DNA Rearrangements (Transposable elements and Chromosomal rearrangements)

• Readings: pp 574-5; 582-7; 595-606
• Problems: Chapter 21: 5, 6, 7, 8, 12, 13, 15, 16

Transposable elements

• Normal and ubiquitous (~15% of Drosophila genome thought to be mobile.)
• • Prokaryotes-transpose to/from cell's chromosome, plasmid, or a phage chromosome.
  • Eukaryotes-transpose to/from same or a different chromosome.
• Nonhomologous recombination:
• • transposable elements insert into DNA that has no sequence homology with the transposon.
• Cause genetic changes
• • Chromosome breaks
  • Knock-out genes

We talked about only 2 kinds: Insertion sequences in E. coli and Ac/Ds elements in corn

(A third major class, Retrotransposons, see RNA intermediate and reverse transcriptase)

Insertion sequence (IS) elements:

• Simplest type of transposable element found in bacterial chromosomes and plasmids.
• • Encode only genes for mobilization and insertion.
  • Ends of all known IS elements show inverted terminal repeats
• Transposition requires transposase, coded by the IS element.
• • uses host replication enzymes for replication.
  • Original copy remains in place; new copy inserts randomly.
• Transposition initiates when transposase recognizes inverted terminal repeats.
• • Staggered cuts are made in DNA at target site, IS element inserts, DNA polymerase and ligase fill the gaps.
  • Small direct repeats (~5 bp) flanking the target site are created.
• See fig 20.2

Ac/Ds elements

McClintock's discovery of transposons in corn:

• Kernel color alleles/traits are "unstable".
• • c/c = white kernels and C/- = purple kernels
• If reversion of c to C occurs in a cell, cell will produce purple pigment and a spot.
• McClintock concluded "c" allele results from a transposon called "Ds" inserted into the "C" gene (Ds = disassociation).
• • DS is non-autonomous (does not produce transposase)
• Autonomous transposon "Ac" controls "Ds" transposon (Ac = activator).
• See Fig. 20.11, 20.12, 20.13

I briefly mentioned Drosophila P elements, a transposable element that has evolved since 1950 and is now worldwide.

Chromosomal Changes

• Duplications/deletions
• Gene families: e.g. globin genes
• Inversions
• Translocations

Unequal Crossing Over leads to duplications and deletions

• One consequence is Multi-gene Families (clusters of similar genes and probably arose through duplications)
• We talked about two examples: the globin gene family in primates and the opsin genes for color vision

Inversions

• 2 Kinds of Inversions
• • Pericentric inversions include the centromere
  • Paracentric inversions do not include the centromere

Fig. 21.10, crossing-over in paracentric inversion hetrozygotes:
You should be able to draw a similar diagram of meiosis in paracentric inversion heterozygotes.

• Produces:
• • 1 normal chromosome
  • 2 deletion chromosomes (inviable)
  • 1 inversion chromosome (all genes present; viable)
• also, convince yourself that individuals homozygous for an inversion experience no problems in meiosis, and that there will be no problems if no crossovers occur

Fig. 21.11, Unequal crossing-over with pericentric inversion:
You should be able to draw a similar diagram of meiosis in paricentric inversion heterozygotes.

• Produces:
• • 1 normal chromosome
  • 2 deletion chromosomes (inviable)
  • 1 inversion chromosome (all genes present; viable)

Translocations

Fig. 21.13, Meiosis in translocation heterozygotes with no cross-over.

Again you should understand and be able to diagram meiosis in translocation heterozygotes.