MECHANISMS OF
TRANSPOSITION
Transposition is the process
by which transposable elements (TEs), also known as "jumping genes,"
move from one location to another within a genome. The mechanisms of
transposition can vary depending on the type of transposable element and whether
it is found in prokaryotes or eukaryotes. Here, I'll outline the general
mechanisms involved in transposition:
1. Cut-and-Paste Mechanism:
The cut-and-paste mechanism
is a way in which certain genetic elements, known as DNA transposons, can move
around in the DNA of living organisms. This process involves several steps:
- Recognition and Binding:
The transposon carries a special enzyme called transposase. This enzyme
recognizes specific sequences at the ends of the transposon called
inverted repeats.
- Excision:
The transposase enzyme then catalyzes the excision of the transposon from
its original location in the DNA. This cutting action creates a
double-strand break in the DNA at that location.
- Transposition:
The transposon, now free as a separate DNA fragment, is moved to a new
location within the organism's genome. It's essentially cut out and
transported.
- Integration:
The transposase enzyme, once again, plays a role by catalyzing the
integration of the transposon into this new genomic site. During this
integration, short repeated sequences may be generated.
In summary, it's like the
transposon is cut out from one place in the DNA and pasted into another by a
special enzyme. This ability to move around can have significant effects on the
organism's genetics and evolution.
2. Replicative Transposition:
Replicative transposition is
a mechanism commonly associated with retrotransposons, a type of genetic
element found in eukaryotes. Here's a simple explanation of the process:
- Transcription:
The retrotransposon gets transcribed into RNA, which is a molecule similar
to DNA but single-stranded.
- Reverse Transcription:
The RNA is then transformed back into a DNA copy by an enzyme called
reverse transcriptase. This newly created DNA copy is called complementary
DNA or cDNA.
- Integration:
The cDNA is then inserted into a new location within the organism's
genome. So, essentially, a copy of the original genetic element is made
and placed in a different spot.
- Formation of Target Site Duplication:
This process often results in the creation of duplicated sequences in the
host genome flanking the retrotransposon. These duplicated sequences are
called target site duplications.
In simple terms, replicative
transposition is like making a copy of a genetic element, converting it into a
different form, and then sticking that copy into a new place in the organism's
genetic material. This copying and pasting process can impact the structure and
function of the genome.
3. Rolling-Circle Mechanism:
Some transposons,
particularly the Helitron class in eukaryotes, use a rolling-circle mechanism.
In this mechanism:
- Circularization:
The transposon is initially in a linear form. After transcription, it
forms a circular DNA molecule.
- Rolling-Circle Replication:
The circular DNA molecule serves as a template for rolling-circle
replication, generating linear copies of the transposon.
- Integration:
The linear copies are integrated into new genomic locations.
4. Recognition of Target
Sequences:
In both cut-and-paste and
replicative transposition, the transposase enzyme or other recognition elements
play a crucial role in recognizing specific sequences at the original and
target sites. These recognition elements can include inverted repeats, direct
repeats, or other sequence motifs.
Understanding these
mechanisms is essential for grasping the dynamic nature of transposition and
its role in genome evolution, genetic diversity, and the potential impact on
the biology of organisms.