Homology
* There is a practically infinite number of
different possible primary structures.
This is the basis for the great diversity of 3D structures,
and functions, of proteins.
* How many different primary structures are possible
for a polypeptide of 200 residues ?
It is apparent that only a fraction of possible primary
structures exist.
* It is very unlikely that two proteins with similar sequences have independently
evolved.
* Such similarities => two proteins must be related
and share a common ancestor - homologous.
Two primary sequences are more than approximately
20%
identical (making reasonable allowance for
insertions and deletions) =>
homologous.
Evolutionary divergence
All Living organisms - from the smallest viruses,
to univellular organism such as bacteria and algae, to plants, and to whales
- are similar at the molecular level.
use the same genetic codes
use the same 20 amino acids
in their proteins
use the same nucleotides in
their DNA and RNA
all the amino acids are L isomer
all the ribose moieties of DNA and RNA are the D
isomer
=> All extant organisms arose from
a
common ancestor that had already acquired
all the basic common biochemical features.
=> Diversity at higher
levels of organization has resulted from the Darwinian
processes of evolutionary divergence.
Evidence for divergent evolution
- macroscopic comparisons of organisms and from fossil record
- sequence comparisons of proteins and nucleic acids
Evolutionary convergence
- Two or more unrelated sequences have become
similar under the pressure of selection for similar functions.
- common evolutionary phenomenon at the macroscopic
level and protein 3D structure.