Problem 10 - AP Biology Premium Prep 2021

Question

Griffith was a researcher who coined the term transformation when he noticed that incubating nonpathogenic bacteria with heat-killed pathogenic bacteria produced bacteria that ultimately became pathogenic, or deadly, in mice. What caused the transformation in his experiment? (A) DNA from the nonpathogenic bacteria revitalized the heat-killed pathogenic bacteria. (B) Protein from the pathogenic bacteria was taken up by the nonpathogenic bacteria. (C) DNA from the pathogenic bacteria was taken up by the nonpathogenic bacteria. (D) DNA in the nonpathogenic bacteria turned into pathogenic genes in the absence of pathogenic bacteria.

Step-by-Step Solution

Answer

The correct answer is (C): DNA from the pathogenic bacteria was taken up by the nonpathogenic bacteria, which caused the transformation observed in Griffith's experiment.

Step 1: Read the question carefully

Make sure you understand the given situation and the possible answers. Griffith's experiment involved nonpathogenic bacteria and heat-killed pathogenic bacteria, and bacterial transformation occurred as a result. We have to find out the cause of this transformation from the four given options.

Step 2: Review Griffith's experiment

In Griffith's transformation experiment, he worked with two strains of bacteria: a nonpathogenic strain (R) and a pathogenic strain (S). He found that when heat-killed pathogenic bacteria were mixed with live nonpathogenic bacteria, the nonpathogenic bacteria became pathogenic, causing the death of mice. This transformation was observed to be a result of a transfer of genetic material.

Step 3: Analyze each possible answer

(A): DNA from nonpathogenic bacteria revitalizing the heat-killed pathogenic bacteria is not a valid explanation since heat-killed bacteria cannot be revived by this method. (B): Protein from the pathogenic bacteria being taken up by the nonpathogenic bacteria is not the correct explanation. While proteins can be taken up by the bacteria, they will not lead to a permanent change in the nonpathogenic bacteria's genetic material, which is essential to cause the transformation. (C): DNA from the pathogenic bacteria being taken up by the nonpathogenic bacteria is the most plausible explanation for this transformation. The uptake of DNA from pathogenic bacteria could cause the nonpathogenic bacteria to acquire new genetic material, including the genes responsible for the pathogenic properties, causing the transformation. (D): DNA in the nonpathogenic bacteria turning into pathogenic genes in the absence of pathogenic bacteria is not a plausible explanation since there is no mechanism for such a conversion to occur without a transfer of genetic material.

Step 4: Choose the correct answer

Based on our analysis, the correct answer is (C): DNA from the pathogenic bacteria was taken up by the nonpathogenic bacteria, which caused the transformation observed in Griffith's experiment.

Key Concepts

Bacterial Transformation

The phenomenon of bacterial transformation refers to the process by which bacteria can acquire new genetic information from their environment. This process is critical in understanding the evolution and adaptation capabilities of bacteria, as it demonstrates their ability to pick up and integrate DNA fragments. The transformative nature of bacteria was first elucidated by Frederick Griffith in 1928.

Griffith's classical transformation experiment involved two strains of pneumococcus bacteria: the nonpathogenic Rough (R) strain and the pathogenic Smooth (S) strain. When Griffith injected mice with the heat-killed S strain alongside the live R strain, he observed that some R bacteria had transformed, gaining the pathogenicity from the S strain. This suggested that a heritable substance from the dead S cells imparted deadly characteristics to the R strain, forever changing our understanding of disease progression and bacteria biology.

Genetic Material Transfer

Genetic material transfer is pivotal for diversity and evolution in many biological organisms, but in bacteria, one of the primary methods of this transfer is through transformation. This occurs when a bacterium takes up DNA from its surroundings, often from lysed cells, and incorporates it into its genome. This can confer new abilities such as antibiotic resistance or, in Griffith's study, the ability to cause disease.

Significantly, the genetic material in question here is not just any type of molecule but specifically deoxyribonucleic acid, or DNA. This was not initially known, as proteins were once thought to be the carriers of genetic information. However, later experiments by Oswald Avery and his colleagues confirmed that DNA was the 'transforming principle' that Griffith had observed. Now, when considering genetic material transfer, scientists focus on DNA as the primary molecule of inheritance and transformation in bacteria.

Pathogenic Bacteria

Pathogenic bacteria are those that can cause disease in hosts such as humans, animals, and plants. They often possess certain genes that give them the ability to invade hosts, overcome host defenses, and cause damage. In Griffith's transformation experiment, it was the pathogenic strain (S strain) that held the genes responsible for causing disease in mice.

These bacteria are equipped with tools like toxins, adhesion factors, and evasion strategies that enable them to colonize and damage their host organisms. When nonpathogenic bacteria acquire these genes through transformation, as illustrated in Griffith's experiment, they too can become pathogenic. The acquisition of such genes is a considerable concern in medical and agricultural industries as it underscores the role of horizontal gene transfer in the spread of disease-causing capabilities among bacteria.

DNA Uptake in Bacteria

DNA uptake in bacteria is part of the transformation process and is a form of horizontal gene transfer. It can occur through various methods, but during transformation, it involves the bacterial cell wall becoming permeable to allow DNA from the environment to enter the cell. This foreign DNA, once inside, can recombine with the bacterium's own DNA, leading to genetic changes.

Important to note from Griffith's experiment is that not all bacteria can naturally take up DNA. This capability depends on the species of bacteria and whether they have a state called 'competence'. Some bacteria can become competent under certain environmental conditions, but for many laboratory experiments, bacteria need to be artificially induced into a competent state. The DNA uptake in bacteria after overcoming these barriers demonstrates a remarkable survivability trait and serves as a mechanism for adaptation and evolution.