TRANSPOSONS

2. MOBILE GENETIC ELEMENTS
• Transposons or Transposable elements (TEs)
• move around the genome
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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
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5. REPLICATIVE TRANSPOSITION
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6. RETROTRANSPOSITION
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7. TRANSPOSABLE ELEMENTS IN
PROKARYOTES
• Insertion sequence (IS) elements
• Tn Transposons
• Composite (e.g. Tn9)
• Non-composite (e.g. Tn3)
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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
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common feature of mobile elements

11. TARGET SITE DUPLICATION
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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
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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.
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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)
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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