Multiple Choice QuestionsDuring translation in prokaryotes, when ribosomes reach the termination codon, the termination codon is recognized by the class I release factors (RF1 or RF2) leading to the release of the polypeptide. A second class II release factor (RF3) facilitates the termination process. Which of the following statements regarding the mechanism of action of the release factors in INCORRECT?
Class I release factors decode the stop codons while the RF3 is a GTPase that stimulates recycling of the class I release factors
Free RF3 has a higher affinity for GTP than GDP
RF1 and RF2 share a conserved segment of ‘GGQ’ sequence which is essential for the polypeptide release.
RF1 and RF2, individually possess another stretch of tripeptide sequences which are involved in the recognition of the terminations codons.
C.
RF1 and RF2 share a conserved segment of ‘GGQ’ sequence which is essential for the polypeptide release.
Free RF3 binds GDP very strongly, and the spontaneous dissociation of the guanine nucleotide is slow, implying that cytoplasmic RF3 is in the GDP, rather than the GTP, conformation.
When RF3·GDP associates with a ribosome in complex with a class 1 RF, the GDP on RF3 can rapidly dissociate and be exchanged for GTP. RF3 in the GTP conformation has a high affinity for the ribosome in the absence of a class 1 RF, and its binding destabilizes that of RF1 or RF2.
The formation of RF3·GTP therefore leads to rapid dissociation of RF1 or RF2 from the ribosome. Subsequent hydrolysis of GTP on RF3 leads to its fast dissociation in the GDP conformation, the normal free state of the factor.
A 1257 bp genomic DNA sequence of a prokaryotic gene was cloned under a strong constitutive promoter along with a suitable polyA signal and used for development of transgenic tobacco plants. Molecular analysis revealed the presence of three types/lengths of transgene derived mRNAs: 555 bp, 981 bp and 1257 bp – in the leaves of transgenic plants. The following statements were proposed to explain the above results.
(a) and (c)
(b) and (d)
(a) and (b)
(c) and (d)
E.coli DNA ligase catalyses formation of a phosphodiester bond between the adjoining 3' hydroxyl, and the 5'phosphoryl ends in DNA duplexs. The energetic need for this reaction is met by the hydrolysis of NAD+ to NMN+ and AMP in a three-step reaction. Following statements are being made about the mechanism of this reaction:
(i) AMP is linked to the 5' phosphoryl end of the nicked DNA.
(ii) Adenylyl group of NAD+ is transferred to the ε-amino group of Lys in DNA ligase to form a phosphoamide adduct.
(iii) DNA ligase catalyses the formation of a phosphodister bond by the nucleophilic attack of the 3' hydroxyl group onto the phosphate and release AMP.
Based on the statements made above, identify the correct sequences of the reaction step.
(i), (ii), (iii)
(i), (iii), (ii)
(ii), (i), (iii)
(iii), (i), (ii)
Given are some statements with reference to the use of genes in plant molecular systematics.
(a) mtDNA are not preferred over cpDNA or rDNA because they generally show slow rate of sequence evolution and fast rate of structural evolution.
(b) cpDNA are not preferred because of their haploidy, uniparental inheritance, and absence of recombination among cpDNA molecules.
(c) rDNA such as ITS are preferred for their higher evolutionary rates as well as shorter sequence length.
(d) rDNA and cpDNA cannot be used simultaneously in molecular systematics since they represent conflicting patterns of inheritance.
Which of the above statements are INCORRECT?
(a), (c) and (d)
(a), (b) and (c)
(a) and (c) only
(b) and (d)
Which one of the following statements regarding restriction/ modifying enzymes used in recombinat DNA technology is correct?
Endonucleases remove nucleases remove nucleotides, one at a time, from the ends of a sequences.
Type II class of restriction enzymes do not recoginse palindromic sequences
Mung bean nuclease acts on double stranded DNA or RNA termini
Type II class of restriction enzymes can generate either "sticky" or blunt ends.
mRNA of a gene was depleted in human cells using siRNA that arrest cells in the G2 phase of the cell cycle. In order to test whether the G2 arrest is due to an off-target or an on-target effect of siRNA mediated mRNA depletion an investigator can:
(a) re-introduce an ectopic copy of the gene coding for the wild-type mRNA and protein.
(b) re-introduce an ectopic copy of the gene that is different in its mRNA sequence at the siRNA target site but encodes for the same protein.
(c) re-introduce an ectopic copy of the gene that codes for different mRNA and protein.
(d) utilize few more siRNAs targeting different regions of the mRNA in question.
Choose the combination with correct statements.
(a),(b),(c) only
(c) and (d) only
(b) and (d) only
(b),(c),(d) only
Presence of selenocysteine in proteins in E.coli is a consequence of:
post-translational modification of cysteine present in special structural regions of the proteins by SeIB and SeIC
post-translational modification of serine present in special structural regions of the proteins by SeIB and SeIC
aminoacylation of special tRNA (tRNASeCys)by serine tRNA synthetase with serine followed by furthuer modification of the attached serine to selenocysteine followed by its transport to the ribosome by SeIB
aminocylation of a special tRNA(tRNA SeCys) by serine tRNA synthetase with
A DNA segment was cloned into active site region of lacZ gene and the recombinant plasmid introduced into lacZ- strain of E.Coli and plated on a medium containing X-gal.The colonies showed blue colour. Which one of the following statements is correct?
The nature of the cloned DNa segment need not be special as cloning of any DNA in lacZ will result in disruption of its reading frame and production of blue colour on X-gal plates
The cloned DNA segment could be a Group I intron whose removal from the precursor lacZ transcript in E.coli results in production of mature lacZ mRNA which can then produce active LacZ protein
The cloned sequence is likely to be lacY sequence which is naturally a part of lac operon in E.coli
The cloned sequence is likely to be an antiterminator sequence which allows full length transcription of lacZ
An intron in a yeast reporter gene carries a mutation in the splice site branch point (UACUAAC to UACA.AAC).To suppress the mutation, a library of point mutants of snRNAs was introduced into the mutant strain. The suppressor is most likely to have a point mutation in:
U1 snRNA
U2 snRNA
RNase P
U6 snRNA
A researcher wanted to identify the enhancer sequences of newly discovered gene. Shown below are the relevant regions of some of the reporter constructs the researcher designed to identify the enhancer.
Which of the above constructs can be used to identify the enhancer?
(a) only
(b) only
Both (a) and(c)
(c) only
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