Transposable Elements Practice Questions

Correct answer: Can move from one genomic location to another

Transposable genetic elements are mobile DNA sequences capable of changing their position within the genome — a process called transposition. This movement can disrupt genes or alter regulatory sequences, making TEs a significant source of mutation and genome evolution.

Correct answer: Barbara McClintock

Barbara McClintock discovered transposable elements while studying kernel color inheritance in maize during the 1940s-50s. She showed that certain genetic loci changed position on chromosomes, a finding so radical it was not widely accepted until molecular evidence confirmed it decades later, earning her the 1983 Nobel Prize in Physiology or Medicine.

Correct answer: Jumping genes

Transposable elements are nicknamed 'jumping genes' because they can move (transpose) to new genomic locations, potentially causing mutations or altered gene expression at the insertion site.

Correct answer: Transposase

Transposase is the enzyme encoded by the transposon itself. It recognizes the inverted repeat sequences at the transposon's ends and catalyzes the cut-and-paste steps: excising the transposon from its original site and integrating it at a new location.

Correct answer: Insertion sequences

Insertion sequences (IS elements) are the simplest prokaryotic transposable elements. Each IS element encodes only the transposase needed for its own movement and is flanked by short inverted terminal repeats.

Correct answer: Inverted repeat sequences

Insertion sequences are defined by short inverted repeat sequences at both ends. These terminal inverted repeats are recognized by transposase, which binds and catalyzes transposition.

Correct answer: Carry additional genes such as antibiotic resistance

Composite transposons are flanked by two insertion sequences and carry additional genes — most famously antibiotic resistance genes such as those on Tn10 (tetracycline resistance). The flanking IS elements supply the transposase needed for movement.

Correct answer: Conservative transposition

Conservative (cut-and-paste) transposition involves excision of the transposon from the donor site and its integration at a new target site. The element moves without leaving a copy behind, so copy number does not increase.

Correct answer: An increase in transposon copies

In replicative transposition, the transposon is copied: one copy is inserted at a new site while the original remains at the donor locus. This increases the total transposon copy number in the genome.

Correct answer: RNA intermediates

Retrotransposons transpose via an RNA intermediate: the element is first transcribed into RNA, then reverse-transcribed back into DNA, which integrates at a new chromosomal site — a mechanism similar to the replication cycle of retroviruses.

Correct answer: Reverse transcriptase

Retrotransposons encode reverse transcriptase, which synthesizes DNA from the RNA intermediate produced by transcription of the element. This cDNA is then integrated into the genome at a new location.

Correct answer: Long interspersed nuclear elements

LINEs (Long Interspersed Nuclear Elements) are autonomous retrotransposons that can exceed 6 kb. They encode reverse transcriptase and an endonuclease needed for target-primed reverse transcription, allowing them to copy themselves into new genomic locations.

Correct answer: SINEs (Short Interspersed Nuclear Elements)

SINEs (Short Interspersed Nuclear Elements) are non-autonomous because they lack the reverse transcriptase gene needed for transposition. They rely on LINE-encoded proteins to mobilize. Alu elements are the most abundant SINEs in the human genome.

Correct answer: Inserting into functional genes

When a transposable element inserts within a gene's coding or regulatory sequence, it disrupts normal function, causing a mutation. This insertional mutagenesis is a primary mechanism by which TEs generate genetic diversity and can cause disease.

Correct answer: Terminal repeat sequences

Most transposable elements possess terminal repeat sequences (inverted or direct) at their ends. These sequences serve as recognition sites for transposase or integrase and are required for the transposition reaction.

Correct answer: A significant portion of the genome

Transposable elements make up a remarkably large fraction of eukaryotic genomes. In humans, TEs (primarily LINEs, SINEs, and DNA transposons) account for approximately 45-50% of the genome, making them the most abundant component of our DNA.

Correct answer: Retrotransposons

Retrotransposons — particularly LINEs and SINEs — are the most abundant transposable elements in the human genome, comprising roughly 34% (LINEs ~21%, SINEs ~13%) of the total genome sequence. DNA transposons are present but far less numerous.

Correct answer: Increasing genetic variation

Transposable elements generate genetic diversity by causing insertional mutations, inducing chromosomal rearrangements, and occasionally capturing and relocating host gene fragments. This variation provides raw material for natural selection and is a major driver of genome evolution.

Correct answer: DNA transposon

DNA transposons move via a cut-and-paste mechanism: transposase excises the element from the donor site and inserts it at a new target. No RNA intermediate is involved, distinguishing them from retrotransposons, which transpose via reverse transcription of an RNA copy.

Correct answer: Eukaryotic elements frequently use RNA intermediates

The most common transposable elements in eukaryotes are retrotransposons, which transpose via an RNA intermediate (transcription followed by reverse transcription). Prokaryotic transposons, such as Tn3, move primarily as DNA without an RNA step.

Correct answer: DNA transposon

DNA transposons move by excising themselves and inserting into a new location. This is known as cut-and-paste transposition.

Correct answer: Transposase gene and inverted repeats

Insertion sequences are simple transposable elements containing a transposase gene and inverted repeats at their ends.

Correct answer: Inserting into genes

Insertion of transposable elements into genes can disrupt their function. This leads to mutations.

Correct answer: Cut-and-paste transposition mechanism

Cut-and-paste transposition involves removal of the transposon and insertion into a new site. It is typical of DNA transposons.

Correct answer: Retrotransposon

Retrotransposons use reverse transcription to convert RNA back into DNA. This DNA is then inserted into the genome.