Genetics

• Transposable elements (transposons): Genetic loci that can move from one location in the genome to another
• Transposition


  A process by which mobile genetic elements move from one location in the genome to another
• Barbara McClintock


  • Discovered mobile genetic elements in maize
  • Nobel Prize 1983
• George Beadle


  • Discovered that genes act by regulating definite chemical events
  • Nobel Prize 1958

  
• Discovery of transposable elements in maize
• Discovery of transposable elements in maize

  1. Linked loci on 9S: Wx, Bz, Sh, C

  2. Kernel sectors uncovering recessive alleles at multiple linked loci

  3. Chromosome breaks repeatedly in the same position during kernel development

  4. Breakage dependent on 2 loci: Ds (dissociation) and Ac (activator)

  
• McClintock suspected mobile genetic elements when she found it impossible to genetically map Ac

  
• McClintock's experiments


  1. Ds moves from near waxy to between shrunken and colorless

  2. Ac maps to different chromosomal locations in different crosses

  3. Ds can jump into C creating c-m1 allele and then jump out to restore functional C

  
• Ac (Activator)


  Autonomous transposable element that encodes the protein(s) necessary for its transposition and for the transposition of nonautonomous elements
• Ds (Dissociation)


  Nonautonomous transposable element that relies on the protein products of autonomous elements for its mobility
• Transposase


  An enzyme encoded by transposable elements that undergo conservative transposition
• Autonomous transposable element


  Transposable element that encodes the protein(s) necessary for its transposition and for the transposition of nonautonomous elements in the same family
• Nonautonomous transposable element


  Transposable element that relies on the protein products of autonomous elements for its mobility
• Transposable elements in bacteria: Insertion sequence (IS) element, Simple and composite transposons, Mechanism of transposition
• Evidence for transposable elements in bacteria

  1. Existence of gal operon mutants that could spontaneously revert to wild type

  2. Lambda phage DNA can insert into the host genome between the gal operon and the neighboring operon
• Insertion sequence (IS) element

  A mobile piece of bacterial DNA (several hundred nucleotide pairs in length) capable of inactivating a gene into which it inserts
• Inverted repeat (IR) sequence

  A sequence found in identical (but inverted) form, for example, at the opposite ends of a DNA transposon
• IS element


  A mobile piece of bacterial DNA (several hundred nucleotide pairs in length) capable of inactivating a gene into which it inserts

  
• The bacterial genome contains segments of DNA, termed IS elements, that can move from one position on the chromosome to another

  
• Evidence for transposable elements in bacteria

  • Mutants of the gal operon of E. coli contain transposable elements
  • The mutants arose spontaneously, and they can revert spontaneously back to wild type
  • The mutants contained an extra segment of DNA inserted into the gal operon
  • The inserted DNA could be found in different locations in the E. coli genome; that is, it moves like Ac and Ds in maize

  
• Inverted repeat (IR) sequence

  A sequence found in identical (but inverted) form—for example, at the opposite ends of a DNA transposon
• Insertion sequence (IS) element

  • Contains the transposase gene flanked by inverted repeat sequences
• Composite transposon


  A type of bacterial transposable element containing a variety of genes that reside between two nearly identical insertion-sequence (IS) elements
• Simple transposon


  A type of bacterial transposable element containing a variety of genes that reside between short inverted repeat sequences
• An R plasmid may contain several transposons carrying resistance genes
• Mechanism of transposition


  1. Excision (leaving) from the original location

  2. Insertion into the new location
• Conservative transposition


  A mechanism of transposition that moves a mobile element to a new location in the genome as it removes that element from its previous location
• Replicative transposition


  A mechanism of transposition that generates a new insertion element integrated elsewhere in the genome while leaving the original element at its original site of insertion
• Cointegrate


  The product of the fusion of two circular elements to form a single, larger circle in replicative transposition
• Target-site duplication


  A short direct-repeat DNA sequence (typically from 2 to 10 bp in length) adjacent to the ends of a transposable element that was generated during the element's integration into the host chromosome
• Class 2 transposable elements

  DNA transposons
• Next week: Final exam
• Unbalanced rearrangements


  A rearrangement in which chromosomal material is gained or lost

  
• Retrotransposons


  A transposable element that uses reverse transcriptase to transpose through an RNA intermediate
• Unbalanced rearrangements


  • Change gene dosage of a chromosomal segment

  
• Unbalanced rearrangements


  • Deletions
  • Deletion loops
  • Pseudodominance
  • Duplications
  • Tandem vs insertional
• Ty element


  A yeast LTR-retrotransposon; the first isolated from any organism
• Balanced rearrangements


  A change in the chromosomal gene order that does not remove or duplicate any DNA

  
• Retrovirus


  An RNA virus that replicates by first being converted into double-stranded DNA

  
• Balanced rearrangements


  • Inversions
  • Paracentric vs pericentric
  • Inversion loops in meiosis