Transposons and Their Statements

Transposons are segments of DNA that can move from one location to another within a genome. This process is called transposition. Several statements about the structure and action of transposons were presented. Let's examine each one to determine its correctness.

Analyzing Transposon Statements

Here is an analysis of each statement:

Statement A: Transposons have inverted terminal repeats and their integration generates inverted repeats at the flanks of the target site in the host genome.

• Most transposons do have inverted terminal repeats (ITRs), which are crucial for their recognition by the transposition machinery.
• However, the integration process of transposons into the host genome typically generates short, direct repeats of the target site sequence flanking the inserted transposon. These are formed due to staggered cuts made in the target DNA and subsequent filling in of the gaps.
• Therefore, this statement is incorrect because it claims inverted repeats are generated at the target site flanks, when it should be direct repeats.

Statement B: A composite transposon can transpose as a unit.

• A composite transposon consists of two insertion sequence (IS) elements, usually in the same orientation, flanking a region of DNA. This central region often contains genes, such as antibiotic resistance genes.
• The IS elements contain the necessary genes (like transposase) and sequences (like ITRs) for transposition.
• The entire structure, including the central region and the two flanking IS elements, can transpose together as a single unit.
• Therefore, this statement is correct.

Statement C: The transposition event may cause deletions or inversions or move a host sequence to a new location.

• Transposon movement can have various effects on the host genome.
• Insertion of a transposon can disrupt genes, leading to mutations.
• Excision of a transposon can sometimes lead to deletions of flanking host DNA.
• Complex transposition events, particularly those involving replication or multiple transposons, can result in inversions or the movement of host DNA sequences along with the transposon to a new site (translocations or gene duplication if part of a host sequence is carried).
• Therefore, this statement is correct.

Statement D: The transposition event may cause deletions or inversions but cannot move a host sequence to a new location.

• This statement is a partial contradiction of statement C.
• As explained for statement C, transposition can indeed lead to the movement of host sequences to new locations.
• Therefore, this statement is incorrect.

Statement E: Replicative transposition proceeds through cointegration.

• Replicative transposition is a mechanism where the transposon is duplicated during the transposition process. One copy remains at the original donor site, and a new copy is inserted into the target site.
• A key intermediate in this process is a cointegrate structure, where the donor DNA molecule carrying the transposon is fused with the target DNA molecule. The transposon is present as two copies within this cointegrate, one at each junction between the donor and target DNA segments.
• The cointegrate is subsequently resolved, often by site-specific recombination mediated by a resolvase enzyme encoded by the transposon, separating the donor and target molecules, with the transposon now duplicated.
• Therefore, this statement is correct.

Summary of Correct Statements

Based on the analysis:

• Statement A is incorrect.
• Statement B is correct.
• Statement C is correct.
• Statement D is incorrect.
• Statement E is correct.

The combination of correct statements is B, C, and E.