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In 1944, Oswald Avery , Colin MacLeod , and Maclyn McCarty were interested in exploring Griffith’s transforming principle further. They isolated the S strain from infected dead mice, heat-killed it, and inactivated various components of the S extract, conducting a systematic elimination study ( [link] ). They used enzymes that specifically degraded proteins, RNA, and DNA and mixed the S extract with each of these individual enzymes. Then, they tested each extract/enzyme combination’s resulting ability to transform the R strain, as observed by the diffuse growth of the S strain in culture media and confirmed visually by growth on plates. They found that when DNA was degraded, the resulting mixture was no longer able to transform the R strain bacteria, whereas no other enzymatic treatment was able to prevent transformation. This led them to conclude that DNA was the transforming principle. Despite their results, many scientists did not accept their conclusion, instead believing that there were protein contaminants within their extracts.

A diagram of Avery, MaLeod and McCarty’s experiment: Determining the identity of the hereditary material. Heat is used to kill S strain of S. pneumonia and capsule components are removed from solution. This produes a solution with DNA, RNA and proteins. This solution is then placed into 4 tubes. In the control, no enzymes are used so DNA, RNA, and proteins are all present. R cells are then added and S cells are found because transformation occurred. In the next experiment proteases degrade proteins in the sample, leaving DNA and RNA. R cells are added and S cells are found. Therefore, transformation occurred in the absence of proteins. In the next experiment ribonucleases degrade RNA in the sample, leaving DNA and proteins. R cells are added and S cells are found. Therefore, transformation occurred in the absence of RNA. In the final experiment deoxyribonucleases degrade DNA in the sample, leaving proteins and RNA. R cells are added and S cells are not found. Therefore, transformation does not occur without DNA. DNA is necessary for transformation.
Oswald Avery, Colin MacLeod, and Maclyn McCarty followed up on Griffith’s experiment and experimentally determined that the transforming principle was DNA.
  • How did Avery, MacLeod, and McCarty’s experiments show that DNA was the transforming principle first described by Griffith?

Hershey and chase’s proof of dna as genetic material

Alfred Hershey and Martha Chase performed their own experiments in 1952 and were able to provide confirmatory evidence that DNA , not protein, was the genetic material ( [link] ). A.D. Hershey, M. Chase. “Independent Functions of Viral Protein and Nucleic Acid in Growth of Bacteriophage.” Journal of General Physiology 36 no. 1 (1952):39–56. Hershey and Chase were studying a bacteriophage , a virus that infects bacteria. Viruses typically have a simple structure: a protein coat, called the capsid, and a nucleic acid core that contains the genetic material, either DNA or RNA (see Viruses ). The particular bacteriophage they were studying was the T2 bacteriophage, which infects E. coli cells. As we now know today, T2 attaches to the surface of the bacterial cell and then it injects its nucleic acids inside the cell. The phage DNA makes multiple copies of itself using the host machinery, and eventually the host cell bursts, releasing a large number of bacteriophages.

Hershey and Chase labeled the protein coat in one batch of phage using radioactive sulfur, 35 S, because sulfur is found in the amino acids methionine and cysteine but not in nucleic acids. They labeled the DNA in another batch using radioactive phosphorus, 32 P, because phosphorus is found in DNA and RNA but not typically in protein.

Each batch of phage was allowed to infect the cells separately. After infection, Hershey and Chase put each phage bacterial suspension in a blender, which detached the phage coats from the host cell, and spun down the resulting suspension in a centrifuge. The heavier bacterial cells settled down and formed a pellet, whereas the lighter phage particles stayed in the supernatant. In the tube with the protein labeled, the radioactivity remained only in the supernatant. In the tube with the DNA labeled, the radioactivity was detected only in the bacterial cells. Hershey and Chase concluded that it was the phage DNA that was injected into the cell that carried the information to produce more phage particles, thus proving that DNA, not proteins, was the source of the genetic material. As a result of their work, the scientific community more broadly accepted DNA as the molecule responsible for heredity.

Questions & Answers

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Ogenrwot
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They formed in two ways first when one sperm and one egg are splited by mitosis or two sperm and two eggs join together
Oluwatobi
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Josephine Reply
Genetics is the study of heredity
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the study of living organisms and their interactions with one another and their environments
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the study of living organisms and their interactions with one another and their environment.
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list any five characteristics of the blood cells
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advantage of electronic microscope is easily and clearly while disadvantage is dangerous because its electronic. advantage of light microscope is savely and naturally by sun while disadvantage is not easily,means its not sharp and not clear
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cell theory state that every organisms composed of one or more cell,cell is the basic unit of life
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is like gone fail us
DENG
cells is the basic structure and functions of all living things
Ramadan
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ISCONT Reply
is organisms that are similar into groups called tara
Yamosa
in what situation (s) would be the use of a scanning electron microscope be ideal and why?
Kenna Reply
A scanning electron microscope (SEM) is ideal for situations requiring high-resolution imaging of surfaces. It is commonly used in materials science, biology, and geology to examine the topography and composition of samples at a nanoscale level. SEM is particularly useful for studying fine details,
Hilary
cell is the building block of life.
Condoleezza Reply
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Diversity of living thing
ISCONT
what is cell division
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Cell division is the process by which a single cell divides into two or more daughter cells. It is a fundamental process in all living organisms and is essential for growth, development, and reproduction. Cell division can occur through either mitosis or meiosis.
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life is defined as any system capable of performing functions such as eating, metabolizing,excreting,breathing,moving,Growing,reproducing,and responding to external stimuli.
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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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