This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “The Multitude of Transposable Elements – 1”.
1. The transposable elements of bacteria are generally retrotransposons.
Explanation: Most transposable elements in bacteria transpose directly as DNA. In contrast, most mobile elements in the eukaryotes are retrotransposons.
2. The IS elements can be identified by the presence of __________
a) Antibiotic resistance gene
b) Endonuclease cleavage site
c) 50 bp inverted repeat
d) Integrase site
Explanation: The IS elements are the most common type of transposons present in the bacterial genome. An inverted repeat of usually 50 bp is the hallmark of such elements present at each end of an insertion sequence.
3. Direct repeats in the IS element are present __________
a) Within the transposon
b) Upstream the inverted repeat
c) Within the inverted repeat
d) Downstream the inverted repeat
Explanation: A set of two direct repeats flank the IS element on either side. They are not transposed with the insertion element and are found within the inverted repeat sequence on either sides.
4. The direct repeat within the IS element has a length of __________
a) 20 bp
b) 11-15 bp
c) 5-11 bp
d) 3-7 bp
Explanation: The direct repeat on the either side of the insertion element is a hallmark of IS element. They are 5 to 11 bp long sequences immediately adjacent to the both ends of the inserted element.
5. The enzyme that catalyzes the transposition of an IS element is called __________
Explanation: The enzyme that catalyzes the transposition of an IS element is called a transposase. The transposase recognizes the inverted repeats and moves the segment of DNA bound by them from one site to another.
6. Transposase restriction mechanism of IS element restricts the transposon and the target DNA in a combination of which of the following?
a) Blunt end cut for transposon and sticky end cut for target DNA
b) Blunt end cut for both transposon and target DNA
c) Sticky end cut for transposon and blunt end cut for target DNA
d) Sticky end cut for both transposon and target DNA
Explanation: The transposase is encoded by the IS element itself. The transposase on production makes blunt-ended cut in the transposable element and staggered/sticky-ended cut in the target DNA.
7. Which of the following functions is not performed by transposase?
a) Restriction of the IS element
b) Integration of the transposon
c) Formation of the RNA intermediate
d) Restriction of the host genome
Explanation: The transposase performs three functions in the transposition of the IS element. First, it precisely excises the IS element from the transposon. Second, it makes staggered cut in the target DNA segment. Last, it ligates the IS element within the target site.
8. The central block of the composite transposable element consists a gene for __________
b) Antibiotic resistance
Explanation: Composite transposons contain one or more protein-coding genes in addition to those required for transposition. Many of the well known composite transposons carry genes for antibiotic resistance. Two of the best known elements are Tn9 (chloramphenicol resistance) and Tn10 (tetracycline resistance).
9. Which of the following is a non-composite transposon?
Explanation: Tn5, Tn10 and Tn9 are examples of composite transposons. Tn3 and Tn21 are examples of non-composite transposons.
10. Which of the following about the P-element is false?
a) 25 bp inverted repeats
b) 2907 bp long element
c) Encodes for transposase
d) DNA transposon
Explanation: The P-element in Drosophila is one of the best examples of transposable elements in eukaryotes. This element is 2907 bp long and features a 31 bp inverted repeat at each end. This element encodes a transposase. P-element does not utilize an RNA intermediate during transposition and are able to insert at many different positions.
11. Hybrid dysgenesis is caused by which of the following transposable element?
a) Non-composite transposon
Explanation: Interbreeding of two incompatible strains of Drosophila displays dysgenic traits. These include a series of defects including mutations, chromosomal aberrations, distorted segregation at meiosis and sterility. This genetic phenomenon is called hybrid dysgenesis.
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