Transposons

Question 1

Insertion sequences

Answer 1

Bacterial DNA cut and paste elements encode own transposition factors

Code for enzymes for transposition, flanked by short inverted terminal repeats

Question 2

Composite transposons

Answer 2

Pair of IS elements flank another gene

May be in same (Tn9) or different orientation (Tn10, Tn5)

Carry resistance genes

IS10 transposes alone

Flanked by pair of IS elements can transpose

Question 3

Non-composite transposons

Answer 3

Large (~5kb)

Not dependent on IS elements

Independent units - gene for transposition, genes encoding drug resistance

e.g. Tn3, Tn1000

Inverted repeats, generates 5bp direct repeat at target site

Question 4

Mechanism of transposition

Answer 4

• Most-DNA only transposons move by cut-and-paste mechanisms
• Element excises completely, inserts into target, join sites repaired by replication
• Some move by nick-and-paste in bacteria

Nick-and-paste - Transposon attached to donor DNA - joined to target to form cointegrate

Question 5

Transposition at molecular level

Answer 5

Terminal inverted repeat DNA recognised by transposase

Transposases oligomerize - transposon ends together

Transposon cleavage activated from background DNA

Question 6

3 main types of eukaryotic transposon

Answer 6

Long terminal repeats (LTR) elements - retrovirus, encode proteins including reverse transcriptase

NON-LTRs
LINE (Long interspersed Elements) - encode proteins that mediate own transposition

SINE - (Short interspersed elements) Don’t encode proteins for movement, rely on LINE