Transposition

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Transposition

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• Honghong liu

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What is transposition ?

• In transposition , a transposable element ,or
  transposon , moves from one DNA address to
  another .

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1.1 Discovery of Bacterial Transposons

• Phenomenon
• Some phage mutations do not behave normally. They
  do not revert readily the way point mutations do,
  and the mutant genes contained long stretches of
  extra DNA.
• Demonstration
• ? Phages pick up the lac genes(both wild-type
  and mutant) during lytic infection of E.Coli ,
  then measure the densities of the sizes of the
  two recombinant DNAs by ultracentrifugation.

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Figure 23.1 demonstration of mutation by
insertion
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1.2 Insertion Sequences

• The simplest Bacterial transposons
• Inverted repeat sequences(at both ends of the
  transposons)genes(collectively known as
  transposase) that code for the enzymes that
  catalyze transposition.

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Figure 23.2 Transposons contain inverted
terminal repeats
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• One other feature of an insertion sequence is
  generation of direct repeats in host DNA flanking
  the transposon.
• How ?

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• Figure 23.3 Generation of direct repeats in host
  DNA flanking a transposon

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1.3 More Complex Transposons

• Insertion sequences and other transposons are
  sometimes called selfish DNA, implying that
  they replicate at the expense of their hosts and
  apparently provide nothing useful in return.
• However, some transposons do carry genes that are
  valuable to the their hosts, the most familiar
  being genes for antibiotic resistance.

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• Figure 23.5 structure of Tn3

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1.4 Mechanisms of Transposition

• Replicative Transposition
• Transposons replicate during transposition
• Copy and Paste
• Nonreplicative Transposition
• Transposons do not replicate during
  transposition
• Cut and Paste

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• Figure 23.7 Detailed scheme of Tn3 transposition
  (Replicative Transposition)

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• Figure 23.8 Nonreplicative transposition

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2.1 Rearrangement of Immunoglobulin Genes

• Rearrangement of the mammalian genes that
  produce antibodies, or immunoglobulins, uses a
  process that closely resembles transposition.

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• Figure 23.12 Structure of an antibody

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• Figure 23.13 Rearrangement of an antibody light
  chain gene

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• Figure 23.14 structure of antibody heavy chain
  coding regions

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• Summary
• The immune systems of vertebrates can produce
  many millions of different antibodies to react
  with virtually any foreign substance. These
  immune systems generate such enormous diversity
  by three basic mechanism
• (1)Assembling genes for antibody light and heavy
  chains from two or three components parts and
  each part selected from heterogeneous pools of
  parts
• (2)Joining the gene parts by an imprecise
  mechanism that can delete bases or even add extra
  bases
• (3)Causing a high rate of somatic mutations

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• How does the recombination machinery determine
  where to cut and paste to bring together the
  disparate parts of an immunoglobulin gene ?
• Recombination signals (RSSs)
• Adjacent to each coding region lies a
  conserved heptamer ,accompanied by a nonamer.
  The heptamer and nonamer are separated by a
  nonconserved spacer containing either 12 bp(a 12
  signal) or 23 bp (a 23 signal).

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• 12/23 rule
• The arrangement of recombination signals is
  such that recombination always joins a 12 signal
  to a 23 signal.
• This 12/23 rule stipulates that 12 signals
  and 23 signals never joined to themselves. So
  only one of each coding region is incorporated
  into the mature immunoglobulin gene.

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Figure 23.15 signals for V(D)J joining
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Mechanism of V(D)J recombination

• V(D)J joining is imprecise, which contributes to
  the diversity of products from the process.
• How do we explain this imprecision ?

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Figure 23.17 Mechanism of cleavage at RSSs
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Figure 23.18 Identifying cleavage products
How do we know hairpins form ?
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2.2 Retrotransposons

• Retrotransposons
• Transposons that replicate through an RNA
  intermediate and therefore depend on reverse
  transcriptase.
• Retrotransposons fall into two groups
• LTR-containing retrotransposons.
• Non-LTR retrotransposons

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• The retrotransposons resemble retroviruses
• As an introduction to the replication scheme of
  the retrotransposons, let us first examine the
  replication of the retroviruses.

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• Figure 23.19 Retrovirus replication cycle

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Figure 23.24 Structures of retroviral RNA and
provirus DNA
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Figure 23.25 A model for the synthesis of the
provirus DNA from a retroviral RNA template
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• Figure 23.27 Model for transposition of
  transposon yeast (LTR-Containing
  Retrotransposons)

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• Non-LTR Retrotransposons Long interspersed
  elements

Figure 23.28 Map of the L1 element
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• We have just seen that the LTR is crucial for
  replication of most retrotransposons with LTRs,
  so how do non-LTR retrotransposons replicate ?
• The answer is that their endonuclease creates a
  single-strand break in the target DNA and their
  reverse transcriptase uses the newly formed DNA
  3, end as a primer .

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• R2Bm, a LINE-like element, resembles the
  mammalian LINEs in that it encodes a reverse
  transcriptase ,but no Rnase H, protease, or
  integrase, and it lacks LTRs.

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Figure 23.29 Dna nicking and cleavage activity of
the R2Bm endonuclease
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Figure 23.30 Evidence for target priming of
reverse transcription of R2Bm
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Figure 23.31 A model for L1 transposition
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• LINEs and LINE-like elements are
  retrotransposons that lack LTRs. These elements
  encode an endonuclease that nicks the target DNA.
  Then the element takes advantage of the new DNA
  3, end to prime reverse transcription of
  element RNA. After second-strand synthesis, the
  element has become replicated at its target site.
  Anew round of transposition begins when the LINE
  is transcribed.

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• Thank you!