101. Which statement best describes the scientific transition that led to the search for genetic material?
ⓐ. Scientists moved from studying metabolism to studying only ecology.
ⓑ. Scientists shifted from cell division to plant taxonomy alone.
ⓒ. Scientists abandoned chromosome studies and focused only on proteins.
ⓓ. Scientists moved from chromosome-level inheritance ideas to molecular evidence about the hereditary substance.
Correct Answer: Scientists moved from chromosome-level inheritance ideas to molecular evidence about the hereditary substance.
Explanation: Classical genetics established that heredity was associated with chromosomes, but it did not reveal the exact molecular basis of inheritance. Later work aimed to identify the specific chemical that carried genetic information. This marked a transition from cytological and genetic observations to molecular investigation. That shift laid the foundation for modern molecular biology.
102. Which question was central to the search for genetic material?
ⓐ. How many ribosomes are present in a bacterial cell?
ⓑ. Which biochemical molecule in the chromosome carries hereditary information?
ⓒ. Why do all cells have exactly the same genome size?
ⓓ. Which lipid forms the strongest plasma membrane?
Correct Answer: Which biochemical molecule in the chromosome carries hereditary information?
Explanation: Once chromosomes were recognized as the carriers of inheritance, the next major problem was to identify the exact hereditary molecule within them. Scientists had to distinguish among candidate substances such as DNA, RNA, and proteins. This question drove several landmark experiments in molecular biology. Its answer transformed genetics into a chemical science.
103. Which statement is most accurate about the stage before direct molecular proof of genetic material was obtained?
ⓐ. Scientists had already proved that RNA was the hereditary material in all cells.
ⓑ. Scientists knew chromosomes were involved in inheritance, but the exact biochemical agent was still uncertain.
ⓒ. Scientists had rejected chromosomes completely as carriers of heredity.
ⓓ. Scientists had already established the full mechanism of translation.
Correct Answer: Scientists knew chromosomes were involved in inheritance, but the exact biochemical agent was still uncertain.
Explanation: The role of chromosomes in inheritance was accepted before the molecular identity of the genetic material was settled. What remained unresolved was whether DNA, protein, or some other molecule was the true hereditary substance. This uncertainty created the need for experimental proof. The next major studies in the chapter address exactly that issue.
104. In Griffith’s experiment with Streptococcus pneumoniae, the smooth (S) strain was described as
ⓐ. virulent and capsulated
ⓑ. non-virulent and without capsule
ⓒ. heat-resistant and non-infective
ⓓ. unable to survive in the host body
Correct Answer: virulent and capsulated
Explanation: The smooth strain of Streptococcus pneumoniae formed smooth colonies because it possessed a polysaccharide capsule. This capsule made the bacteria virulent and able to cause disease in mice. In contrast, the rough strain lacked the capsule and was non-virulent. The difference between these two strains was central to Griffith’s observations.
105. Which property correctly characterizes the rough (R) strain used by Griffith?
ⓐ. It was virulent because it had a protective capsule.
ⓑ. It was non-virulent because it lacked a capsule.
ⓒ. It could kill mice only after being heat-killed.
ⓓ. It produced smooth colonies due to capsule formation.
Correct Answer: It was non-virulent because it lacked a capsule.
Explanation: The rough strain lacked the polysaccharide capsule that protects the bacteria from the host’s defenses. Because of this, it was non-virulent and did not cause disease in mice under normal conditions. Its rough colony appearance reflected the absence of the capsule. This strain became important because it was later transformed into a virulent form.
106. Which result was obtained when live S strain bacteria were injected into mice in Griffith’s experiment?
ⓐ. The mice remained healthy and live R bacteria were recovered.
ⓑ. The mice remained healthy and no bacteria were recovered.
ⓒ. The mice died, showing that the strain was virulent.
ⓓ. The mice died only if the strain had first been heat-killed.
Correct Answer: The mice died, showing that the strain was virulent.
Explanation: Live S bacteria were pathogenic and could cause fatal infection in mice. Their capsule protected them from destruction in the host body, allowing disease to develop. This result established the virulence of the S strain clearly. It served as one of the control observations in Griffith’s study.
107. What happened when live R strain bacteria were injected into mice?
ⓐ. The mice died and live S bacteria were recovered.
ⓑ. The mice died and heat-killed S bacteria were recovered.
ⓒ. The mice showed delayed death due to capsule formation.
