MCQs on Database Screening and Screening by Nucleic Acid Hybridisation – 2
1 - Question
The process of baking or cross-linking of the membrane is carried out by ___________
a) By heating at 80 degrees
b) By irradiating with UV
c) Both irradiation and heating method are used
d) By addition of agarose
Explanation: The cross-linking or baking of membrane is carried out by either heating it at 80 degrees or irradiating with UV. It is important to carry out this step because it binds the DNA irreversibly to the membrane.
2 - Question
Choose the incorrect statement for prehybridization mixture.
a) The function of it is to carry out specific binding
b) It constitutes of non-labelled DNA of non-specific sequence which is obtained from salmon or herring testes or sperm
c) It is added in a small amount
d) It is carried out before the hybridization is carried out
Explanation: Prehybridization mixture is meant for blocking sites which are meant for non-specific binding. It constitutes of non-labelled DNA of non-specific sequence which is obtained from salmon or herring testes or sperm. They are added in a small amount and it is added before the hybridization is carried out. So that when hybridization is carried out, only specific binding takes place.
3 - Question
After hybridization how the position of the labelled DNA probe is determined?
a) By staining
b) By UV irradiation
c) By using chemiluminescence
d) By electronic microscope
Explanation: After the hybridization has been carried out and the membrane has been washed, the position of the labelled DNA probe is determined by using chemiluminescence. Commonly the probe is labelled by incorporation of fluoroscein conjugated molecules.
4 - Question
The important parameters for precise annealing of the probe and the subsequent washing don’t include ___________
a) Size of the probe
b) Proportion of only A
c) Ionic concentration of hybridization buffer
d) Other agents which alter the base pairing
Explanation: The probe should anneal precisely and the parameters which decide these are size of the probe, proportion of all A, G, T and C. Ionic concentration of hybridization buffer and other agents such as formamide which alter base pairing. These parameters define the maximum temperature at which probe and target DNA bind fully.
5 - Question
If hybridization is carried out with a clone having a sequence similar to that of the vector from which library has been constructed, then it is of no use. Choose the correct statement in respect to it.
a) PCR amplification of the probe can be carried out
b) Excision of the probe should be carried out which should be followed polyacrylamide gel electrophoresis
c) If the vector sequence and the DNA probe are having similarities, some of the vector would be hybridized
d) Probe should be in a vector that is having some sequence similarity with the library to be screened
Explanation: If hybridization is carried out with a clone having a sequence similar to that of the vector from which the library has been constructed then all the vector would hybridize. To make the sequence different, PCR amplification of the probe can be carried out. Also, excision of the probe can be carried out by using suitable restriction enzymes and it is followed by agarose gel electrophoresis. Or probe in a vector should not have any sequence similarity with the library to be screened.
6 - Question
At what temperatures the hybridization and washing of the DNA probes should be carried out?
a) At melting temperature
b) At a temperature lower than the melting temperature
c) At a temperature higher than the melting temperature
d) At 100 degrees
Explanation: The hybridization and washing of the DNA probes should be carried out at a temperature below than the melting temperature. It is so because if it is carried out a temperature lower than the melting temperature, some amount of mismatch is allowed. Melting temperature depends on the composition of the probes.
7 - Question
Which library should be used, genomic or cDNA when the screening is carried out by oligonucleotide probes?
c) Both genomic and cDNA library can be used equivalently
d) None of the libraries are preferred if oligonucleotide probes are used
Explanation: If oligonucleotide probes are used, then cDNA libraries should be used. It is so because cDNA libraries are enriched for coding sequences and thus fewer sequences have to be screened.
8 - Question
Sometimes successive rounds of screening of a genomic library are carried out and an ordered collection of clones is done in a linear fashion, then the process is called as ___________
a) chromosome jumping
b) chromosome sorting
c) chromosome walking
d) transposon tagging
Explanation: Chromosome walking is the process of successive rounds of screening of a genomic library and it leads to the collection of clones in a linear order.
9 - Question
If the clone for a gene is obtained on the basis of its position in the gene map, then it is called as ___________
a) locational cloning
b) positional cloning
c) chromosome walking
d) transposon tagging
Explanation: If the clone for a gene is obtained on the basis of its position in the gene map then it is called positional cloning.
10 - Question
Mobile portions of a chromosome which can be transferred from one portion to another are termed as ___________
a) mobile part
c) walking elements
d) transferrable genetic element
Explanation: Transposons are the mobile portions of the chromosomes which can be transferred from one portion of the chromosome to another either on the same chromosome or different chromosome.