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Dna replication

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Solved MCQs(1-27) along with explanation concerned with DNA replication.

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Dna replication

  1. 1. DNA REPLICATIONSOLVED MCQS WITH EXPLANATION(1-27)BYDr Ichha PurakUniversity ProfessorDepartment of BotanyRanchi Women’s College,Ranchi
  2. 2. Which of the following reactions is required for proofreading (i.e. correctingreplication errors) during DNA replication by DNA polymerase III?a) 3 - 5 exonuclease activityb) 5 - 3 exonuclease activityc) 3 - 5 endonuclease activityd) 5 - 3 endonuclease activityAnswer c) 3 - 5 endonuclease activityExplanationThe proofreading activity possessed by many DNA polymerases is anexonuclease activity that degrades mismatched bases that have been wronglyincorporated into the growing chain. This is, therefore, an exonucleaseactivity (exonucleases digest from theend of a DNA chain) and it operatesbackwards from the 3 growing end, i.e. 3 - 5.6/15/20132SolvedMCQDNAREPLICATION
  3. 3. How does the mismatch repair system distinguish between the parental (i.e. correct)DNA strand and the newly synthesised strand containing the mismatched base?a) Thymine in the parental strand of the helix is methylated at GATC.b) Thymine in the new strand of the helix is methylated at GATC.c) Guanine in the parental strand of the helix is methylated at GATC.d) Guanine in the new strand of the helix is methylated at GATC.Answer d) Guanine in the new strand of the helix is methylated at GATC.ExplanationMismatch repair is a system that repairs mismatches that have slippedevaded proofreading during DNA replication. DNA becomes methylatedat the G of GATC sequences after replication; however, this does notoccur immediately so the new strand, containing the error, can bedistinguished from the methylated parental strand6/15/20133SolvedMCQDNAREPLICATION
  4. 4. What is the name of the DNA repair system in E. coli in which dual incisionsare made in the damaged part of the double helix, and a 12-13 basesegment is removed and replaced with new DNA?a) Mismatch repairb) Base excision repairc) Nucleotide excision repaird) AP site repairAnswer c) Nucleotide excision repairExplanationNucleotide excision repair is an almost universal repair mechanism in whicha section of damaged DNA is removed and replaced with new DNA by aDNA polymerase. It is used to repair photoproducts caused by UV damageand bulky DNA lesions caused by a variety of mutagens.6/15/20134SolvedMCQDNAREPLICATION
  5. 5. Which of the following is the name of the human genetic disorder resulting from defects innucleotide excision repair?a) Hereditary nonpolyposis colorectal cancer (HNPCC)b) Xeroderma pigmentosum (XP)c) Lynch syndromed) DiabetesAnswer b) Xeroderma pigmentosum (XP)ExplanationPeople born with the disorder, xeroderma pigmentosum, have a mutationin one of the genes coding for nucleotide excision repair enzymes.Therefore they are unable to carry out efficient repair on sunlight damageand they are hypersensitive to sunlight. They have to protect their skinfrom daylight or risk getting skin cancer.6/15/20135SolvedMCQDNAREPLICATION
  6. 6. In which of the following would you find telomeres?a) Human mitochondrial DNAb) Human chromosomesc) Bacterial chromosomesd) The influenza virus genomeAnswer b) Human chromosomesExplanationTelomeres are found at the ends of the linear double-stranded DNAmolecules in human chromosomes. They protect chromosome ends fromnucleases and they also provide a special mechanism for replication ofchromosome ends, using the enzyme telomerase. Circular molecules suchas bacterial chromosomes and most mitochondrial genomes do not needthese specialised DNA ends6/15/20136SolvedMCQDNAREPLICATION
  7. 7. Which of the following enzymes are used to join bits ofDNA?a) DNA ligaseb) DNA polymerasec) primased) EndonucleaseAnswer DNA ligaseExplanationDNA ligase enzyme joins bits of DNA on lagging strand6/15/20137SolvedMCQDNAREPLICATION
  8. 8. Why is an RNA primer considered essential during DNA synthesis byDNA polymerase III?a) The enzyme requires a free 3-PO4 group.b) The enzyme requires a free 5-PO4 group.c) The enzyme requires a free 5-OH group.d) The enzyme requires a free 3-OH group.e) There is no particular reason, that is simply the observation.Answer d) The enzyme requires a free 3-OH groupExplanationPolymerase III requires a free 3-OH group to begin synthesis of DNA. AnRNA primer provides a 3 hydroxyl group, from which the DNA polymerasecan start synthesis.6/15/20138SolvedMCQDNAREPLICATION
