Problem 11 - Cell and Molecular Biology: Concepts and Experiments

Question

Suppose you were a scientist living in the 1890 s and were studying a disease of tobacco crops that stunted the growth of the plants and mottled their leaves. You find that the sap from a diseased plant, when added to a healthy plant, is capable of transmitting the disease to that plant. You examine the sap in the best light microscopes of the period and see no evidence of bacteria. You force the sap through filters whose pores are so small that they retard the passage of the smallest known bacteria, yet the fluid that passes through the filters is still able to transmit the disease. Like Dimitri Ivanovsky, who conducted these experiments more than a hundred years ago, you would probably conclude that the infectious agent was an unknown type of unusually small bacterium. What kinds of experiments might you perform today to test this hypothesis?

Step-by-Step Solution

Answer

Use techniques like electron microscopy and PCR to identify viral particles or genetic material.

Step 1: Evaluate Filtered Sap

Begin by re-evaluating the properties of the sap that has passed through the fine filter. Modern technology can allow you to investigate the material using techniques such as electron microscopy, which can potentially reveal particulate matter smaller than the resolution limits of light microscopes.

Step 2: Use Electron Microscopy

Employ electron microscopy to observe any sub-microscopic entities. This approach allows for observing structures such as viruses, which are capable of passing through bacterial filters due to their smaller size.

Step 3: Test for Viral Genetic Material

Perform tests such as PCR (Polymerase Chain Reaction) to detect the presence of viral RNA or DNA in the sap. This would help identify the pathogen as a virus if genetic material specific to known viruses is found.

Step 4: Conduct Koch's Postulates

Isolate the suspected pathogen and conduct Koch's postulates using modern methodology. This involves using the isolated pathogen to infect a healthy plant and observing if it causes the same disease, thus confirming pathogenicity.

Key Concepts

Electron Microscopy

Electron microscopy is a powerful technique that allows scientists to see objects at a much smaller scale than traditional light microscopes. This type of microscopy uses a beam of electrons instead of light to achieve high magnification and resolution.
Unlike light waves, electrons have shorter wavelengths, making it possible to visualize extremely small entities, such as viruses, which are much tinier than bacteria. This makes electron microscopes crucial tools in the study of viruses and other microscopic agents.

• They can reveal the shape, size, and even some structures of viruses.
• Scanning Electron Microscopes (SEMs) provide 3D images of the virus surface.
• Transmission Electron Microscopes (TEMs) can show fine details of internal structures.

Utilizing electron microscopy could help confirm the presence of a virus in a scenario where bacteria are eliminated as the cause of a disease.

Viral Genetic Material

Viruses, unlike living cells, possess a very simple structure and depend on a small amount of genetic material to replicate. This material can be either DNA or RNA, which are the essential components for understanding how viruses replicate and cause disease.
Modern technology allows detection of this genetic material using techniques such as Polymerase Chain Reaction (PCR), which is a method used to amplify small quantities of DNA or RNA sequences.

• Detecting viral genetic material confirms that a virus is present in the sample.
• It helps in identifying the type of virus by comparing the sequences to known databases.
• PCR is a precise technique that can target specific genetic markers of the virus, making it highly reliable.

Detecting the genetic material can prove crucial in identifying unknown viral infections, even when their physical or morphological characteristics are hard to observe.

Koch's Postulates

Koch's Postulates are a series of criteria set to establish a causative relationship between a microbe and a disease. They were developed in the late 19th century but continue to be foundational in microbiology studies today.
The steps involve isolating a microbe from a diseased organism, cultivating it in pure form, and using it to induce the disease in a healthy organism. Here is how they work:

• The pathogen must be found in all individuals suffering from the disease, and not in healthy ones.
• The pathogen must be isolated and grown in pure culture.
• When introduced to a healthy host, the pathogen should cause the disease.
• The pathogen must be re-isolated from the newly diseased host and identified as identical to the original.

Although these postulates were initially intended for bacteria, they have been adapted to study viruses by using cell cultures and genetic testing.

Polymerase Chain Reaction (PCR)

Polymerase Chain Reaction (PCR) is a revolutionary lab technique used to amplify small segments of DNA and is instrumental in virus detection and diagnosis. It works by producing millions of copies of a specific DNA sequence from a small initial sample.
The PCR process involves three main steps repeated in cycles:

• Denaturation: This step involves heating the DNA to separate its two strands.
• Annealing: Cooling allows primers to attach to the specific DNA sequence at the starting point for replication.
• Extension: The DNA polymerase enzyme adds new nucleotide bases, extending the DNA strands and making copies.

PCR is quick and highly accurate, allowing for easy detection even if only a small amount of viral DNA/RNA is present, making it invaluable for diagnosing viral infections and conducting genetic research.