Avery Experiment: DNA as the Transforming Principle

Summary

This video explains the groundbreaking Avery, MacLeod, and McCarty experiment from 1944, which helped prove that DNA, not proteins, is the molecule responsible for heredity. Building on Griffith's earlier work, the experiment identified DNA as the 'transforming principle' by testing the effects of enzymes that destroyed proteins, RNA, and DNA in bacteria. The results demonstrated that only when DNA was destroyed did bacterial transformation stop, establishing DNA's role in genetic inheritance. Despite skepticism at the time, this experiment paved the way for later confirmation by the Hershey-Chase experiment in 1952.

Takeaways

• 😀 The Avery, MacLeod, and McCarty experiment in 1944 helped prove that nucleic acids, specifically DNA, are the molecules of heredity.
• 😀 Before the 1940s, scientists believed proteins were responsible for inheritance, not nucleic acids.
• 😀 DNA is a long, double-stranded molecule composed of just four bases (adenine, thymine, cytosine, and guanine).
• 😀 Proteins are structurally complex and consist of combinations of 20 amino acids, which differ greatly from the simplicity of DNA.
• 😀 The Griffith experiment laid the groundwork for the Avery team by showing that a 'transforming principle' could change bacterial properties.
• 😀 Avery and his team identified the transforming principle as DNA through a series of tests eliminating other biomolecules like carbohydrates and lipids.
• 😀 The transforming material was found to be soluble in salt solutions, stable at low temperatures, and resistant to heat up to 65°C.
• 😀 Avery's experiment involved testing proteins, RNA, and DNA by removing them with specific enzymes to identify which was responsible for transformation.
• 😀 The addition of DNases, enzymes that destroy DNA, led to the survival of mice, confirming that DNA was the transforming principle.
• 😀 Despite Avery's evidence, the scientific community remained skeptical, and DNA wasn't fully accepted as the genetic material until the 1952 Hershey-Chase experiment.

Q & A

What was the primary objective of the Avery, MacLeod, and McCarty experiment?

The primary objective was to identify the molecule responsible for heredity, ultimately proving that DNA, not proteins, was the transforming principle in bacterial transformation.

What did the Griffith experiment contribute to the Avery, MacLeod, and McCarty experiment?

The Griffith experiment provided the foundational concept of bacterial transformation, where a 'transforming principle' from dead bacteria could change the properties of live bacteria, setting the stage for Avery, MacLeod, and McCarty's work.

How did Avery, MacLeod, and McCarty isolate the 'transforming principle'?

They isolated the transforming principle from the S-strain bacteria and subjected it to a series of tests, including solubility, stability, and UV absorption spectra, to determine its identity.

Why was the UV absorption spectrum important in the experiment?

The UV absorption spectrum, with peaks matching DNA, provided evidence that the transforming principle shared properties with DNA, helping to rule out other biomolecules as candidates.

What role did enzymes play in the experiment?

Enzymes were used to destroy specific biomolecules (proteins, RNA, and DNA) in order to determine which one was responsible for bacterial transformation. The results showed that DNA was essential for transformation.

What happened when proteins and RNA were destroyed during the experiment?

Even after proteins and RNA were destroyed using specific enzymes, transformation still occurred, indicating that neither proteins nor RNA were responsible for the transforming principle.

What was the significance of using DNase in the experiment?

When DNase (an enzyme that destroys DNA) was introduced, transformation did not occur, suggesting that DNA was the key component responsible for the bacterial transformation process.

Why were carbohydrates and lipids eliminated as potential candidates for the transforming principle?

Carbohydrates and lipids were eliminated because their characteristics, such as their UV absorption and nitrogen to phosphorus ratios, did not match those of the transforming material, which was identified as DNA.

How did the scientific community initially react to Avery's findings?

The scientific community was initially skeptical of Avery's findings. Even Avery himself was cautious, fearing that contamination of DNA samples might have influenced the results.

What event helped confirm Avery's conclusions about DNA as the molecule of heredity?

The Hershey and Chase experiment in 1952, which provided further evidence supporting DNA as the hereditary material, helped to solidify Avery's conclusions and the acceptance of DNA as the transforming principle.