  9. 9. Which molecule serves to destabilize the DNA helix in order to open itup, creating a replication fork?a)DNA helicaseb) DNA ligasec) DNA polymerased) SSBPse) DNA gyraseAnswer a) DNA HelicaseExplanationThe DNA helicase enzyme destabilizes the DNA helix by breaking hydrogenbondsFor DNA Replicaion, unwinding of DNA is done bya) Helicaseb) ligasec) Hexonucleased) TopoisomeraseAnswer a) Helicase6/15/20139SolvedMCQDNAREPLICATION
  10. 10. Semi conservative replication of DNA was first demonstrated ina) Escherichia colib) Streptococcus pneumonaec) Salmonella typhimuriamd) Drosophila melanogasterAnswer a) Escherichia coliExplanationThe experiment was performed by Maselson and Stahl (1959 ) to know about mode ofreplication. The experiment was performed using Escherichia coliMode of DNA replication in E.coli isa) Conservative and unidirectionalb) Semiconservative and unidirectionalc) Conservative and bidirectionald) Semiconservative and bidirectionalAnswer d) Semiconservative and bidirectional6/15/201310SolvedMCQDNAREPLICATION
  11. 11. During the replication of DNA, the synthesis of DNA on laggingstrand takes place in segments, these segments are calleda) Satellite segmentsb) Double helix segmentsc) Kornbeg segmentsd) Okazaki segmentsAnswer d) Okazaki segmentsExplanation6/15/201311SolvedMCQDNAREPLICATION
  12. 12. The elongation of the leading strand during DNA synthesisa) Progresses away from the replication forkb) Occur in 3’-5’ directionc) Produces Okazaki fragmentd) Depend on the action of DNA polymeraseAnswer d) Depend on the action of DNA polymeraseExplanationTakes place with advancement of DNA polymerase on thetemplate ,On leading strand it is towards the neck of Replicationforlk in 5’----3’ direction6/15/201312SolvedMCQDNAREPLICATION
  13. 13. Eukaryotes differ from prokaryote in mechanism of DNA replicationdue to:a) Different enzyme for synthesis of lagging and leading strandb) Use of DNA primer rather than RNA primerc) Unidirectional rather than bidirectional replicationd) Discontinuous rather than semidiscontinuous replicationAnswerd) Discontinuous rather than semidiscontinuous replicationExplanationOkazai fragments are smaller and are formed discontinuously6/15/201313SolvedMCQDNAREPLICATION
  14. 14. True replication of DNA is possible due toa) Hydrogen bondingb) Phosphate backbonec) Complementary base pairing ruled) None of the aboveAnswer c) Complementary base pairing ruleExplanationAdenine pairs with Thymine only( A = T ) and Guanine pairs withCytosine ( G ≡C ) only ,because of strict base pairing patternthe new daughter strand is assembled according to basespresent on the template6/15/201314SolvedMCQDNAREPLICATION
  15. 15. All of the following are differences between eukaryotic andprokaryotic DNA replication except __________.a) the type and number of polymerases involved in DNA synthesisb) multiple vs. single replication originsc) the rate of DNA synthesisd) the ability to form a replication forkAnswerd) the ability to form a replication forkExplanationBoth prokaryotes and eukaryotes form replication forks during DNAreplication6/15/201315SolvedMCQDNAREPLICATION
  16. 16. Telomeres have all of the following properties or characteristicsexcept __________.a) the telomerase enzymeb) forming "hairpin" loopsc) a link to the aging process in eukaryotic cellsd) being found in both eukaryotic and prokaryotic chromosomesAnswer d) being found in both eukaryotic and prokaryoticchromosomeExplanationThe telomere sequence occurs at the ends of linear eukaryoticchromosomes. Prokaryotes have circular chromosomes so donothave the terminities or telomeres6/15/201316SolvedMCQDNAREPLICATION
  17. 17. Assume that in the following cross, d+ x e+, the only gametes formedconsist of de alleles. This may be due to a phenomenon called__________.a) gene conversionb) reciprocal recombinationc) nonreciprocal recombinationd) genetic exchangeAnswer a) Gene conversionExplanationGene conversion is a situation in which the products of meiosis do not fit thenormal Mendelian patterns of inheritance. One or more alleles are notrepresented, and it is as if one gene has been converted into another6/15/201317SolvedMCQDNAREPLICATION
  18. 18. In the Meselson-Stahl experiment, which mode of replicationcan be eliminated based on data derived after one generationof replication?a)dispersiveb) semiconservativec) conservatived) all three modesAnswer c) ConservativeExplanationConservative replication theory says that parental strands re-anneal with parental strands, and daughter strands with daughterstrands after DNA replication. This experiment showed this to benot true.6/15/201318SolvedMCQDNAREPLICATION
