GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Transposable and Repeated Elements

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #1 : Help With Transposable And Repeated Elements

Which of the following is not true about transposable elements?

Possible Answers:

Transposable elements often move around the genome

Transposable elements are primarily considered non-coding DNA

Transposable elements are only found in eukaryotes

Transposable elements can cause disease

Correct answer:

Transposable elements are only found in eukaryotes

Explanation:

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.

Example Question #2 : Help With Transposable And Repeated 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

Possible Answers:

I, II, and III

None of these

II and III

III only

I and II

Correct answer:

II and III

Explanation:

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.

Example Question #3 : Help With Transposable And Repeated Elements

What differentiates a LTR retrotransposon and a retrovirus?

Possible Answers:

LTR retrotransposons cannot move between organisms

Retroviruses are the only ones present in eukaryotes

Retroviruses do not insert DNA into their host

Retroviruses encode an envelope protein

None of these are correct

Correct answer:

Retroviruses encode an envelope protein

Explanation:

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.

Example Question #4 : Help With Transposable And Repeated Elements

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?

Possible Answers:

Other types of transposons

None of these

A transposase inhibitor

The immune system

RNAi and piRNAs

Correct answer:

RNAi and piRNAs

Explanation:

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.

Example Question #26 : Gene Regulation And Genomics

How do transposons rapidly propogate through and between species?

Possible Answers:

Bacterial infections

Transposons cannot move between species

None of these are correct

Horizontal transfer

Vertical transfer

Correct answer:

Horizontal transfer

Explanation:

It is hypothesized that transposons can rapidly move through populations and species by horizontal transfer, most likely through viruses.

Example Question #4 : Help With Transposable And Repeated Elements

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)?

Possible Answers:

Genes that are interrupted by a transposon

A transposon that suppresses another transposon

None of these are correct

Defective transposons that served as sites of chromosome breakage

Transposons that are mobile and insert all over the genome

Correct answer:

Defective transposons that served as sites of chromosome breakage

Explanation:

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.

Example Question #5 : Help With Transposable And Repeated Elements

Transposable elements can be significant factors in causing newly resistant bacterial strains. How do transposons cause resistance to develop?

Possible Answers:

Transposons are not actually involved in creating resistance

An individual transposon incorporates into the genome and is adapted

A composite of two transposons and a gene insert into the bacterial genome

Transposons change gene expression levels

A transposon modifies a bacterial resistance plasmid

Correct answer:

A composite of two transposons and a gene insert into the bacterial genome

Explanation:

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.

Example Question #6 : Help With Transposable And Repeated Elements

What makes an LTR retrotransposon unique among other transposons?

Possible Answers:

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

None of these are correct

LTR retrotransposons are not present in eukaryotes like humans

LTR retrotransposons are derived from short RNAs

Correct answer:

The transposon is bound by repeats that contain a series of proteins

Explanation:

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.

Example Question #7 : Help With Transposable And Repeated Elements

How do non-LTR retrotransposons insert into the genome?

Possible Answers:

None of these are correct

The ends of the transposon mimic ribosomal RNA gene sequences

Host DNA repair functions are tricked into integrating the transposon

The transposon uses the standard transposase insertion method

The transposon carries an RNA polymerase promoter

Correct answer:

Host DNA repair functions are tricked into integrating the transposon

Explanation:

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.

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