Hershey-Chase experiment

src: i.ytimg.com

The Hershey-Chase experiment is a series of experiments conducted in 1952 by Alfred Hershey and Martha Chase who help to confirm that DNA is a genetic material. While DNA has been known by biologists since 1869, many scientists still assume at the time that proteins carry information for inheritance because DNA looks simpler than proteins. In their experiments, Hershey and Chase showed that when bacteriophages, which are composed of DNA and proteins, infect bacteria, their DNA enters host bacterial cells, but most of their proteins do not. Although the results are not conclusive, and Hershey and Chase are careful in their interpretations, previous discoveries, contemporaries, and further all serve to prove that DNA is a hereditary material.

Hershey shared the 1969 Nobel Prize in Physiology or Medicine with Max DelbrÃÆ'¼ck and Salvador Luria for their "discovery of the genetic structure of the virus."

Video Hershey-Chase experiment

​​â € <â €

At the beginning of the twentieth century, biologists think that proteins carry genetic information. This is based on the belief that proteins are more complex than DNA. The "tetranucleotide" hypothesis proposed by Phoebus Levene, which mistakenly argues that DNA is a recurrent collection of recurrent nucleotides, supports this conclusion. The results of the Avery-MacLeod-McCarty experiment, published in 1944, show that DNA is a genetic material, but there are still doubts in the general scientific community to accept this, which set the stage for the Hershey-Chase experiment.

Hershey and Chase, along with others who have conducted related experiments, confirmed that DNA is a biomolecule that carries genetic information. Prior to that, Oswald Avery, Colin MacLeod, and Maclyn McCarty have shown that DNA causes the transformation of one strain of Streptococcus pneumoniae into another. The results of this experiment provide evidence that DNA is a biomolecule that carries genetic information.

Maps Hershey-Chase experiment

Methods and results

Hershey and Chase should be able to examine the different parts of the phage they studied separately, so they need to distinguish the phage part. Viruses are known to consist of protein and DNA shells, so they choose unique labels each with different isotope elements. This allows each to be observed and analyzed separately. Since phosphorus is contained in DNA but not amino acids, radioactive phosphor-32 is used to label DNA contained in the T2 freak. Radioactive sulfur is used to label the protein part of the T2 phag, because sulfur is contained in proteins but not DNA.

Hershey and Chase incorporate radioactive elements in bacteriophages by adding isotopes to a separate medium where bacteria are allowed to grow for 4 hours before the introduction of bacteriophage. When bacteriophages infect bacteria, the progeny contains radioactive isotopes in their structure. This procedure is performed once for the sulfur labeled phage and once for phosphor labeled phage. Labeled progeny is then allowed to infect unlabeled bacteria. The phage coat stays outside the bacteria, while the genetic material enters. The phage disturbance of bacteria by agitation in a blender followed by centrifugation allows for the separation of the phage mantle from bacteria. These bacteria are released to release phag progeny. Phage progeny labeled with radioactive phosphors is still labeled, while the phage progeny labeled with radioactive sulfur is not labeled. Thus, the Hershey-Chase experiment helps confirm that DNA, not protein, is a genetic material.

Hershey and Chase showed that the introduction of deoxyribonuclease (referred to as DNase), an enzyme that breaks DNA, into a solution containing labeled bacteriophage does not introduce ³² P into the solution. This indicates that the phage is resistant to the enzyme intact. In addition, they were able to plasmolyze bacteriophages so they became osmotic shock, which effectively created a solution containing mostly ³² P and a heavier solution containing a structure called "ghost" containing ³⁵ S and viral protein layer. It was found that these "ghosts" can be absorbed into bacteria susceptible to T2, even though they contain no DNA and only the remains of native bacterial capsules. They conclude that proteins protect DNA from DNAse, but when both are separated and the phage is inactivated, DNAse can hydrolyze the phage DNA.