  19. 19. The discovery of Okazaki fragments suggested that DNA synthesisis __________.a) discontinuousb) continuousc) 3 to 5d) semiconservativeAnswer a) discontinuousExplanationDNA must be synthesized on both strands of the double helix at thesame time. However, one strand runs in a 5 to 3 direction and one runsin the 3 to 5 direction. In order for replication to occur on both strands inthe same direction simultaneously, one strand must be made in adiscontinuous fashion (in short pieces—Okazaki fragments) andreannealed later.6/15/201319SolvedMCQDNAREPLICATION
  20. 20. In vivo synthesis of DNA is __________a) 3 to 5b) 5 to 3c) both 3 to 5 and 5 to 3d) neither 3 to 5 nor 5 to 3Answer b) 5 to 3‘ExplanationDNA polymerase must add nucleotides in a 5 to 3 direction.Therefore, DNA synthesis must also occur in this direction6/15/201320SolvedMCQDNAREPLICATION
  21. 21. Semiconservative replication of DNA means thata) Each daughter duplex will have one of the original strand and one newstrandb) One daughter duplex will be entirely new and other will have both originalparental strandsc) Both daughter duplexes will be entirely new and the parental duplex will bedegradedd) Each strand of each daughter duplex will have parts of parental strands andpart of new strandsAnswer a) Each daughter duplex will have one of the original strand and onenew strandExplanationExperiment conducted by Maselson and Stahl (1959) clearly demonstrates thatdensity of DNA after first generation is intermediate between n N 15 and N14which supports Semiconservative replication6/15/201321SolvedMCQDNAREPLICATION
  22. 22. The theta structure that results from autoradiography of replicating bacterialchromosome illustrates all the following excepta) The unreplicated portion of the chromosomeb) The pair of daughter molecules in the process of being formedc) Relication forksd) The replication originAnswer d) The replication originExplanationWhen replication starts from origin it proceeds bidirectionally and when tworeplication forks meet each other on other side theta like structure is formed6/15/201322SolvedMCQDNAREPLICATION
  23. 23. A replicating prokaryotic chromosome has --------replicaton forks and a replicating eukaryoticchromosome has ------------ replication forks.a) one:manyb) Two :twoc) Two : manyd) Many : manyAnswer c) two : manyExplanationAs prokaryotic chromosome (DNA ) is circular ,it has only one origin (ori) of replicationwhich consists of two Y shaped replicationwhereas eukaryotic chromosome is large andlinear having many orgins each of which consistsof 2 replication forks6/15/201323SolvedMCQDNAREPLICATION
  24. 24. Which of the following forms of DNA can serve as a template forDNA polymerasea) Partially double stranded DNAb) Circular double stranded DNAc) Intact double stranded DNAd) Circular single stranded DNAAnswer a) Partially double stranded DNAExplanationDNA double helix gradually opens by DNA helicse by dissolving Hbonds between base pairs giving single strands which behaveas template for DNA polymerase6/15/201324SolvedMCQDNAREPLICATION
  25. 25. 6/15/2013SolvedMCQDNAREPLICATION25All input of energy is not required for which of the following steps ofreplication ?a) Separation of two strandab) Unwinding of DNA duplexc) Linking together of the individual deoxynucleotidesd) All of the above require energyAnswer d) All of the above require energyExplanationMost of the steps of DNA replication require energy which isgenerally provided by hydrolysis of energy rich comounds likeATP
  26. 26. 6/15/2013SolvedMCQDNAREPLICATION26Why should patients with Xeroderma pigmentosa avoid sunlighta) The UV wavelengths do irrepairable damage to DNAb) Sunlight inhibits any residual DNA repair activity of the cellc) These patients lack pigmentation to protect them from burningd) Sunlight inhibits DNA polymerasesAnswer a) The UV wavelengths do irrepairable damage toDNA
  27. 27. 6/15/2013SolvedMCQDNAREPLICATION27The exonuclease activity of the DNA polymerases functions toa) Remove the RNA primer sequencesb) Proofread the new DNA strand and remove inappropriatenucleotidesc) Maximize the fidelity of DNA replicationd) All of the aboveAnwer d) All of the aboveExplanationBy having exonuclease activity it can remove the nucleotides from theends of Okazaki fragments and also by moving backwards for proofreading
  28. 28. 6/15/2013SolvedMCQDNAREPLICATION28The last DNA to be replicated in the eukaryotic chromosome isa) Telomeres at the end of the chromosomesb) Heterochromatinc) Euchromatin in the arms of the chromosomed) Facultative heterochromatinAnswer b) heterochromatinExplanationAs heterochromatin is densely packed it is always late replicating
  29. 29. 6/15/2013DNAREPLICATION29

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