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
What are simple transposons?
-are flanked by very short IR
sequences of less than 50 bp
-The IR sequences do not encode transposase; this enzyme is encoded by the simple transposon itself
-There are additional genes in the central element
What two groups are Eukaryotic transposable elements divided into?
- DNA transposons
- Retrotransposons
How are DNA transposons transposed?
through conservative or replicative transposition
How are retrotransposons transposed?
transcribed, then reverse
transcriptase produces a double-stranded DNA copy of the element, which is then inserted into the
genome (copy and paste transposition)
What is a P element/what genome carries it?
-The Drosophila melanogaster genome carries several dozen copies of a transposable element
called a P element
-However, none of the Drosophila melanogaster captured before 1960s have P elements
-Today, all wild-caught D. melanogaster contain P elements
What are retroviruses?
-Retroviruses infect eukaryotic cells, and have genomes composed of single-stranded RNA
- On infection the RNA is transcribed into double-
stranded DNA by reverse transcriptase, allowing the DNA to integrate into the host’s genome
-The viral genes gag and env, encoded by the integrated virus, are needed to produce new
retroviral particles; pol encodes reverse transcriptase
Are retrotransposons related to retroviruses?
-Retrotransposons are related to retroviruses; all carry pol and some contain gag
-None encode env, thus they can be reverse transcribed and inserted into host DNA, but are unable to produce viral particles
What are the gene(s) carried on retrotransposons flanked by?
Long terminal repeats(LTRs)
What percent of the human genome is composed of repetitive DNA sequences derived from former transposable elements that can no longer move within the genome?
45%
What two elements are relatively abundant, and are among the remaining functional transposable elements in humans?
Long interspersed elements
(LINEs) and short interspersed
elements (SINEs)
What are L1 Elements?
-L1 elements are particularly common LINEs (about 600,000 per genome) that vary in length from 6.5 to 8.0 kb
-Full-length L1 elements encode a protein with nuclease and reverse transcriptase function, and may also encode an RNA-binding protein
-L1 elements are associated with spontaneous human mutations
What are Alu Elements?
-Alu elements are the most common of the SINEs
-They vary in length from 100 to 300 bp and are flanked by 7-20 bp direct repeats
-the human genome contains about 1.2 million of these elements; these are relatively recent additions
to the genome and are found in close primate relatives but no other mammals
