Chapter 12, Problem 1TYU

When Griffith injected mice with a combination of live rough-strain and heat-killed smooth-strain pneumococci, he discovered that (a) the mice were unharmed (b) the dead mice contained living rough-strain bacteria (c) the dead mice contained living smooth-strain bacteria (d) DNA had been transferred from the smooth-strain bacteria to the mice (e) DNA had been transferred from the rough-strain bacteria to the smooth-strain bacteria

Summary Introduction

Introduction: Transformation is a process by which bacterial cells uptake naked DNA from its environment. It is a process of horizontal gene transfer by which the genome of the bacteria gets altered due to the uptake of foreign gene. The experimental demonstration of transformation was done by bacteriologist Frederick Griffith.

Answer to Problem 1TYU

Correct answer: When Griffith injected mice with a combination of live rough-strain and heat-killed smooth-strain of pneumococci, he discovered that the dead mice contained living smooth-strain bacteria.

Hence, the correct answer is option (c).

Explanation of Solution

Reason for the correct answer:

Frederick Griffith’s principle of transformation states that a harmless strain of bacteria can be made virulent by exposing it to heat-killed bacterial strains. Griffith started the experiment to find out a vaccine against pneumonia. He selected two strains of pneumococci:

1. 1. Smooth (S) strain that has a polysaccharide coat which makes them virulent strain.
2. 2.  Rough (R) strain that lacks capsule which makes them non-virulent strain.

The mice died when they were injected with the virulent S form of pneumococci. Pneumonia was not caused when the non-virulent R strain injection was given to the mice. Again mice survived when they were injected with the heat-killed S form. Then a mixture of heat-killed S and R strain was injected to the mice. The mice died by causing pneumonia. By all these observations, it concluded that virulent genes from the heat-killed S strain that causes pneumonia in mice got transformed to the non-virulent living R strain. Hence, he concluded that the transformation of the virulent gene made the harmless bacteria dangerous.

Option (c) is given as “the dead mice contained living smooth-strain bacteria”.

The mice died and live smooth cells were found in the dead mice.

Hence, the correct answer is option (c).

Reasons for incorrect answers:

Option (a) is given as, “the mice were unharmed”.

In the Griffith experiment, the mice died when injected with a combination of live rough cells and heat-killed smooth cells.

Hence, option (a) is incorrect.

Option (b) is given as, “the dead mice contained living rough-strain bacteria”.

The dead mice contained living smooth cells but not living rough cells.

Hence, option (b) is incorrect.

Option (d) is given as, “DNA had been transferred from the smooth-strain bacteria to the mice”.

The DNA from bacteria does not usually transfer to the host in which it lives. The DNA of mice was unaffected due to the presence of bacteria.

Hence, option (d) is incorrect.

Option (e) is given as, “DNA had been transferred from the rough-strain bacteria to the smooth-strain bacteria”.

The DNA was transferred from the smooth-strain bacteria to rough-strain bacteria.

Hence, option (e) is incorrect.

Hence, the options (a), (b), (d), and (e) are incorrect.

Conclusion

The DNA was transferred from the smooth-strain bacteria to rough-strain bacteria. Thus, the mice die and live smooth cells were found in the dead mice.