Biology Reference
In-Depth Information
However, this is very much an average figure arrived at by comparison of the 4-
fold degenerate sites in the protein-coding sequences of 337 human/rodent gene
comparisons. Sequence conservation does vary quite considerably between pro-
teins: indeed a study of 1196 orthologous mouse and human protein sequences
revealed sequence conservation of between 36% and 100% with an average of 85%
(Makalowski
et al
., 1996).
Large scale genomic DNA sequence comparisons between human and mouse
are still difficult owing to the paucity of orthologous pairs of sequences >20kb in
length. However, Koop (1995) noted three distinct patterns of sequence diver-
gence in the noncoding DNA sequences of human and rodent genomes:
(i)
A high level of sequence similarity in gene regions contrasting with divergent
non-coding regions, for example
-globin (
HBB
; 11p15.5) and
-crystallin
(
CRYGA
; 2q33-q35) genes.
(ii) A conserved pattern of sequence similarity in noncoding regions, for example
T-cell receptor-
(
TCRA
; 14q11.2) and -
(
TCRD
; 14q11.2) genes and
- and
-myosin heavy chain (
MYH6
,
MYH7
; 14q11.2-q13) genes (Koop and Hood,
1994). At least some of this conservation may be attributed to regions which
bind T-cell nuclear proteins which may play a role in the control of gene tran-
scription (Koop
et al
., 1994).
(iii) A mixed pattern of sequence similarity, for example the immunoglobulin
heavy chain J-C
portion exhibits ~64%
sequence homology between human and mouse whilst the C
µ
-C
gene region (14q32.33): the J-C
µ
region shows
little if any sequence conservation.
This 'mosaic model' of genome evolution may reflect differing rates of mutation
or differential repair efficiencies between different regions of the genome. As the
sequences of further syntenic regions become available [e.g. the Bruton's tyrosine
kinase (
BTK
; Xq21.33-q22) gene region; Oeltjen
et al
., 1997], this question can be
addressed.
2.3 Primate evolution
I believe that our Heavenly Father invented man because he was disappointed
in the monkey.
Mark Twain
2.3.1 Adaptation and adaptive radiation
The order of primates originated in the Palaeocene some 50-60 Myrs ago and
contains the monkeys, apes and humans (Martin 1993;
Figure 2.3
). Originally
adapted for arborial life, extant primates include low canopy runners (guenons),
high canopy acrobats (spider monkeys) and the brachiating great apes whilst
some have become exclusively terrestrial (baboons, mandrills, and humans).
These habits are responsible for the adaptation of the skeleto-muscular system to
allow jumping, swinging and grasping. Primates exhibit a wide variety of char-
acteristics which have equipped them to exploit their various niches optimally.
