MCQs on cDNA Libraries, Polyadenylated RNA and cDNA Synthesis – 1
1 - Question
Choose the incorrect statement for cDNA libraries.
a) They constitute of DNA copies produced from the RNA sequences and usually mRNA
b) They represent expressed sequences
c) Introns are not represented
d) Comparison of cDNA sequences with genomic sequences leads to the determination of polyadenylation sites
Explanation: cDNA libraries constitute of DNA copies produced from RNA sequences and usually mRNA. They are not only the collection of expressed sequences but post transcriptional changes are also recorded. Introns can also be represented. Comparison of cDNA sequences with genomic sequences can lead to the determination of positions of introns, polyadenylation sites etc.
2 - Question
A times partial sequencing of cloned cDNAs is carried out. These cDNA are known as ___________
a) expressed RNA sequences
b) expressed sequence tags (ESTs)
c) expressed cDNA sequences
Explanation: The partial sequencing of cloned cDNA is the first step of genome characterization project. These cloned cDNA sequences are known as expressed sequence tags (ESTs).
3 - Question
Polyadenylation of RNA species is an important criterion for the production of cDNA species. Which of the following holds true?
a) Polyadenylation should be at 3’ end
b) Eukaryotic mRNAs are mostly non-polyadenylated
c) Bacterial mRNAs and organelle mRNAs are polyadenylated
d) It is carried out by the addition of T residues after synthesis
Explanation: Polyadenylation of RNA species is an important criterion for the production of cDNA species and polyadeylation should be at the 3’ end. Mostly eukaryotic mRNAs are polyadenylated. It is carried out by the addition of A residues after synthesis. Usually, bacterial mRNAs and organelle mRNAs are non-polyadenylated residues.
4 - Question
Choose the incorrect statement for oligo-dT cellulose.
a) It is used for separation of polyadenylated mRNA from another mRNA
b) oligo-dT are covalently attached to the solid support via OH bonds
c) A solution containing RNA is passed through the column
d) Poly A tail attaches to the oligo-dT by ionic bonds
Explanation: oligo-dT cellulose is used for separation of polyadeylated mRNA from another mRNA. These are covalently attached on a solid support via OH bonds. For separation, a solution containing RNA is passed through the column and the molecules having poly A tails are attached to the column via hydrogen bonds.
5 - Question
Poly A tail from the column is eluted by using high salt concentration.
Explanation: All non-specific mRNA are firstly eluted from the column. The poly A tail is eluted from using low salt concentration, it is so because low salt concentration destabilizes nucleic acids.
6 - Question
At times streptavidin is used in place of cellulose. Choose the correct statement for this alternative.
a) oligo-dT streptavidin conjugate can be extracted by magnetic beads
b) The magnetic beads are attached with activin
c) Recovery is not based on the magnetic properties of the beads
d) The interaction between polyA tail and the oligo-dT is reduced
Explanation: If streptavidin is used in the place of cellulose, oligo-dT streptavidin conjugate can be extracted by magnetic beads. These magnetic beads are having biotin attached to them. Recovery is based on the magnetic properties of the beads. The interaction between the polyA tail and the oilgo-dT still remains the basic principle and it is not affected.
7 - Question
Choose the correct with respect to RNA molecules.
a) They are less labile than DNA molecules
b) The 2’ hydroxyl group of ribose group decreases the activity
c) There is no less of activity while boiling
d) Baking of glassware, treatment with UV can be used for protection against degradation
Explanation: RNA molecules are more labile as compared to the DNA molecules and they are more reactive because of the 2’ hydroxyl group of ribose group. Baking of glassware, treatment with UV can be used as methods for protection against degradation. If precautions are not taken, there is subsequent loss of activity while boiling.
8 - Question
What is the basis of RNaseH method?
a) It is based on RNA synthesis by DNA strand
b) It is based on complementary DNA synthesis by RNA strand through reverse transcriptase
c) It is based on complementary DNA synthesis by RNA strand through RNaseH enzyme
d) It is based on getting double strand RNA from a single strand
Explanation: RNase H method is based on the synthesis of the complementary DNA synthesis by RNA strand through reverse transcriptase enzyme and it leads to the formation of the duplex of RNA and DNA.
9 - Question
The first step for RNaseH method is to anneal a chemically synthesized oligo-dT primer to the 3’ polyA tail of RNA.
Explanation: A chemically synthesized oligo-dT primer is annealed to the 3’ polyA tail of RNA. After this primer is attached the RNA DNA duplex is formed with the help of reverse transcriptase.
10 - Question
For the synthesis of the second DNA strand, incorporated oligo-dT is of no use. Why?
a) It is so because it is unstable
b) It is so because it is at the 3’ end of the template molecule
c) It is so because it would be unable to initiate replication
d) It is so because it detaches itself after first strand synthesis
Explanation: For the second DNA strand synthesis, oligo-dT is of no use. It is so because it is present at 5’ end of the template and replication could not start from 5’ end.