ⓓ. The mice survived because the R strain was non-virulent.
Correct Answer: The mice survived because the R strain was non-virulent.
Explanation: Live R bacteria did not kill mice because they lacked the protective capsule needed for virulence. As a result, the host defenses could eliminate them effectively. This made the R strain a useful comparison against the virulent S strain. The survival of mice in this case helped Griffith interpret the mixed-treatment result later.
108. In Griffith’s experiment, injection of heat-killed S strain bacteria into mice resulted in
ⓐ. death of mice with recovery of live S cells
ⓑ. conversion of the mice into carriers of R strain
ⓒ. death of mice due to toxic capsule alone
ⓓ. survival of mice because the bacteria were no longer virulent
Correct Answer: survival of mice because the bacteria were no longer virulent
Explanation: Heating killed the S bacteria, so they could no longer multiply or establish infection in the mice. Even though these bacteria had once been virulent, they lost their disease-causing ability after being killed. Therefore, mice injected with heat-killed S cells survived. This control result was essential for interpreting the later transformation experiment.
109. Which combination killed mice in Griffith’s experiment and led to the recovery of live S bacteria?
ⓐ. heat-killed S strain mixed with live R strain
ⓑ. live R strain mixed with heat-killed R strain
ⓒ. heat-killed R strain mixed with live S strain
ⓓ. heat-killed S strain alone
Correct Answer: heat-killed S strain mixed with live R strain
Explanation: This was the key result in Griffith’s experiment. The mixture of heat-killed S bacteria and live R bacteria unexpectedly killed the mice, and live S bacteria were later recovered from them. This showed that some factor from the dead S cells had transformed the live R cells into virulent S-type cells. The result could not be explained by the original properties of the R strain alone.
110. Recovery of live S bacteria from mice injected with heat-killed S and live R cells indicated that
ⓐ. heat alone converted S bacteria into R bacteria
ⓑ. R bacteria had destroyed all S bacterial material
ⓒ. only proteins can transmit hereditary change
ⓓ. some heritable factor from dead S cells transformed live R cells
Correct Answer: some heritable factor from dead S cells transformed live R cells
Explanation: The appearance of live S bacteria after injection of the mixture showed that the live R cells had undergone a stable change. They had acquired the ability to form capsules and become virulent. Griffith concluded that a transforming principle from the dead S bacteria had brought about this change. At that stage, he did not identify the chemical nature of the transforming substance.
111. The main conclusion drawn by Griffith from his experiment was the existence of
ⓐ. semiconservative DNA replication
ⓑ. RNA as the universal genetic material
ⓒ. a transforming principle capable of changing bacterial type
ⓓ. protein synthesis on ribosomes
Correct Answer: a transforming principle capable of changing bacterial type
Explanation: Griffith’s experiment provided evidence that one bacterial form could be converted into another by some transferable factor. He called this the transforming principle. The experiment did not reveal whether this factor was DNA, RNA, protein, or some other substance. Its importance lay in demonstrating that hereditary properties could be transferred between cells.
112. Which statement about Griffith’s experiment is correct?
ⓐ. It demonstrated bacterial transformation but did not identify DNA chemically.
ⓑ. It proved directly that DNA enters bacterial cells from bacteriophages.
ⓒ. It showed that only heat-killed R cells can become virulent.
ⓓ. It established the detailed structure of the DNA double helix.
Correct Answer: It demonstrated bacterial transformation but did not identify DNA chemically.
Explanation: Griffith discovered the phenomenon of transformation, not the exact chemical identity of the transforming agent. His work showed that non-virulent bacteria could acquire virulence from material derived from dead virulent bacteria. The identification of DNA as the transforming substance came later through the work of Avery, MacLeod, and McCarty. This distinction is one of the most important historical points in the chapter.
113. A student claims that Griffith proved DNA to be the genetic material. Which correction is most accurate?
ⓐ. Griffith showed that proteins are the transforming material in bacteria.
ⓑ. Griffith showed transformation, but the chemical identity of the transforming substance was determined later.
ⓒ. Griffith established that RNA alone causes virulence in pneumococcus.
ⓓ. Griffith discovered the double-helix model while working on bacterial strains.
Correct Answer: Griffith showed transformation, but the chemical identity of the transforming substance was determined later.
Explanation: Griffith’s experiment was a landmark because it revealed that a heritable change could pass from dead virulent cells to live non-virulent cells. However, he did not isolate the molecule responsible for that change. Later experiments identified DNA as the transforming substance. So Griffith’s contribution was the discovery of transformation, not the final chemical proof of DNA as genetic material.
