Welcome back to bogobiology! In this video, we're going to discuss how the Avery Mcleod
and Mccarty experiment help to prove that nucleic acids are the molecules of heredity.
Today we know that dna is the molecule that transfers genetic information
from parent to offspring. However for many years scientists believed that proteins were responsible .
Avery, Mcleod and McCarty's classic 1944 experiment helped to prove that nucleic
acids are the molecules of heredity and we'll be going over how they did it.
If you'd like some guided notes to help you, I've left a link to them in the video description.
Okay here we go! There are four major groups of biomolecules; carbohydrates,
lipids, proteins, and nucleic acids. Scientists in the 1940s were unsure which was responsible
for inheritance, but they believed that the top two contenders were proteins and nucleic acids.
Nucleic acids and proteins are both biomolecules but they're quite different from one another.
DNA is a long double-stranded molecule composed of just four bases; adenine, thymine, cytosine
and guanine. RNA, another nucleic acid, is also composed of just four bases although it uses
uracil instead of thymine. By contrast proteins are incredibly structurally complex and contain
combinations of 20 different building blocks called amino acids. DNA is almost always the
same shape, whereas proteins have an endless variety of structure and size. Protein structure
is always closely tied to their function and changing the structure often disables the protein.
At the time researchers believed that DNA was far far too simple to be responsible for a complex
process like inheritance. The Griffith experiment laid the groundwork for Avery, Mcleod and Mccarty.
This experiment proved that some type of critical material could be transferred
between bacteria and change its properties, a process now called bacterial transformation.
Griffith called this mysterious substance that passed between deadly s strain pneumococcus
bacteria and innocuous are strained bacteria the "transforming principle ".
Mice who received r-strain bacteria that had been transformed by deadly s strain bacteria
all died. However, scientists were still unsure if this transformation was caused
by carbohydrates, lipids, proteins, RNA or DNA coming out of the s strain.
The Avery Mcleod Mccarty experiment expanded upon Griffith's ideas and eventually identified
nucleic acids and not proteins as the molecule of inheritance. The Avery experiment isolated the
mysterious material and then analyzed it using an extensive series of tests.
They examined its solubility stability and its contents very carefully and compared them to
known samples of biological molecules. From this series of tests they concluded the following:
The transforming material was soluble in salt solutions, it could be stored at low
temperatures for long periods and withstand 30 minutes of heating at 65 degrees celsius,
it was disabled if exposed to a ph below five. The transforming principle also had ultraviolet
absorption curves with a maximum of 2600 and a minimum of 2350 which matched DNA. It also had
a nitrogen to phosphorus ratio of about 1.67 which matched dna and effectively eliminated
carbohydrates and lipids. These biomolecules can sometimes contain phosphorus and nitrogen but
it's rare. Finally it precipitated out of solution in alcohol which also eliminated carbohydrates.
While this information was extremely useful several of the above characteristics could
apply to a number of different biomolecules. Proteins for instance are notoriously
particular about the temperature and pH of their environment, so Avery and his
colleagues needed to explore further in order to figure out what the transforming principle was.
The next phase of the experiment is by far the most famous and it has come to be known as the
Avery experiment. Avery Mcleod and McCarty had previously eliminated carbohydrates and lipids
leaving proteins, RNA and DNA as the remaining contenders. One of these biomolecules was the
component responsible for bacterial transformation and therefore for
heredity. The researchers disabled each of these components in s-strain bacteria,
mixed it with innocuous r-strain bacteria that was primed for transformation, incubated the
mixture, and then injected the combination into mice. If the mouse still died despite
removing a component they would know that the biomolecule was not the transformation principle.
As Avery noted in the paper, carbohydrates and lipids were not directly tested in the second
part of the experiment; they were removed via process of elimination. The researchers added
special enzymes each meant to destroy a specific component to several separate vials of bacteria.
The first group was the control which had no component that was destroyed.
As expected transformation still took place the r-strain bacteria absorbed something from the
s strain bacteria and the mice died. In the second group they also tried protein-destroying enzymes
called proteases, specifically trypsin and chemotrypsin. They also tried an enzyme called
pepsin but discovered that the low pH needed for pepsin to work destroyed the transformation
material completely. In the presence of trypsin and chemotrypsin the mice still died, meaning
that transformation had still taken place and proteins did not appear to be responsible.
They also tried rna-destroying enzymes called rnases each with the same result. The mice died
meaning that transformation had still taken place even in the absence of proteins or RNA. Avery
Macleod and Mccarty finally tried dnases enzymes that destroyed DNA, in one instance the mice
survived. Adding phosphatases and esterases to the mixture did not disrupt the transformation process
but adding dna de-polymerases did. When Avery Macleod and Mccarty added depolymerases to the mixture
the mice survived. This told them that DNA was likely to be the transforming principle. The Avery
Macleod and Mccarty experiment further supported the theory that DNA was the transforming principle .
However despite this evidence, the scientific community was skeptical of Avery's results.
Even oswald avery himself was cautious because he believed it was possible that
trace amounts of the transforming principle could have contaminated their DNA samples,
and that DNA might not be wholly responsible. DNA was not widely
accepted as the transforming principle until Hershey and Chase's experiment later in 1952.
That pretty much wraps up our discussion of the Avery experiment, feel free to check out
the study guide I linked in the video description, or the list of references that I consulted when
making this video. Thanks again for watching and please remember to like, comment, and subscribe!