DNA: The Genetic Material

Identifying the Genetic Material DNA was identified as the genetic material through a series of experiments. Frederick Griffith, 1928 Oswald Avery, 1944 Alfred Hershey & Martha Chase, 1952

Griffith’s Experiments The discovery of the chemical nature of the gene began in 1928 with British scientist Frederick Griffith, who was trying to figure out how certain types of bacteria produce pneumonia. Griffith was studying two different strains of the same bacterial species: the S form (deadly) and the R form (not deadly).

Griffith’s Experiments The following tests were conducted.

Griffith’s Experiments Griffith reasoned that some chemical factor or molecule was transferred from the heat-killed cells of the S strain into the live cells of the R strain changing the harmless bacteria into a disease-causing bacteria. He called this process transformation because one type of bacteria was changed (transformed) into another. recovered

The Molecular Cause of Transformation In 1944, a group of scientists at the Rockefeller Institute in New York, led by the Canadian biologist Oswald Avery, wanted to determine which molecule in the heat-killed S bacteria caused transformation.

The Molecular Cause of Transformation Avery and his team extracted (removed) a mixture of various molecules from the heat-killed S bacteria and treated this mixture with enzymes that destroyed proteins (protease). Transformation still occurred. Avery’s team repeated the experiment using enzymes that would break down DNA (DNAase). When they destroyed the DNA in the mixture, transformation did not occur. Therefore, they concluded DNA was the transforming factor.

The Hershey-Chase Experiment In 1952, Alfred Hershey and Martha Chase performed the most important of the experiments relating to Avery’s discovery. Hershey and Chase studied viruses—nonliving particles that can infect living cells. They wanted to determine which part of the virus - the protein coat or the DNA core - entered the bacterial cell. Their results would either support or disprove Avery’s finding that genes were made of DNA.

Bacterial Viruses The kind of virus that infects bacteria is known as a bacteriophage, which means “bacteria eater.” A typical bacteriophage is shown. When a bacteriophage enters a bacteria, it attaches to the surface of the bacterial cell and injects its genetic information into it. The viral genes use the bacteria to produce many new bacteriophages, which gradually destroy the bacterium. When the cell splits open, hundreds of new viruses burst out.

The Hershey-Chase Experiment Hershey and Chase wanted to answer the following question, “Which part of the bacteriophage (DNA or protein) actually entered the bacterium? They used two groups of viruses. One group had the DNA marked with phosphorus-32 (P-32) and the other had the protein coat marked with sulfur-35 (S-35).

The Hershey-Chase Experiment They then allowed the bacteriophage to infect the bacteria. If they found radioactivity of the S-35 in the bacteria, it would mean that the virus’s protein coat had been injected into the bacteria. If they found P-32 in the bacteria, then it would mean that the DNA core had been injected. Results: Nearly all the radioactivity in the bacteria was from phosphorus P-32 , the marker found in DNA. Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.

The Structure of DNA The discovery of DNA’s structure was important because it explained how DNA could serve as the genetic material. Erwin Chargaff, 1949 Maurice Wilkins & Rosalind Franklin, 1952 James Watson & Francis Crick, 1953

Chargaff’s Observation Chargaff showed that different species have different amounts of the four nucleotides that make up DNA, but that the amount of A was equal to the amount of T and that the amount of G was equal to the amount of C.

X-ray Diffraction Wilkins and Franklin developed X-ray diffraction photographs of strands of DNA that suggested that the DNA molecule resembled a tightly coiled helix and was composed of at least two or three chains of nucleotides.

The DNA Model Watson & Crick determined that the structure of the DNA molecule was a “double helix”. It was made of two strands of nucleotides, running in opposite directions, and they were held together by hydrogen bonds.