Experiments and conclusions

Hershey and Chase are also able to prove that the DNA of the phage is inserted into the bacteria as soon as the virus attaches to its host. Using high-speed blenders they are able to force bacteriophages from bacterial cells after adsorption. Lack of ³² P labeled the remaining DNA in solution after bacteriophage has been allowed to absorb into the bacteria indicating that the phage DNA is transferred to the bacterial cell. The presence of almost all the radioactive ³⁵ S in the solution indicates that the protein layer that protects DNA before adsorption is outside the cell.

Hershey and Chase concluded that DNA, not protein, is a genetic material. They decide that a protective protein layer is formed around the bacteriophage, but it is this internal DNA that gives its ability to produce offspring in bacteria. They show that, in growth, proteins have no function, while DNA has several functions. They determine this from the amount of radioactive material left outside the cell. Only 20% of ³² P stays outside the cell, indicating that it is incorporated with DNA in the cell's genetic material. All the ³⁵ S in the protein layer remains outside the cell, indicating that it is not inserted into the cell, and that the protein is not a genetic material.

The Hershey and Chase experiments concluded that few sulfur-containing materials enter bacterial cells. But no specific conclusions can be made about whether sulfur-free materials enter bacterial cells after phag adsorption. Further research is needed to conclude that it is only the bacteriophage DNA that enters the cell and not the combination of proteins and DNA in which proteins do not contain sulfur.

src: upload.wikimedia.org

Discussion

Confirm

Hershey and Chase conclude that proteins are unlikely to be genetic material of offspring. However, they did not make any conclusions about the specific function of DNA as a derived material, and merely said that it must have an undefined role.

Confirmation and clarity came a year later in 1953, when James D. Watson and Francis Crick correctly hypothesized, in their journal article "The Molecular Structure of Nucleic Acids: Structures for Deoxyribose Nucleic Acid", the double helix structure of DNA, and suggests a copy mechanism whereby DNA serves as a derived material. Furthermore, Watson and Crick suggest that DNA, the genetic material, is responsible for the synthesis of thousands of proteins found in cells. They have made this proposal based on the structural similarities that exist between two macromolecules: both proteins and DNA are the linear sequences of monomers (amino acids and nucleotides, respectively).

Other experiments

After the Hershey-Chase experiment was published, the scientific community generally acknowledged that DNA is the material of the genetic code. This discovery led to a more detailed investigation of DNA to determine its composition and its 3D structure. Using X-ray crystallography, the structure of DNA was discovered by James Watson and Francis Crick with the help of experimental evidence documented earlier by Maurice Wilkins and Rosalind Franklin. Knowledge of the structure of DNA leads scientists to examine the nature of genetic coding and, in turn, understand the process of protein synthesis. George Gamow proposes that the genetic code is composed of a series of three base pairs of DNA known as a triplet or codon representing one of twenty amino acids. Genetic coding helps researchers to understand the mechanisms of gene expression, the process by which information from genes is used in protein synthesis. Since then, much research has been done to modulate the steps in the process of gene expression. These steps include transcription, RNA connection, translation, and post-translational modification used to control the chemical and structural properties of proteins. In addition, genetic engineering gives engineers the ability to manipulate directly the genetic material of organisms using recombinant DNA techniques. The first recombinant DNA molecule was invented by Paul Berg in 1972 when it combined DNA from the SV40 monkey virus with the lambda virus.

Experiments on hereditary materials during the Hershey-Chase experiment often used bacteriophages as model organisms. Bacteriophages themselves use experiments on hereditary materials because they incorporate their genetic material into the genetic material of their host cell (making them useful tools), they multiply rapidly, and they are easily collected by the researchers.

src: i.ytimg.com

Legacy

The Hershey-Chase experiment, its predecessors, is like the Avery-MacLeod-McCarty experiment, and the successors firmly state that hereditary information is carried by DNA. This finding has many applications in forensic science, criminal investigation and genealogy. This provides background knowledge for further applications in DNA forensics, where DNA fingerprints use data derived from DNA, not a protein source, to infer genetic variation.

src: ib.bioninja.com.au

References

src: i.ytimg.com

External links

• Hershey-Chase experiment animation
• Clear descriptions and simple summaries

Source of the article : Wikipedia