Genetics BIOL 2120 - lecture notes

I. Transposable Elements in Bacteria
     A. IS Elements
          1. E. coli
          2. seen as mutation with high reversion rate
          3. contains inverted repeat at ends 9-40 base pairs
          4. only has genes for transposition
               a. transposase
               b. binds at or near ends of IS elements
               c. cuts both DNA strands
          5. usually <2500 base pairs
          6. target site duplication
               a. direct repeat 2-13 base pairs
               b. staggered cleavage to produce
          7. plasmids may also have IS
               a. allows for recombination with sites on chromosome with same IS
               b. this is how Hfr strains are made
     B. Composite Transposons
          1. created when 2 IS insert near each other
          2. region between two sequences is captured
          3. can jump when flanking elements transpose
          4. many produce transposase
          5. repressor: prevents transposition (truncated transposase)
     C. Tn3 Elements
          1. inverted repeats of 38-40 base pairs
          2. produce target site duplications when insert
          3. contain transposase, resolvase/repressor
          4. beta-lactamase (ampicillin resistance)
          5. transposition of Tn3 is a two step process
               a. fusion of two molecules ‘cointegrate'
                    (1) Tn is replicated
                    (2) 1 copy inserted at each junction
                    (3) oriented in same direction
               b. resolvase allows site specific recombination event at resolution site
                    (1) repressor binds to resolution site
                    (2) resolution site is between transposase and resolvase
                    (3) keeps both molecules from being made
                    (4) Tn3 is fairly immobile
     D. Medical Significance of Bacterial Transposons
          1. antibiotic resistance genes in transposons
          2. jump from one species to another
          3. if two bacteria in body at same time, chance to transfer resistance
II. Transposable Elements in Eukaryotes
     A. General Characteristics
          1. wide variety
          2. inverted repeat
          3. create target site duplication
          4. some encode transposase
     B. Maize
          1. Ac and Ds Elements
          2. cause stripes and spots on corn kernels
          3. result in chromosomal breakage when jump
          4. if gene lies on telomere side of dissociation element (Ds)
               a. chromosomal breakage at Ds
               b. acentric fragment is lost
          5. Ds won't dissociate without Ac (activator) sequence
          6. one of many instabilities in maize
               a. Ds found in numerous locations in different maize strains
               b. all are activated by Ac
               c. Ac can move by itself
               d. both can cause mutations when insert
          7. called ‘controlling elements'
          8. Ac
               a. 4563 base pairs
               b. 11 base pair inverted repeat at ends
               c. flanked by 8 nucleotide direct repeats (target site duplication)
          9. Ds
               a. heterogeneous
               b. modified Ac elements
               c. missing part of internal sequence
               d. may have additional DNA inserted
               e. may be piggyback: one Ds inserted inside another
          10. Ds can't make transposase; needs Ac to jump
     C. Drosophila
          1. P Elements and Hybrid Dysgenesis
          2. cross between M strain female and P strain male
               a. hybrids have frequent mutation, chromosome breakage, sterility
               b. called hybrid dysgenesis
          3. P elements vary in size
               a. up to 2907 base pairs
               b. terminal repeats of 31 base pairs
               c. complete elements carry transposase
               d. incomplete P elements also exist
               e. some strains of Drosophila contain up to 50 copies of P elements
          4.Strains isolated from wild before 1950
               a. no P elements
               b. suggest that P element introduced by virus after this date
          5. cytotype
               a. cellular condition transmitted from mother to offspring
               b. P cytotype inhibits transposition
               c. M cytotype permits transposition
          6. P elements only move in germ line
     D. Mariner
          1. Ancient
          2. Widespread - Many Unrelated Species
               a. insects, worms, fungi, humans
          3. sequence often more similar in unrelated species
          4. suggests transmission across species boundaries
          5. perhaps caused by virus with very wide host range
     E. Retrotransposons:
          1. general characteristics
               a. RNA to DNA to insert
               b. LTR on either side of central region
               c. LTR run in same direction
               d. LTR have short inverted repeats at ends
          2. Retrovirus-like Elements
               a. contain two genes
                    (1) similar to retrovirus genes
                         (a) gag: capsule protein of virus
                         (b) pol: reverse transcriptase/integrase
                    (2) lack env gene (envelope protein)
               b. Ty element
                    (1) Saccharomyces cerevisiae
                    (2) 5 base pair target site duplication
                    (3) most yeast strains have about 35 copies
                    (4) solo delta sequence: LTR left by itself
                         (a) occurs when circular transposon excises
                         (b) can form virus-like particles inside yeast
               c. Drosophila
                    (1) copia: makes copious amounts of RNA
                    (2) gypsy (wanders around genome)
          3. Retroposons
               a. these have long polyadenylated sequences
               b. probably originated from mRNA
               c. no inverted repeats at ends
               d. Drosophila
                    (1) F, G and I elements
                    (2) located at telomeres
                    (3) involved in replication: to maintain length of telomere
                    (4) consist of 12-15% of genome
               e. mammals
                    (1) LINES (long interspersed nuclear elements)
                    (2) human: L1
                         (a) only known active human transposon
                         (b) 50,000 - 100,000 copies (about 5% of genome)
III. Genetic and Evolutionary Significance of Transposable Elements
     A. Transposons and Genome Organization
          1. translocations
          2. insertions and deletions (crossovers at unmatched tsn sequences)
     B. Transposons and Mutation
          1. insert within gene
          2. turn on or off gene expression nearby
     C. Evolutionary Issues Concerning Tranposable Elements
          1. increased variety
          2. novel gene combinations

For questions, comments and additional information, contact mfhicks@pstcc.edu
Last Updated: June 24 2001
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