Thus, Griffith concluded that something had passed from the heat-killed S strain to the R strain, transforming the R strain into S strain in the process. They isolated the S strain from the dead mice and isolated the proteins and nucleic acids RNA and DNA as these were possible candidates for the molecule of heredity. They used enzymes that specifically degraded each component and then used each mixture separately to transform the R strain.
They found that when DNA was degraded, the resulting mixture was no longer able to transform the bacteria, whereas all of the other combinations were able to transform the bacteria. This led them to conclude that DNA was the transforming principle. Forensic Scientists used DNA analysis evidence for the first time to solve an immigration case.
The story started with a teenage boy returning to London from Ghana to be with his mother. Immigration authorities at the airport were suspicious of him, thinking that he was traveling on a forged passport. After much persuasion, he was allowed to go live with his mother, but the immigration authorities did not drop the case against him. All types of evidence, including photographs, were provided to the authorities, but deportation proceedings were started nevertheless.
Around the same time, Dr. The immigration authorities approached Dr. Jeffreys for help. Forensic scientists analyze many items, including documents, handwriting, firearms, and biological samples. They analyze the DNA content of hair, semen, saliva, and blood, and compare it with a database of DNA profiles of known criminals. Analysis includes DNA isolation, sequencing, and sequence analysis. Forensic scientists are expected to appear at court hearings to present their findings. They are usually employed in crime labs of city and state government agencies.
Geneticists experimenting with DNA techniques also work for scientific and research organizations, pharmaceutical industries, and college and university labs. Although the experiments of Avery, McCarty and McLeod had demonstrated that DNA was the informational component transferred during transformation, DNA was still considered to be too simple a molecule to carry biological information. Proteins, with their 20 different amino acids, were regarded as more likely candidates.
The decisive experiment, conducted by Martha Chase and Alfred Hershey in , provided confirmatory evidence that DNA was indeed the genetic material and not proteins. Chase and Hershey 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.
The bacteriophage infects the host bacterial cell by attaching to its surface, 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 selected radioactive elements that would specifically distinguish the protein from the DNA in infected cells. They labeled one batch of phage with radioactive sulfur, 35 S, to label the protein coat. Another batch of phage were labeled with radioactive phosphorus, 32 P.
Because phosphorous is found in DNA, but not protein, the DNA and not the protein would be tagged with radioactive phosphorus. Likewise, sulfur is absent from DNA, but present in several amino acids such as methionine and cysteine. Each batch of phage was allowed to infect the cells separately. After infection, the phage bacterial suspension was put in a blender, which caused the phage coat to detach from the host cell.
Cells exposed long enough for infection to occur were then examined to see which of the two radioactive molecules had entered the cell. The phage and bacterial suspension was spun down 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 that contained phage labeled with 35 S, the supernatant contained the radioactively labeled phage, whereas no radioactivity was detected in the pellet.
In the tube that contained the phage labeled with 32 P, the radioactivity was detected in the pellet that contained the heavier bacterial cells, and no radioactivity was detected in the supernatant. Hershey and Chase concluded that it was the phage DNA that was injected into the cell and carried information to produce more phage particles, thus providing evidence that DNA was the genetic material and not proteins Figure 3.
Figure 3. At the same time that Griffith was conducting his experiments, researcher Oswald Avery and his colleagues at the Rockefeller University in New York were performing detailed analyses of the pneumococcal cell capsule and the role of this capsule in infections. Modern antibiotics had not yet been discovered, and Avery was convinced that a detailed understanding of the pneumococcal cell was essential to the effective treatment of bacterial pneumonia.
Over the years, Avery's group had accumulated considerable biochemical expertise as they established that strains of pneumococci could be distinguished by the polysaccharides in their capsules and that the integrity of the capsule was essential for virulence. Thus, when Griffith's results were published, Avery and his colleagues recognized the importance of these findings, and they decided to use their expertise to identify the specific molecules that could transform a nonencapsulated bacterium into an encapsulated form.
In a significant departure from Griffith's procedure, however, Avery's team employed a method for transforming bacteria in cultures rather than in living mice, which gave them better control of their experiments.
Avery and his colleagues, including researchers Colin MacLeod and Maclyn McCarty, used a process of elimination to identify the transforming principle Avery et al. In their experiments Figure 3 , identical extracts from heat-treated S cells were first treated with hydrolytic enzymes that specifically destroyed protein , RNA , or DNA. After the enzyme treatments, the treated extracts were then mixed with live R cells.
Encapsulated S cells appeared in all of the cultures, except those in which the S strain extract had been treated with DNAse, an enzyme that destroys DNA. These results suggested that DNA was the molecule responsible for transformation. Avery and his colleagues provided further confirmation for this hypothesis by chemically isolating DNA from the cell extract and showing that it possessed the same transforming ability as the heat-treated extract.
