan
(1916). Right: Molecular structure of the Holliday junction that leads to crossing over
the phenomenon of crossing over (see Fig. 4.3, center), the exchange of gene sequences between two chromosomes (Morgan, 1916). The swapping of maternal and
paternal DNA enhances genetic variation and makes any offspring unique, otherwise
the genes residing on a parental chromosome would be inherited all together. The
structure enabling crossing over, a knot formed by two partially separated strands
of two double spirals, called the Holliday junction, is shown in the right-hand panel
of Fig. 4.3. The frequency of separation between particular genes due to crossing
over could be used to indicate the distance between them along the chromosome;
this was a useful tool before modern fluorescence-based sequencing methods were
developed.
Another complication, leading to a still higher genetic variation and influencing evolution, is the horizontal gene transfer common among single-cell organisms.
On these occasions, genes are transferred from a donor species to a possibly unrelated receiver species. This plays havoc with the phylogenetic tree connecting its
separated branches to a network of a richer structure (Fig. 4.4), and complicates
attempts to establish the origin of the various genetic lines.
It is easy now to forget that the genetics of the first half of the 20th century was
advancing in the dark, with a wrong idea about the nature of the carrier of inheritable traits. It was supposed that inheritance resided in a certain kind of particles,
which Darwin called gemmules, and it was established early in the 20th century that
this carrier in turn resided in the chromosomes – but it was assumed that genetic
information was contained in proteins. This was quite natural: proteins are ubiquitous in cells, and versatile. The very term, coined in 1838 by the great chemist
Jacob Berzelius after the Greek word proteios (primary), tells of their primary importance. The true culprit was only identified when Oswald Avery and coworkers
induced bacterial transformation using pure DNA (Avery et al, 1944). This quite
naturally attracted attention to nucleotides, culminating in the discovery of the double helix structure of DNA by James Watson and Francis Crick (1953).
In retrospect, it should be clear that proteins are unfit to serve as memory elements. They can, in principle, form autocatalytic replication cycles, but mistakes
in their composition are unforgivable. Each protein has its own conformations, and
4 Life Evolves
Fig. 4.3 Left: A scheme of Mendelian inheritance. Center: Illustration of crossing over by Morgan
(1916). Right: Molecular structure of the Holliday junction that leads to crossing over
the phenomenon of crossing over (see Fig. 4.3, center), the exchange of gene sequences between two chromosomes (Morgan, 1916). The swapping of maternal and
paternal DNA enhances genetic variation and makes any offspring unique, otherwise
the genes residing on a parental chromosome would be inherited all together. The
structure enabling crossing over, a knot formed by two partially separated strands
of two double spirals, called the Holliday junction, is shown in the right-hand panel
of Fig. 4.3. The frequency of separation between particular genes due to crossing
over could be used to indicate the distance between them along the chromosome;
this was a useful tool before modern fluorescence-based sequencing methods were
developed.
Another complication, leading to a still higher genetic variation and influencing evolution, is the horizontal gene transfer common among single-cell organisms.
On these occasions, genes are transferred from a donor species to a possibly unrelated receiver species. This plays havoc with the phylogenetic tree connecting its
separated branches to a network of a richer structure (Fig. 4.4), and complicates
attempts to establish the origin of the various genetic lines.
It is easy now to forget that the genetics of the first half of the 20th century was
advancing in the dark, with a wrong idea about the nature of the carrier of inheritable traits. It was supposed that inheritance resided in a certain kind of particles,
which Darwin called gemmules, and it was established early in the 20th century that
this carrier in turn resided in the chromosomes – but it was assumed that genetic
information was contained in proteins. This was quite natural: proteins are ubiquitous in cells, and versatile. The very term, coined in 1838 by the great chemist
Jacob Berzelius after the Greek word proteios (primary), tells of their primary importance. The true culprit was only identified when Oswald Avery and coworkers
induced bacterial transformation using pure DNA (Avery et al, 1944). This quite
naturally attracted attention to nucleotides, culminating in the discovery of the double helix structure of DNA by James Watson and Francis Crick (1953).
In retrospect, it should be clear that proteins are unfit to serve as memory elements. They can, in principle, form autocatalytic replication cycles, but mistakes
in their composition are unforgivable. Each protein has its own conformations, and