Biology Reference
In-Depth Information
and centromere function, the assembly of synaptonemal complexes (the pro-
tein structure that forms between homologous chromosomes during meio-
sis and that is involved in pairing and crossing over), and cell division. Thus,
although TEs are silenced, they also are maintained in genomes, and, according
to Biemont (2009), “The presence of numerous TEs in centromeric and pericen-
tromeric regions can be considered to be a key to success in the evolution of
eukaryote organisms owing to their function in cell division.”
TEs initially originally were divided into two classes according to their struc-
ture and mechanism of transposition (
Table 4.1
). Class I elements transpose by
reverse transcription of an RNA intermediate. Class I elements include elements
Table 4.1: A Classification of Transposable Elements by their Method of Transposition.
Class I Transposable Elements Transpose by Means of RNA Intermediates
A.
Viral superfamily (retrovirus-like retrotransposons)
Have long direct-terminal repeats (LTRs), encode reverse transcriptase from open reading
frames (ORFs) in DNA between LTRs, able to generate 4- to 6-bp target-site duplications,
have no 3
′
terminal poly(A) tract, are dispersed in genome.
Examples:
Copia
-like elements in
Drosophila melanogaster
Gypsy
-like elements in
D. melanogaster
Pao
in
Bombyx mori
B.
Nonviral superfamily (nonviral retroposons)
Have no terminal repeats, have ORFs, do not encode enzymes responsible for their
transposition, have 3
′
terminal poly(A) tract, are dispersed in genome.
Examples:
F
family in
D. melanogaster
R2 retroposons in many insects
HeT-A retroposons in telomeres of
D. melanogaster
Class II Transposable Elemen
ts Transpose Directly from DNA to DNA
All have a transposase and terminal inverted repeats (IRs)
A.
With short inverted repeats (SIRs)
Examples:
P
and
hobo
in
D. melanogaster
mariner
in many insect species
B
.
With long inverted repeats (LIRs)
Example:
FB
(
foldback
) in
D. melanogaster
Rolling-Circle Transposons (
Helitrons
)
Helitrons
do not have inverted repeats and move by a different method than class I and II
elements.
Helitrons
have been found in diverse organisms, including insects. Transposition is
by a semireplicative model; only one strand of the transposed transposon is transferred from
one genomic site to another where it serves as a template for DNA synthesis catalyzed by the
host repair mechanism.
(From
Finnegan 1989, Robertson 1993, Xiong et al. 1993, Kapitonov and Jurka 2007.
)
