Identifying the Genetic Material Griffith Experiments In 1928, Frederick Griffith, a bacteriologist, was trying to prepare a vaccine against pneumonia. A vaccine is a substance that is prepared from killed or weakened disease-causing agents, including certain bacteria. The vaccine is introduced into the body to protect the body against future infections by the disease-causing agent. Griffith discovered that harmless bacteria could turn virulent when mixed with bacteria that cause disease. A bacteria that is virulent is able to cause disease.
Griffith had discovered what is now called transformation, a change in genotype caused when cells take up foreign genetic material. Avery Experiments In 1944, a series of experiments showed that the activity of the material responsible for transformation is not affected by protein-destroying enzymes. The activity is stopped, however, by a DNA-destroying enzyme. Thus, almost 100 years after Mender’s experiments, Oswald Avery and his co-workers demonstrated that DNA is the material responsible for transformation. Viral Genes and DNA In 1952, Alfred Hershey and Martha Chase used the bacteriologic TO to prove that
DNA carried genetic material. A bacteriologic, also referred to as phage, is a virus that infects bacteria. When phages infect bacterial cells, the phages are able to produce more viruses, which are released when the bacterial cells rupture. Dona’s Role Revealed Hershey and Chase carried out the following experiment: Step 1 TO phages were labeled with radioactive isotopes. Step 2 The phages infect E. Coli bacterial cells. Step 3 Bacterial cells were spun to remove the virus’s protein coats. Hershey and Chase concluded that the DNA of viruses is injected into the bacterial cells, while cost of the viral proteins remain outside.
The injected DNA molecules causes the bacterial cells to produce more viral DNA and proteins. This meant that the DNA, rather than proteins, is the hereditary material, at least in viruses. Section 2 : The Structure tot D A Winding Staircase Watson and Crick determined that a DNA molecule is a double helix-?two strands twisted around each other, like a winding staircase. Nucleotides are the subunits that make up DNA. Each nucleotide is made of three parts: a phosphate group, a five- carbon sugar molecule, and a nitrogen-containing base. The five-carbon sugar in
DNA nucleotides is called didgeridoos The nitrogen base in a nucleotide can be either a bulky, double-ring Purina, or a smaller, single-ring pyridine Charges Observations In 1949, Erwin Charge observed that for each organism he studied, the amount of adenine always equaled the amount of thymine (A=T). Likewise, the amount of guanine always equaled the amount of cytosine (G=C). However, the amount of adenine and thymine and of guanine and cytosine varied between different organisms. Wilkins and Franklins Photographs By analyzing the complex patterns on X-ray diffraction photo, scientists can determine the structure of the molecule.
In 1952, Maurice Wilkins and Roseland Franklin developed high-quality X-ray diffraction photographs of strands of DNA. These photographs suggested that the DNA molecule resembled a tightly coiled helix and was composed of two or three chains of nucleotides Watson and Cricks DNA Model In 1953, Watson and Crick built a model of DNA with the configuration of a double helix, a “spiral staircase” of two strands of nucleotides twisting around a central axis. The double-helical model of DNA takes into account Charges observations and the tatters on Franklins X-ray diffraction photographs.
Pairing Between Bases An adenine on one strand always pairs with a thymine on the opposite strand, and a guanine on one strand always pairs with a cytosine on the opposite strand. These base-pairing rules are supported by Charges observations. The strictness of base- pairing results in two strands that contain complementary base pairs. Section 3 The Replication tot D Roles of Enzymes in DNA Replication The complementary structure of DNA is used as a basis to make exact copies of the DNA each time a cell divided. The process of making a copy of DNA is called DNA replication.
DNA replication occurs during the synthesis (S) phase of the cell cycle, before a cell divides. DNA replication occurs in three steps: Step 1 DNA helices open the double helix by breaking the hydrogen bonds that link the complementary nitrogen bases between the two strands. The areas where the double helix separates are called replication forks. Step 2 At the replication fork, enzymes known as DNA polymerases move along each of the DNA strands. DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base- airing rules.
Step 3 Two DNA molecules form that are identical to the original DNA molecule. Checking for Errors In the course of DNA replication, errors sometimes occur and the wrong nucleotide is added to the new strand. An important feature of DNA replication is that DNA polymerases have a “proofreading” role. This proofreading reduces errors in DNA replication to about one error per 1 billion nucleotides. The Rate of Replication Replication does not begin at one end of the DNA molecule and end at the other. The circular DNA molecules found in prokaryote usually have two replication forks that egging at a single point.