114. In the Avery, MacLeod, and McCarty experiment, the material used to test the transforming principle was obtained from
ⓐ. live R strain bacteria
ⓑ. heat-killed S strain bacteria
ⓒ. living mouse blood cells
ⓓ. actively growing bacteriophages
Correct Answer: heat-killed S strain bacteria
Explanation: Avery, MacLeod, and McCarty worked with extracts prepared from heat-killed virulent S bacteria. Griffith had already shown that some component from dead S cells could transform live R cells. The later researchers separated and tested different biochemical fractions from this source. Their aim was to identify the exact substance responsible for transformation.
115. Which treatment prevented transformation in the Avery, MacLeod, and McCarty experiment?
ⓐ. treatment with protease
ⓑ. treatment with RNase
ⓒ. treatment with lipase
ⓓ. treatment with DNase
Correct Answer: treatment with DNase
Explanation: DNase specifically destroys DNA. When the transforming preparation was treated with DNase, it lost its ability to convert R bacteria into S bacteria. This showed that DNA was essential for the transformation process. Protease and RNase did not abolish transformation in the same way.
116. If a transforming extract from virulent pneumococci is treated with protease and still transforms R cells, the most direct inference is that
ⓐ. protein is not the transforming substance
ⓑ. DNA has been completely destroyed
ⓒ. RNA is absent from all bacterial cells
ⓓ. the R strain has become heat-killed
Correct Answer: protein is not the transforming substance
Explanation: Protease breaks down proteins. If transformation still occurs after protease treatment, protein is unlikely to be the material carrying the transforming activity. This was one of the key logical steps in the Avery, MacLeod, and McCarty experiment. It helped eliminate protein as the transforming principle.
117. In the Avery, MacLeod, and McCarty study, RNase treatment did not stop transformation. This finding showed that
ⓐ. RNA is the universal genetic material
ⓑ. RNA makes bacterial capsules directly
ⓒ. RNA was not the transforming substance in that experiment
ⓓ. RNA protects DNA from digestion by DNase
Correct Answer: RNA was not the transforming substance in that experiment
Explanation: RNase degrades RNA. Since transformation still occurred after RNase treatment, RNA could not be the critical transforming material in that system. This result narrowed the possibilities further. By comparing the effects of different enzymes, the investigators identified the most likely chemical basis of transformation.
118. Which statement best summarizes the conclusion of Avery, MacLeod, and McCarty?
ⓐ. Protein entered bacterial cells and directed capsule formation.
ⓑ. DNA was identified as the transforming substance.
ⓒ. Heat itself converted R strain into S strain.
ⓓ. RNA and DNA acted equally as transforming agents.
Correct Answer: DNA was identified as the transforming substance.
Explanation: Their experiments provided strong evidence that DNA carried the transforming activity. Only when DNA was destroyed did transformation fail. This meant that DNA, rather than protein or RNA, was responsible for transferring the hereditary change. The work gave direct biochemical support to DNA as genetic material.
119. Which set of enzymes was used mainly to test whether proteins or RNA were responsible for transformation?
ⓐ. DNase and ligase
ⓑ. helicase and polymerase
ⓒ. amylase and lipase
ⓓ. protease and RNase
Correct Answer: protease and RNase
Explanation: Protease was used to destroy proteins, and RNase was used to destroy RNA. The transforming activity remained even after these treatments, showing that neither protein nor RNA was the responsible substance in that experiment. This stepwise elimination was crucial to the logic of the study. It made the effect of DNase especially meaningful.
120. A purified extract from heat-killed S bacteria is divided into three parts. One is treated with protease, one with RNase, and one with DNase. Transformation fails only in the third part. Which conclusion is most appropriate?
ⓐ. Proteins are necessary for transformation.
ⓑ. RNA is the hereditary material in pneumococcus.
ⓒ. DNA is required for the transforming activity.
ⓓ. The capsule itself acts independently of all biomolecules.
Correct Answer: DNA is required for the transforming activity.
Explanation: The only treatment that abolished transformation was DNase, which specifically destroys DNA. Since the other enzyme treatments did not stop transformation, DNA must be the critical component needed for the activity. This is the core result of the Avery, MacLeod, and McCarty experiment. It provides direct evidence linking DNA with hereditary change.
