Chapter 18 &8

• Transposons, or transposable elements (TEs), are mobile DNA elements that can move around the genome

  
• About 45% of our DNA appears to be left-over transposon sequences

  
• Transposons


  Mobile DNA elements that can move around the genome

  
• Transposons


  • Create flanking direct repeats during transposition
  • Many contain terminal inverted repeats as part of their structure

  
• Types of transposons


  • DNA transposons (class II) - Direct DNA transposition
  • Retrotransposons (class I) - DNA -> RNA -> DNA
• DNA transposons


  Require transposase for excision & insertion
• Autonomous DNA transposons


  Encode their own transposase
• Non-autonomous DNA transposons


  Rely on transposase from other transposons
• Retrotransposons


  Require reverse transcriptase & transposase

  
• Autonomous retrotransposons


  Encode their own reverse transcriptase & transposase
• Non-autonomous retrotransposons


  Rely on reverse transcriptase & transposase from other transposons
• Mutation from insertion


  • Insertion of the Gret1 retrotransposon disrupts VvmybA1 gene expression in white grapes

  
• Transposons create repeat sequences across multiple chromosomes, increasing risk of chromosome rearrangements

  
• Effects of transposition


  • Deletions
  • Inversions
  • Translocations
• 45% of the human genome is made up of transposable elements
• Transposable elements in humans

  • Alu (most common, >1 million copies, 11% of genome)
  • LINEs (fewer copies but larger, ~20% of genome)

  
• Transposable elements (TEs) have been "domesticated" or coopted by their host as adaptations
• TEs have aided in the evolution of the placenta in mammals
• Hybrid dysgenesis


  Sudden appearance of numerous mutations, chromosome mutations, and sterility in hybrid offspring between two populations
• Hybrid dysgenesis may play a role in speciation
• P elements in Drosophila

  Encode both a transposase and a repressor of transposition, with the repressor deposited into the egg by the female

  
• If no repressor is deposited, a burst of transposition occurs, causing sterility of the offspring
• Reciprocal crosses produce different outcomes in hybrid dysgenesis
• Mutations are inherited changes in the DNA sequence of genetic information

  
• Types of mutations


  • Gene mutations (change to base sequence of one gene)
  • Chromosomal mutations (change in number or structure of a chromosome)
• Karyotype


  Representation of the number and structure of an organism's chromosomes

  
• Karyotypes


  • Arranged in descending order based on size, centromere location, and banding pattern

  
• Chromosome banding patterns


  Distinctive for each chromosome, allow analysis of chromosome structure

  
• Types of chromosome mutations

  • Duplications
  • Deletions
  • Inversions
• Duplications


  Result in extra copies of genes

  
• Deletions


  Result in fewer copies of genes
• Inversions


  Change the orientation of genes, but don't change the number

  
• Duplications and deletions often arise from unequal crossing over
• Inversion heterozygotes


  Have one chromosome with an inversion and one without, forming an inversion loop during meiosis
• Recombination in inversion heterozygotes can result in acentric or dicentric chromosomes with deletions and duplications, leading to no viable recombinant gametes
• Pericentric inversion


  Includes the centromere
• Paracentric inversion


  Does not include the centromere
• Usually little or no genetic material is lost in inversions
• Effects of inversions


  • An inversion may occur in the middle of a gene, disrupting function
  • A new location may cause position effects on gene expression
• Inversion heterozygotes and meiosis

  1. Chromosomes will form an inversion loop to allow pairing to occur

  2. Recombinant chromosomes will be acentric or dicentric and have deletions and duplications

  3. No viable recombinant gametes