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Scientists Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944) were interested in exploring this transforming principle further. They isolated the S strain from the dead mice and isolated the proteins and nucleic acids, namely RNA and DNA, as these were possible candidates for the molecule of heredity. They conducted a systematic elimination study. 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.

Career connection

Forensic scientists and dna analysis

DNA evidence was used 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. Alec Jeffreys of Leicester University in the United Kingdom had invented a technique known as DNA fingerprinting. The immigration authorities approached Dr. Jeffreys for help. He took DNA samples from the mother and three of her children, plus an unrelated mother, and compared the samples with the boy’s DNA. Because the biological father was not in the picture, DNA from the three children was compared with the boy’s DNA. He found a match in the boy’s DNA for both the mother and his three siblings. He concluded that the boy was indeed the mother’s son.

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; most forensic DNA analysis involves polymerase chain reaction (PCR) amplification of short tandem repeat (STR) loci and electrophoresis to determine the length of the PCR-amplified fragment. Only mitochondrial DNA is sequenced for forensics. 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. Students wishing to pursue a career as a forensic scientist should have at least a bachelor's degree in chemistry, biology, or physics, and preferably some experience working in a laboratory.

Experiments conducted by Martha Chase and Alfred Hershey in 1952 provided confirmatory evidence that DNA was the genetic material and not proteins. Chase and Hershey were studying a bacteriophage, which is 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 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.

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 be detached from the host 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 ( [link] ).

Illustration shows bacteria being infected by phage labeled with ^{35}S, which is incorporated into the protein coat, or ^{32}P, which is incorporated into the DNA. Infected bacteria were separated from phage by centrifugation and cultured. The bacteria that had been infected with phage containing ^{32}P-labeled DNA made radioactive phage. The bacteria that had been infected with ^{35}S-labeled phage produced unlabeled phage. The results support the hypothesis that DNA, and not protein, is the genetic material.
In Hershey and Chase's experiments, bacteria were infected with phage radiolabeled with either 35 S, which labels protein, or 32 P, which labels DNA. Only 32 P entered the bacterial cells, indicating that DNA is the genetic material.

Around this same time, Austrian biochemist Erwin Chargaff examined the content of DNA in different species and found that the amounts of adenine, thymine, guanine, and cytosine were not found in equal quantities, and that it varied from species to species, but not between individuals of the same species. He found that the amount of adenine equals the amount of thymine, and the amount of cytosine equals the amount of guanine, or A = T and G = C. This is also known as Chargaff’s rules. This finding proved immensely useful when Watson and Crick were getting ready to propose their DNA double helix model.

Section summary

DNA was first isolated from white blood cells by Friedrich Miescher, who called it nuclein because it was isolated from nuclei. Frederick Griffith's experiments with strains of Streptococcus pneumoniae provided the first hint that DNA may be the transforming principle. Avery, MacLeod, and McCarty proved that DNA is required for the transformation of bacteria. Later experiments by Hershey and Chase using bacteriophage T2 proved that DNA is the genetic material. Chargaff found that the ratio of A = T and C = G, and that the percentage content of A, T, G, and C is different for different species.

Questions & Answers

what is deglutition
Nalumansi Reply
what is biology
Sierrah Reply
biology is a type of science that studies living things
viruses are composed of what
Daniel Reply
what are some disease caused by virus
what are some disease caused by virus
what are some disease caused by virus
Ebola,rabies,polio,hepatitis and small pox
Corona (viral disease)
viruses kinda in a sense not "living" what mean that a virus molecule that attaches to living organisms cells to destroy/kill them. so to answer your question they are composed/made up of molecules (please note that I am not 100% sure, please feel free to correct me :] )
what is the use of biology
Tarkaa Reply
To widen our reasoning abilities about the past and the present events in biology and life at large
To understand our biography
to study living things
Among all these which one is the major difference between platyhelminthes and coelenterates? 1. are multicellular 2. have developed a mesoderm 3. reproduce sexually 4. reproduce asexually
Habeeb Reply
what is evolution?
Emmanuelfray Reply
how can you define biology in terms of a burning fire in the bush?
what is a cell
Kagelelo Reply
cell is a functional and unit of life
it is the building block n basic unit of life
describe the process of testing the presence of starch in a leaf
Estel Reply
give me somes of examples.
Manu Reply
give example of what.
example of what
what are chromosomes?
what is Biology
Biology is the study of animals and plants on their characters and death
No biology is the study of life (living thing's)
Chromosomes refers to one of the small parts like threads in the NUCLEI (central part )of plants and animals cell that carry the GENES
Yeah okol....Bio-means life and -logy means study/knowledge
Ecology is the study of
Abdul Reply
Ecology is the study of the interaction of organism in each other and their environment.
Ecology is the study of the interaction of organism in each other and their environment
oh yes!!! exactly
thank much
what is biome?
a geographical area with dinstic species
thanks much it very helpful
my pleasure
characteristics of living things
Jane Reply
excretion, movement, sensitivity, reproduction,growth, respiration,nutrition
what are there function
what are their function
what is the two type of reproduction?
sexual reproduction and asexual reproduction
what are their function
What is light independent reaction
Benjamin Reply
correct word or phrase. ______A____ uses carbon dioxide and _____B______ to make sugar during______C______. _____D______ traps light energy during this process. *
Andrea Reply
A_plant ,B_water,C_photosynthesis,D_chlorophyll
by which of the following feature c3 and c4 plant similar A .enzyme used to fix carbon dioxide B.cell where calvin cycle takes place C.cell where carbon dioxide fixation takes place
Kaleab Reply
correct word or phrase. ______A____ uses carbon dioxide and _____B______ to make sugar during______C______. _____D______ traps light energy during this process. *
what is biology ?
Bizone Reply
study of organisms
is the study of living organisms
what is reproductive system
Is the giving birth of offspring of the same one
it is the study of living organism

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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