This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “The Multitude of Transposable Elements – 2”.
1. Dysgenesis occurs in every cell of the Drosophila.
Explanation: Hybrid dysgenesis is principally a phenomenon of the germ cells. In crosses involving the P-M system, the F1 generation hybrid flies have normal somatic tissues. Their gonads, however, do not develop.
2. P-strains of the Drosophila have active P-elements.
Explanation: P-elements reside passively in the P-strains because they express a repressor of transposition that keeps the transposons silent. The repressor is also inherited maternally through the egg cytoplasm thus the offspring thus produced will be normal.
3. Hybrid dysgenesis occurs in which of the following cases?
a) Crossing between P-female and P-male
b) Crossing between M-female and M-male
c) Crossing between P-female and M-male
d) Crossing between M-female and P-male
Explanation: Hybrid dysgenesis is observed when crossing P-males with M-females. The eggs of M-strain do not contain the repressor protein that prevents the transposition. This leads to transposition of the P-elements from the sperm of the P-strain leading to hybrid dysgenesis in the progeny.
4. Retrotransposons transposes through the RNA intermediates.
Explanation: The elements that transpose through an RNA intermediate utilizing a reverse transcriptase are called the retrotransposon. They are primarily found in the eukaryotic organisms.
5. Retrotransposons are of how many types?
Explanation: Retrotransposons can be divided into two types on the basis of presence of long terminal repeats. If the LTR is present on either side of the transposon they are known as LTR-transposons and if they are absent then it is known as non LTR-Transposons.
6. LTR-retrotransposons are abundantly found in __________
Explanation: LTR-retrotransposons are also known as viral retrotransposons. They are most abundantly found in lower eukaryotes such as yeast and Drosophila.
7. LINE and SINE are examples of __________
a) LTR retrotransposon
b) Composite transposon
c) Non-LTR retrotransposon
Explanation: LINE and SINE are the most abundantly found non LTR-retrotransposons in the vertebrates. They lack the terminal LTR sequences and have a stretch of A/T at one end.
8. Which of the following about the L1 LINE family is correct?
a) Highly accumulated in autosomes
b) About 37% of the total human genome
c) Presence of short direct repeats on either end
d) Approximately 2 kb in length
Explanation: The corrected characteristics are:
i) Highly accumulated in sex chromosomes than autosomes
ii) Makes up about 17% of the total human genome
iii) Approximately 6 kb in length.
9. The consensus sequence of the L1 LINE family has three open reading frames.
Explanation: The consensus sequence co the L1 LINE family contains two long open reading frames. ORF1 encodes an RNA-binding protein and ORF2 encodes a protein with both endonuclease and reverse transcriptase activity.
10. SINES are transcribed by __________
a) Klenow fragment
b) Polymerase I
c) Polymerase II
d) Polymerase III
Explanation: SINEs are transcribed by RNA polymerase III. They require the machinery of the LINE elements as they are around 100-400 bp long non-autonomous sequences.
11. The SINE commonly contains the restriction site for the endonuclease __________
a) Pvu I
b) Spa I
c) Alu I
d) Xma I
Explanation: The most common SINE elements are Alu elements in the human genome. These are about 300 bp long sequences that are scattered throughout the human genome, constituting of almost 10% of the total DNA.
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