Final answer:
Insertion Sequences (IS elements) are simple mobile genetic elements with a transposase gene, flanked by inverted repeats and low copy numbers. Composite Transposons (Tn elements) consist of two IS elements flanking additional genes, often conferring antibiotic resistance, and also exist in low copy numbers. Both can move via cut-and-paste or replicative transposition mechanisms.
Step-by-step explanation:
Differentiating Insertion Sequences and Composite Transposons
Bacterial Insertion Sequences (IS elements) are mobile genetic elements that consist of a single gene encoding transposase flanked by inverted repeats. These elements can insert into bacterial genomes or plasmids and are typically found in low copy numbers due to the compact nature of bacterial DNA. Each IS element is also surrounded by direct repeats of host genome DNA, a consequence of the transposition process.
In contrast, Composite Transposons (Tn elements) include a pair of IS elements that flank one or more genes, often those providing antibiotic resistance. When these Tn elements move, they carry the interspersed genes along with them, contributing to horizontal gene transfer and the spread of antibiotic resistance. Similar to IS elements, Tn elements have low copy numbers within the bacterial genome.
The major difference between the two elements lies in the additional genetic material that Tn elements carry between the IS sequences, which can include important genes like those conferring drug resistance. Both IS elements and Tn elements function by cut-and-paste or replicative transposition mechanisms, facilitated by the transposase enzyme that these mobile elements encode. Cut-and-paste transposition involves the excision and reinsertion of the transposon, whereas replicative transposition results in copying of the element within the genome.