DNA rearrangements: recombination and transposition.
Study questions.
1. Which of the following statements about genetic rearrangements are correct?
a) They generate new combinations of genes.
b) They can move a segment of DNA from one chromosome to another.
c) They are mediated by the breakage of DNA and the rejoining of the resulting fragments.
d) They generate genome sequence variability, upon which natural selection can act.
e) They can regulate gene expression.
f) They always require extensive regions of sequence similarity between the interacting DNA molecules in order to juxtapose the recombining sites. Neither site-specific nor transpositional recombination requires extensive similarity between the recombining DNA molecules
ANSWERS:
A, B, C, D and E2. Outline the steps of homologous recombination.
See page 802 in Textbook. You need to know the type of enzymes and other activities used by this process.
3. Which of the following statements about the RecA protein of E. coli are correct?
a) It is an ATP-dependent nuclease that generates single-stranded DNA.
b) It catalyzes an ATP-dependent strand-assimilation reaction in which a single-stranded DNA molecule associates with duplex DNA.
c) It hydrolyzes ATP to promote branch migration.
d) It binds to single-stranded DNA to form a filament.
e) It facilitates the search of duplex DNA for regions with sequence similarity to the invading single-stranded DNA.
ANSWER:
B, C, D and E4. Homologous recombination is likely to require which of the following?
a) DnaB protein g) Single-strand binding protein
b) Topoisomerase I h) DNA-dependent RNA polymerase
c) RecA protein i) DNA polymerase I
d) RecBCD complex j) DNA ligase
e) ATP k) dATP
f) NAD+
ANSWER:
B, C, D, E,, G, I, J, KHomologous recombination involves some DNA repair synthesis between recombining molecules. It also requires some of the proteins used for DNA replication; DnaB is not required.
5. Which of the following statements about transposons is/are correct?
a) They contain insertion sequences.
b) They contain inverted terminal repeat sequences.
c) They contain one or more genes specifying one or more enzymes that catalyze the transposition event.
d) They require the rec gene products to complete their movements between or within genomes.
e) They can lead to the duplication of short sequences of DNA in the recipient genome.
On either side of the inserted DNA.ANSWER:
A, B, C, and E6. The kind of recombination displayed by transposons, sometimes called RecA-independent recombination, has been regarded as having far greater evolutionary significance than does general, or RecA-dependent, recombination. Why do you think this may be the case?
ANSWER:
Because it allows horizontal transfer of new gene cassettes, e.g., groups of genes that specify drug resistance. Homologous recombination has more of a role in reassorting the alleles of homologous genes in pairs of chromosomes.7. Describe four different ways by which an antibiotic resistance gene might move from the genomic DNA of one bacterium to the genomic DNA of another.
ANSWER:
Through plasmid transfer in bacterial conjugation, through a virus by transduction, through uptake of DNA released from dying cells (i.e., transformation) or through transposition (which may be associated with conjugation, transduction or transformation).True or false?
8. Transposases recognize sufficiently extensive sequences surrounding the integration sites so that the transposon avoids becoming integrated into the middle of a gene, because gene disruption could be lethal to the cell. False. Transposons often do integrate inside and inactivate ("knock out") genes.
9. The capacity of plasmids to replicate indefinitely without being part of a host chromosome distinguishes them from transposable elements.
True