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Overzicht van transpositie en recombinatie
Transposons vormen een aanzienlijk deel van de genomen van verschillende organismen. Daarom wordt aangenomen dat transpositie een belangrijke evolutio…
Transposition is a specialized form of recombination in which genetic elements, such as chromosomal segments, are relocated from one position in the genome to another. These mobile elements are called transposons, or jumping genes.
Every transposon contains a coding sequence for an enzyme called transposase, in addition to other genes, as well as short flanking sequences that are reverse complements of each other. There are three types of transposition.
In the first type, known as non-replicative or conservative transposition, the transposase encoding gene produces the dimeric enzyme that cleaves at short inverted sequences that flank a DNA transposon. Then, the inverted sequences come together to form a DNA loop which can be inserted into a target chromosome by transposase-mediated cuts.
In the second type, called replicative transposition, transposase cleaves both the transposon terminals and the target DNA. Then the 3’ ends of the transposon and the 5’ ends of the target DNA are covalently attached in a step called strand transfer.
This creates an intermediate where the 5’ end of the transposon is still attached to the donor DNA. The unligated ends are used as primers by DNA polymerase to replicate the transposon. This intermediate is called a cointegrate.
Enzymes called resolvases cleave the intermediate at the internal resolution site, generating donor and target DNAs that each have one copy of the transposon.
In the third type of transposition, the transposable element is first transcribed into an RNA intermediate known as a retrotransposon. The RNA is copied back into a DNA sequence by reverse-transcription and then inserted into a target site.
Despite their different mechanisms, all three of these processes can alter the genomic structure and potentially the function of the target DNA.
Q1: What are transposons and how do they move within the genome?
Transposons, also called jumping genes, are mobile genetic elements that relocate from one genomic position to another through transposition. Each transposon contains a transposase gene encoding an enzyme that catalyzes movement, plus flanking sequences and other genes. This specialized recombination process can alter genomic structure and potentially affect target DNA function.
Q2: What is the difference between non-replicative and replicative transposition?
Non-replicative transposition involves transposase cleaving inverted flanking sequences to form a DNA loop inserted into a target chromosome without copying. Replicative transposition creates a cointegrate intermediate where the transposon is copied during insertion, resulting in both donor and target DNA retaining one transposon copy after resolvase cleavage.
Q3: How does retrotransposition differ from DNA transposition mechanisms?
Retrotransposition first transcribes the transposable element into an RNA intermediate called a retrotransposon. Reverse-transcriptase then copies this RNA back into DNA, which is inserted into a target site. This RNA-mediated mechanism contrasts with direct DNA-based transposition, yet all three transposition types can alter genomic structure and function.
Q4: Why do transposons rarely move within genomes?
Transposons rarely move because transposition can have deleterious effects on genome stability and gene function. The frequency of transposition correlates with sequence specifications and structural motifs at donor and target sites. This low frequency means genetic selection is required to detect transposition outcomes, such as color variegation in maize or white patches on Snapdragon flowers.
Q5: What evolutionary roles have transposons played in organisms?
Transposons comprise significant portions of many organism genomes and likely drove speciation by changing genome sizes and modifying gene expression patterns. In bacteria, transposition confers antibiotic resistance by transferring resistant genetic elements. In eukaryotes, transposons regulate target genes under physiological stress conditions, a mechanism extensively studied in plants.
Q6: What structural features do all transposons share?
All transposons contain a transposase-encoding gene and short flanking sequences that are reverse complements of each other. These inverted sequences are critical recognition sites where transposase cleaves DNA during movement. Additionally, transposons carry other genes beyond transposase, enabling their diverse functions across different genomic contexts.
Q7: How does the cointegrate intermediate form during replicative transposition?
During replicative transposition, transposase cleaves both transposon terminals and target DNA. The 3' ends of the transposon are covalently attached to the 5' ends of target DNA through strand transfer, creating a cointegrate where the transposon's 5' end remains attached to donor DNA. DNA polymerase then uses unligated ends as primers to replicate the transposon.