Biology Reference
In-Depth Information
First
Alu
sequence
Oldest (J)
Jo
(81)
Jb
(81)
S
(48)
Sq
(44)
Intermediate (S)
Sp
(37)
Sx
(37)
Sc
(35)
Sg
(31)
Youngest (Y)
Y
(19)
Yb8
(3)
Ya 5
(4)
Ya 8
(4)
Figure 8.6.
The proposed evolution of the 12 human
Alu
subfamilies. Numbers in
parentheses represent approximate times (in Myrs) of insertion of different subfamilies
into the human genome (redrawn from Mighell
et al.
, 1997).
by dimerization of a free left arm monomer with a free right arm monomer
(
Figure 8.5
), an event which is thought to have occurred about 60 Myrs ago, before
the divergence of prosimians (Zietkiewicz
et al.
, 1998). Subsequently, many
rounds of sequential amplification took place to generate the 12 human
Alu
sub-
families seen today (Mighell
et al.
, 1997;
Figure 8.6
).
The total numbers of copies of
Alu
sequences in four of the great apes have
been estimated by Hwu
et al
. (1986): human, 910 000; chimpanzee, 330 000;
gorilla, 410 000; and orangutan, 580 000. As with the LINE elements, it would
appear that numerous insertions and deletions of these sequences have occurred
during the evolution of the great apes. At the chromosomal level,
Alu
sequences
insert preferentially into R bands (Wichman
et al
., 1992) whereas at the DNA
level, they preferentially integrate into A-rich sequences (Batzer
et al
., 1990;
Daniels and Deininger, 1985; Matera
et al
., 1990).
During mammalian evolution, the introduction of
Alu
sequences in the vicin-
ity of genes has sometimes altered gene expression as a consequence of their being
recruited to perform a regulatory function; examples of this phenomenon are
given in Chapter 5, section 5.1.12,
Alu sequences
.
Alu
sequences may also have been
involved in, or mediated, many other different types of gene rearrangement dur-
ing gene evolution including gross deletions (Section 8.1), duplications (Section
8.5), transpositions (Chapter 9, section 9.2), gene fusions (Chapter 9, section 9.3),
recombination (Chapter 9, section 9.4) and gene conversion events (Chapter 9,
