Transposition
Download
1 / 31

Transposition - PowerPoint PPT Presentation


  • 142 Views
  • Uploaded on

Transposition. Evidence Mechanisms: DNA-mediated RNA-mediated. Transposable elements. Mobile genetic elements - they move from one location in the genome to another Found in all organisms (so far studied) Effects: Insertion near or within a gene can inactivate or activate the target gene.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Transposition' - joann


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Transposition

Transposition

Evidence

Mechanisms:

DNA-mediated

RNA-mediated


Transposable elements
Transposable elements

  • Mobile genetic elements - they move from one location in the genome to another

  • Found in all organisms (so far studied)

  • Effects:

    • Insertion near or within a gene can inactivate or activate the target gene.

    • Cause deletions, inversions, and translocations of DNA

    • Lead to chromosome breaks



Observations of b mcclintock 1930 s 1950 s
Observations of B. McClintock (1930’s-1950’s)

  • Certain crosses in maize resulted in large numbers of mutable loci.

    • The frequency of change at those loci is much higher than normally observed.

  • Studies of these plants revealed a genetic element called “Dissociation” or Ds on the short arm of chromosome 9.

  • Chromosome breaks occurred at the Ds locus, which could be observed cytologically

    • i.e. by looking at chromosome spreads from individual cells, e.g. sporocytes.

  • Frequency and timing of these breaks is controlled by another locus, called “Activator” or Ac.


Breaks are visible cytologically on morphologically marked chromosome 9
Breaks are visible cytologically on morphologically marked chromosome 9

2 homologous chromosomes are distinguishable

C

Sh

Bz

Wx

Ds

knob

CEN

Heterochromatin

beyond knob

c

sh

bz

wx

At pachytene of meiosis, see:

Ds

c

sh

bz

wx

OR

Ds

c

sh

bz

wx


Mcclintock s chromosome breaks 1952 cshsqb
McClintock’s chromosome chromosome 9breaks, 1952CSHSQB

Chromosome 9,

short arm,

pachytene phase

of meiosis


Ds activity can appear at new locations on chromosome 9
Ds chromosome 9 activity can appear at new locations on chromosome 9

I

Sh

Bz

Wx

Ds

knob

Can find transpositions in the progeny

Ds

I

Sh

Bz

Wx

knob

Ds

I

Sh

Bz

Wx

knob

Ds

I

Sh

Bz

Wx

knob


Appearance of ds at a new location is associated with breaks e g duplications and inversions
Appearance of chromosome 9Ds at a new location is associated with breaks: e.g. Duplications and Inversions

I

Sh

Bz

Wx

Ds

knob

Ds

Bz

Sh

inversion

Sh

Bz

Wx

Wx

Ds

I

OR

Ds

duplication

Ds

I

Sh

Bz

Wx

I

Sh

Bz

Wx


In the presence of ac ds events lead to variegation in sectors of kernels
In the presence of chromosome 9Ac,Ds events lead to variegation in sectors of kernels

I>C, colorless

Ac

I

Sh

Bz

Wx

Ds

CEN

C

sh

bz

wx

After breakage and loss of acentric chromosomes, “recessive” markers are revealed in sectors of kernels.

C = Colored

Ds

C

sh

bz

wx






Transposable elements that move via dna intermediates
Transposable elements that move via DNA intermediates staggered breaks

  • Bacterial insertion sequences

    • Inverted repeat at ends

    • Encode a transposase

  • Bacterial transposons:

    • Inverted repeat at ends

    • Encode a transposase

    • Encode a drug resistance marker or other marker

    • TnA family: transposase plus resolvase


Is elements and transposons
IS elements and transposons staggered breaks


Ac ds transposons in maize
Ac staggered breaks/Ds transposons in maize

  • Ac is autonomous

    • Inverted repeats

    • Encodes a transposase

  • Ds is nonautonomous

    • Inverted repeats

    • Transposase gene is defective because of deletions in coding region


Structure of ac and ds
Structure of staggered breaksAc and Ds

CAGGATGAAA

TTTCATCCCTA

transposase

Ac

CAGGATGAAA

TTTCATCCCTA

deletion

Ds

Nonfunctional transposase


Replicative vs nonreplicative transposition
Replicative staggered breaksvs. Nonreplicative transposition


Mechanism for dna mediated transposition
Mechanism for DNA-mediated transposition staggered breaks

  • Transposase nicks at ends of transposon (note cleavage is at the same sequence, since the ends are inverted repeats).

  • Transposase also cuts the target to generate 5’ overhangs

  • The 3’ end of each strand of the transposon is ligated to the 5’ overhang of the target site, forming a crossover structure.



Replicative transposition from the crossover structure
Replicative transposition from the crossover structure staggered breaks

  • The 3’ ends of each strand from the staggered break (at the target) serve as primers for repair synthesis.

  • Copying through the transposon followed by ligation leads to formation of a cointegrate structure.

  • Copying also generates the flanking direct repeats.

  • The cointegrate is resolved by recombination.


Nonreplicative transposition from the crossover structure
Nonreplicative transposition from the crossover structure staggered breaks

  • Crossover structure is released by nicking at the other ends of the transposon (i.e. the ones not initially nicked).

  • The gap at the target (now containing the transposon) is repaired to generate flanking direct repeats.





Transposable elements that move by rna intermediates
Transposable elements that move by RNA intermediates four classes

  • Called retrotransposons

  • Common in eukaryotic organisms

    • Some have long terminal repeats (LTRs) that regulate expression

      • Yeast Ty-1

      • Retroviral proviruses in vertebrates

    • Non-LTR retrotransposons

      • Mammalian LINE repeats ( long interspersed repetitive elements, L1s)

      • Similar elements are found even in fungi

      • Mammalian SINE repeats (short interspersed repetitive elements, e.g. human Alu repeats)

      • Drosophilajockey repeats

      • Processed genes (have lost their introns). Many are pseudogenes.



Mechanism of retrotransposition
Mechanism of retrotransposition four classes

  • The RNA encoded by the retrotransposon is copied by reverse transcriptase into DNA

  • Primer for this synthesis can be generated by endonucleolytic cleavage at the target

  • Both reverse transcriptase and endonuclease are encoded by SOME (not all) retrotransposons

  • The 3’ end of the DNA strand at the target that is not used for priming reverse transcriptase can be used to prime 2nd strand synthesis


Events in l1 transposition
Events in L1 transposition four classes

ORF2

RT’ase

endonuclease

promoter

3’ UTR

ORF1

transcribe

Staggered break at target

Priming of synthesis by RT’ase at staggered break

Priming of synthesis by RT’ase at staggered break

2nd strand synthesis and repair of staggered break

FDR

FDR

RT’ase works preferentially on L1 mRNA


Recombination between two nearly identical sequences e g transposons will lead to rearrangements
Recombination between two nearly identical sequences (e.g transposons) will lead to rearrangements

  • Deletion if the repeats are in the same orientation

  • Inversion if the repeats are in the opposite orientation


Consequences of recombination between two transposons
Consequences of recombination between two transposons transposons) will lead to rearrangements


ad