HMB265H1 Lecture Notes - Lecture 18: Transposase, Wild Type, Restriction Enzyme

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HMB265 Lecture 16 transposable elements
Transposition
- Movement of small segments of DNA called transposable elements from one position to
another in the genome
- It was first noticed in a particular strain of maize with frequent breaks in chromosome 9
- It was found that Ds (dissociation) elements help cause the break
- Another element known as Ac (activator) is needed to activate break at the Ds locus
- Ac element is hard to map
Unusual phenotypes caused by the Ds element
- Chromosome 9 has four loci which control phenotype in maize
- Wild-type maize has pigmented, plump, shiny phenotype this is the dominant
phenotype
- Chromosomal breakage at the Ds locus gives colorless, shrunken, not shiny phenotype
thus revealing the recessive phenotype
- Breakage occurs only in some cells
- In another instance, Ac activates Ds loss from the C gene in one chromosome leading to
a spotted phenotype with purple spots on a colorless ground
o the spots represent where the Ds has left the gene whereas the colorless ground
shows that Ds breakage has occurred
transposable elements in corn
- Ds nonautonomous element
- Ac autonomous element
- Autonomous elements encode info required for own movements and for movement of
nonautonomous elements
- Three phenotypes can be observed:
o Wild type, pigmented phenotype where the C gene is present
o Colorless phenotype due to Ds presence on C gene which disrupts the reading
frame
o Spotted phenotype which is due to:
Ds element leaving the gene and Ac element present
Ac elements jumps into locus thus disrupting the function of the C gene
and acting as Ds element
Mechanism of transposition in corn
- Ac encodes a single protein which is a transposase
- Ds does not encode intact transposase, and this is the reason why it can’t transpose on
its own
- Transposase from Ac binds to either Ds or Ac end and promotes cleavage and
integration into a new target site
- The transposase also cuts the target-site on DNA thus acting like a restriction enzyme
- The transposable elements are inserted, and the host repairs the gaps
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