Chapter 16(3) Flashcards
What are transposable elements?
DNA sequences that can move within the genome by an enzyme-driven process called transposition
What is an insertional inactivation?
When a transposable element causes a mutation if it inserts into a wild-type allele and disrupts its function
How was transposition discovered?
Barbara McClintock discovered
transposition using crosses
involving three linked genes, C,
Sh and Wx, in maize
What was located at the site of chromosome breakage?
dissociation (Ds) element
What element do Ds need to cause chromosome breakage?
Activator (Ac) element
What were McClintock’s 2 observations?
- McClintock observed that the
break points of chromosome 9
all occurred in the same position of the affected chromosome - McClintock’s second observation
was colorless kernels with varied
patterns of purple spotting
* She concluded that the unstable
mutant alleles were caused by insertion of Ds into the C locus to produce a kernel lacking pigmentation
What did random transposition of Ds out of the gene cause?
caused reversion back to wild-type and the array of purple spots on the kernels
What are autonomous transposable elements? Example?
carry a transposase gene and all DNA sequences needed to carry out transposition (e.g., Ac)
What are non-autonomous transposable elements? Example?
have no transposase gene and may lack the sequences needed for transposition (e.g, Ds)
What two categories of transposable elements do bacterial genomes contain?
-IS(insertion sequence) elements
-transposons
What are IS elements?
(insertion sequence) elements are simple transposable elements containing only the genes and sequences needed for autonomous transposition
What is the IR sequence?
-They contain a transposase gene that is bracketed by a short, inverted repeat (IR) sequence
* Different IS elements have different IR sequences
* The inverted repeats are integral to transposase- mediated transposition
What are the role of transposons?
in bacteria, they carry multiple genes and confer new traits on bacteria that contain them
What two types are Transposons(Tn) composed of?
composite and simple
What are composite transposons?
-have a central region of several kb containing one or more functional genes
-The central region is flanked by complete IS elements in opposite orientation; at least one of these (sometimes both) contains a copy of the transposase gene
* The transposon Tn10 has a typical composite transposon structure