We now consider these experiments, which were published in , as providing definitive proof that DNA is the hereditary material.
However, the team's results were not well received at the time, most likely because popular opinion still favored protein as the hereditary material. Coli cell and injects its chromosome purple line.
These experiments involved the T2 bacteriophage , a virus that infects the E. At the time, bacteriophages were widely used as experimental models for studying genetic transmission because they reproduce rapidly and can be easily harvested.
In fact, d uring just one infection cycle , bacteriophages multiply so rapidly within their host bacterial cells that they ultimately cause the cells to burst, thus releasing large numbers of new infectious bacteriophages Figure 4. The T2 bacteriophage used by Hershey and Chase was known to consist of both protein and DNA, but the role that each substance played in the growth of the bacteriophage was unclear.
Electron micrographs had shown that T2 bacteriophages consist of an icosahedral head, a cylindrical sheath, and a base plate that mediates attachment to the bacterium, shown schematically in Figure 5. After infection, phage particles remain attached to the bacterium, but the heads appear empty, forming "ghosts. Figure 5 To determine the roles that the T2 bacteriophage's DNA and protein play in infection, Hershey and Chase decided to use radioisotopes to trace the fate of the phage's protein and DNA by taking advantage of their chemical differences.
Proteins contain sulfur, but DNA does not. Conversely, DNA contains phosphate, but proteins do not. Thus, when infected bacteria are grown in the presence of radioactive forms of phosphate 32 P or sulfur 35 S , radioactivity can be selectively incorporated into either DNA or protein. Hershey and Chase employed this method to prepare both 32 P-labeled and 35 S-labeled bacteriophages, which they then used to infect bacteria.
To determine which of the labeled molecules entered the infected bacteria, they detached the phage ghosts from the infected cells by mechanically shearing them off in an ordinary kitchen blender. The ghosts and bacterial cells were then physically separated using a centrifuge.
The larger bacterial cells moved rapidly to the bottom of the centrifuge tube, where they formed a pellet. The smaller, lighter phage ghosts remained in the supernatant, where they could be collected and analyzed. During analysis, Hershey and Chase discovered that almost all of the radioactive sulfur remained with the ghosts, while about one-third of the radioactive phosphate entered the bacterial cells and could later be recovered in the next generation of bacteriophages.
From these experiments, Hershey and Chase determined that protein formed a protective coat around the bacteriophage that functioned in both phage attachment to the bacterium and in the injection of phage DNA into the cell. Interestingly, they did not conclude that DNA was the hereditary material, pointing out that further experiments were required to establish the role that DNA played in phage replication.
In fact, Hershey and Chase circumspectly ended their paper with the following statement: "This protein probably has no function in the growth of intracellular phage. The DNA has some function. However, a mere one year later, the structure of DNA was determined , and this allowed investigators to put together the pieces in the question of DNA structure and function. Avery, O. Studies on the chemical nature of the substance inducing transformation of pneumococcal types.
Journal of Experimental Medicine 79 , — Griffith, F. The significance of pneumococcal types. Journal of Hygiene 27 , — Hershey, A. Independent functions of viral protein and nucleic acid in growth of bacteriophage. Journal of General Physiology 36 , 39—56 Steinman, R. A triple tribute to the experiment that transformed biology.
Journal of Experimental Medicine , — Restriction Enzymes. Genetic Mutation. Functions and Utility of Alu Jumping Genes.
Transposons: The Jumping Genes. DNA Transcription. What is a Gene? Colinearity and Transcription Units. Copy Number Variation. Copy Number Variation and Genetic Disease. Copy Number Variation and Human Disease. Tandem Repeats and Morphological Variation. Chemical Structure of RNA. Eukaryotic Genome Complexity. RNA Functions.
Citation: O'Connor, C. Nature Education 1 1 How did scientists determine that DNA is the hereditary material? Groundbreaking experiments by Griffith, Avery, Hershey, and Chase disproved the notion that proteins were genetic material.
Aa Aa Aa. In the first half of the twentieth century, Gregor Mendel's principles of genetic inheritance became widely accepted, but the chemical nature of the hereditary material remained unknown. Scientists did know that genes were located on chromosomes and that chromosomes consisted of DNA and proteins. At the time, however, proteins seemed to be a better choice for the genetic material, because chemical analyses had shown that proteins are more varied than DNA in their chemical composition, as well as in their physical properties.
Therefore, the eventual identification of DNA as the hereditary material came as a surprise to scientists. This breakthrough resulted from a series of experiments with bacteria and bacteriophages, or viruses that infect bacteria.
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