GRE Subject Test: Biochemistry, Cell, and Molecular Biology › Help with Transposable and Repeated Elements
What differentiates a LTR retrotransposon and a retrovirus?
Retroviruses encode an envelope protein
LTR retrotransposons cannot move between organisms
Retroviruses are the only ones present in eukaryotes
Retroviruses do not insert DNA into their host
None of these are correct
The only difference between most LTR retrotransposons and retroviruses are that retroviruses can encode an envelope protein. Phylogenetic analyses have shown that retrotransposons and retroviruses are extremely closely related, and may be direct ancestors of one another.
Barbara McClintock initially discovered transposons in her work on corn at Cold Spring Harbor Lab, which she called the Ac / Ds system. What were dissociators (Ds)?
Defective transposons that served as sites of chromosome breakage
Transposons that are mobile and insert all over the genome
Genes that are interrupted by a transposon
A transposon that suppresses another transposon
None of these are correct
Barbara McClintock named the transposons that are defective, and serve as sites of chromosomal breakage where other transposons insert (the associator, Ac) the dissociators. These were likely transposons that lacked the transposase that catalyzes their movement.
How do non-LTR retrotransposons insert into the genome?
Host DNA repair functions are tricked into integrating the transposon
The transposon uses the standard transposase insertion method
The ends of the transposon mimic ribosomal RNA gene sequences
The transposon carries an RNA polymerase promoter
None of these are correct
Non-LTR retrotransposons use an endonuclease that nicks thymine-rich host DNA, which eventually leads to incorporation of the transposon by host DNA repair functions. These other methods are all associated with different specializations of transposon.
How do transposons rapidly propogate through and between species?
Horizontal transfer
Vertical transfer
Bacterial infections
Transposons cannot move between species
None of these are correct
It is hypothesized that transposons can rapidly move through populations and species by horizontal transfer, most likely through viruses.
What makes an LTR retrotransposon unique among other transposons?
The transposon is bound by repeats that contain a series of proteins
The LTR regions allows insertion of the transposon anywhere in the genome, versus other restricted transposons
LTR retrotransposons are not present in eukaryotes like humans
LTR retrotransposons are derived from short RNAs
None of these are correct
LTR stands for Long Terminal Repeats, which are 250-500 base pair repeats located on the ends of a transposon. These repeats encode a series of proteins, most significantly transposase. These are very likely to be early evolutionarily stages of retroviruses.
Which of the following is not true about transposable elements?
Transposable elements are only found in eukaryotes
Transposable elements often move around the genome
Transposable elements can cause disease
Transposable elements are primarily considered non-coding DNA
Transposable elements are portions of the DNA that are free to move around the genome and are generally considered non-coding DNA. This can be potentially dangerous, however. Transposable elements can insert themselves in the coding regions of genes, thus making them non-functional. This can lead to disease. Both eukaryotic and prokaryotic genomes contain transposable elements.
Transposable elements, or transposons, are separated into two classes. Which of these categories of life have class I transposons in their genomes?
I. Bacteria
II. Yeast
III. Eukaryotes
II and III
I, II, and III
None of these
I and II
III only
Class I transposable elements are RNA-mediated elements of a single evolutionary origin, and are found in yeast, which only have class I elements, and in eukaryotes, which have both class I and class II elements. Bacteria only have class II elements, and hence are not included in the correct answer to this question.
Transposable elements can be significant factors in causing newly resistant bacterial strains. How do transposons cause resistance to develop?
A composite of two transposons and a gene insert into the bacterial genome
Transposons are not actually involved in creating resistance
An individual transposon incorporates into the genome and is adapted
A transposon modifies a bacterial resistance plasmid
Transposons change gene expression levels
Two transposons flanking an antibiotic resistance gene can easily move between bacteria and confer new resistance. A mix of transposons and new genes such as this is called a composite transposon. Recall that bacteria exchange genetic information via conjugation, transduction, and transformation.
The hybrid dysgenesis phenomenon was observed in Drosophila flies. It was determined that this was caused by a transposon no longer under control in wild type - lab strain crosses. What are transposons commonly controlled by in their hosts?
RNAi and piRNAs
Other types of transposons
The immune system
A transposase inhibitor
None of these
Movement of transposons is very commonly controlled by RNA interference. The RNAi system cuts up problematic RNAs, and uses these small pieces to target transposons for destruction.