TRANSPOSONS ;THE JUMPING GENES

TRANSPOSONS

1. TRANSPOSONS

2. Mobile Genetic Elements Transposons or Transposable elements ( TEs ) move around the genome 2

3. Three different mechanisms for transposition Conservative transposition : The element itself moves from the donor site into the target site. Also called cut-and-paste transposons Replicative transposition : The element moves a copy of itself to a new site via a DNA intermediate Retrotransposition : The element makes an RNA copy of itself which is reversed-transcribed into a DNA copy which is then inserted ( cDNA ) 3

4. Conservative transposition 4

5. Replicative transposition 5

6. Retrotransposition 6

7. Transposable elements in prokaryotes Insertion sequence (IS) elements Tn Transposons Composite (e.g. Tn9) Non-composite (e.g. Tn3) 7

8. Insertion sequence (IS) elements Simplest type of transposable element found in bacterial chromosomes and plasmids Encode only genes for mobilization and insertion Range in size from 768 bp to 5 kb IS1 first identified in E. coli ’s glactose operon is 768 bp long and is present with 4-19 copies in the E. coli chromosome Ends of all known IS elements show inverted terminal repeats ( ITRs ) 8

9. 9

10. Generation of short direct repeats flanking the newly inserted element This results from a staggered cut being made in the DNA strands at the site of insertion 10 common feature of mobile elements

11. Target site duplication 11

12. 12

13. Tn Transposons Similar to IS elements but are more complex structurally and carry additional genes 2 types of transposons : Composite transposons Noncomposite transposons 13

14. Composite Transposons The transposon is a composite transposon , composed of IS-elements flanking an included sequence, in this case containing an antibiotic resistance gene Composite transposons probably evolved from IS elements by the chance location of a pair in close proximity to one another. 14

15. 15 IS10R is an autonomous element, while IS10L is non-autonomous

16. Noncomposite transposons Carry genes (e.g., a gene for antibiotic resistance) Ends are non-IS element repeated sequences Tn3 is 5 kb with 38-bp ITRs and includes 3 genes; bla ( - lactamase ) , tnpA ( transposase ), and tnpB ( resolvase , which functions in recombination) 16

17. Transposition of Tn3

18. Transposable elements in eukaryotes Ac / Ds Elements in maize AC is a full-length autonomous copy DS is a truncated copy of AC that is non-autonomous, requiring AC in order to transpose P elements in Drosophila Paternal P type mating with maternal M type causes Hybrid Dysgenesis in offspring

19. Ac / Ds Elements in maize When  Ac is present, Ds may be transposed to a region adjacent to W. Ds can induce chromosome breakage, leading to loss of function of the  W gene – no anthocyanine pigment is produced

20. P elements in Drosophila P elements code a repressor, which makes them stable in the P strain in male (but unstable when crossed to the wild type female/; female lacks repressor in cytoplasm)

21. Retrotransposons Retrovirus-like transposons Long terminal repeats Homologs of gag, pol , and env genes E.g. LTR retrotransposons Retroposons Poly(A) tails HeT -A and TART (telomere associated retroposon ) in drosophila counter telomere shortening

22. Retrotransposition

23. Ty1 elements in Yeast is an example of LTR retrotransposons

24. Transposable elements in Humans – mostly retroposons Long Interspresed Nuclear Repeats – LINEs L1 elements 6kb 2 ORFs 3000 to 5000 copies Short Interspresed Nuclear Repeats – SINEs Alu elements 350 base pairs long do not contain any coding sequences Can be recognized by the restriction enzyme  AluI 5% of the human genome

25. Transposon mediated chromosomal rearrangements Deletions – intrachromosomal recombination between two transposons in the same orientation Inversions - intrachromosomal recombination between two transposons in the opposite orientation Duplications – unequal crossing over