What are Transposons? Explain the general structure and mechanism of transposition of Tn3 elements in prokaryotes. (IFS 2020, 15 Marks)

Transposons

• Transposons, also known as "jumping genes," are DNA sequences capable of moving or "transposing" from one location in the genome to another. This movement can occur within the same chromosome or between different chromosomes.
• Types: Transposons are generally categorized into two main types:
  
  1. Class I (Retrotransposons): Move through an RNA intermediate.
  2. Class II (DNA Transposons): Move directly as DNA.
• Role in Genetic Diversity: Transposons can cause mutations and chromosomal rearrangements, contributing to genetic diversity and evolution.
• Presence Across Life Forms: Found in almost all life forms, including bacteria, plants, and animals, transposons play various roles in genome structure and function.
• Impact on Genome Size and Composition: They can make up a large part of some genomes and influence gene regulation and expression patterns.

General Structure of Tn3 Transposon Elements in Prokaryotes

• Tn3 Transposon Family: Tn3 is a well-studied DNA transposon family in prokaryotes, especially bacteria. It is known for its capability to carry antibiotic resistance genes.
• Size: The Tn3 element is typically around 4.95 kilobases in length, making it relatively large for bacterial transposons.
• Components:
  
  • Inverted Repeat Sequences (IRs): Found at both ends of the transposon, these sequences are crucial for the transposition process, as they are recognized by transposase enzymes.
  • Transposase (tnpA): This enzyme facilitates the "cut-and-paste" movement of the transposon within the genome.
  • Resolvase (tnpR): Works in recombination and helps resolve intermediate structures formed during the transposition.
  • Antibiotic Resistance Genes: Many Tn3 elements carry genes that confer resistance to antibiotics, aiding bacterial survival.
• Insertion Sites: The Tn3 element can insert itself into various sites in the genome, showing little to no site specificity, which enhances its ability to spread.

Mechanism of Transposition of Tn3 Elements in Prokaryotes

• Formation of Cointegrate Structure:
  
  • The Tn3 transposon mechanism follows a "replicative" pathway, where it initially duplicates itself.
  • Transposase cuts and joins the transposon to the target DNA, creating a structure with two copies of the transposon, known as a cointegrate.
• Single-Strand Nicking:
  
  • Transposase makes a single-strand break on both the transposon and target DNA.
  • The ends of the transposon are joined to the target DNA.
• DNA Synthesis:
  
  • DNA replication machinery fills in the single-stranded regions, resulting in the formation of a duplicated copy of the transposon.
• Resolution of Cointegrate:
  
  • Resolvase enzyme (tnpR) acts at the res sites of the two transposon copies in the cointegrate, cutting them apart.
  • This resolves the intermediate structure, separating the two transposons into distinct DNA molecules.
• Result: This mechanism results in one copy of the transposon remaining at the original site and another inserted into the new site, thereby increasing the number of transposons in the genome.

Conclusion

Transposons are important genetic elements that contribute to genome evolution and diversity. The Tn3 element in prokaryotes has a specific structure and mechanism of transposition that allows it to move within the genome and potentially confer selective advantages to the host organism.