How Do Genes Work

How do genes work? To answer that question, the first thing you need to know is what genes are made of. The DNA molecule must somehow specify how to assemble proteins, which are needed to regulate the various functions of each cell. Understanding the structure of DNA has been the key to understanding how genes work. Bacterial Transformation 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. The lungs of these mice were filled with the disease-causing bacteria. How could that happen if the S strain cells were dead? Griffith reasoned that some chemical factor that could “transform” harmless bacteria into diseasecausing bacteria was transferred from the heat-killed cells of the S strain into the live cells of the R strain. He called this process transformation, because one type of bacteria had been changed permanently into another. Because the ability to cause disease was inherited by the offspring of the transformed bacteria, Griffith concluded that the transforming factor had to be a gene. The Molecular Cause of Transformation 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 bacteria was most important for transformation. • • Transforming factor could be: (chromosomes) Protein or DNA Avery treated Griffith’s deadly bacteria with: 1) protein-destroying enzymes • Bacteria were still transformed (in absence of protein) 2) DNA –destroying enzymes • Bacteria failed to transform Conclusion (1944): DNA is genetic material What clues did bacterial transformation yield about the gene? By observing bacterial transformation, Avery and other scientists discovered that the nucleic acid DNA stores and transmits genetic information from one generation of bacteria to the next. Several different scientists repeated Avery’s experiments. Alfred Hershey and Martha Chase performed the most important of the experiments relating to Avery’s discovery. Hershey and Chase studied bacteriophage viruses, particles that can infect living cells. The Hershey-Chase Experiment American scientists Alfred Hershey and Martha Chase studied a bacteriophage that was composed of a DNA core and a protein coat. They wanted to determine which part of the virus-the protein coat or the DNA core-entered the bacterial cell Hershey and Chase grew viruses in cultures containing radioactive isotopes of phosphorus-32 (P-32) sulfur-35 (S-35) The Hershey-Chase Experiment (1952) What role did bacterial viruses play in identifying genetic material? Hershey and Chase’s experiment with bacteriophages confirmed Avery’s results, convincing many scientists that DNA was the genetic material found in genes-not just in viruses and bacteria, but in all living cells. What is the role of DNA in heredity? The DNA that makes up genes must be capable of storing, copying, and transmitting the genetic information in a cell. Genes control patterns of development, which means that the instructions that cause a single cell to develop into an oak tree or a dog must somehow be stored in the DNA of each of these organisms. Before a cell divides, it must make a complete copy of every one of its genes, similar to the way that a book is copied. When a cell divides, each daughter cell must receive a complete copy of the genetic information. Careful sorting / transmission is especially important during the formation of reproductive cells in meiosis. The Components of DNA What are the chemical components of DNA? DNA is a nucleic acid made up of nucleotides (monomers) joined into long strands or chains (polymer) by covalent bonds. DNA’s nucleotides are made up of three basic components: a 5-carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base. The nucleotides can be joined together in any order, meaning that any sequence of bases is possible. Solving the Structure of DNA What clues helped scientists solve the structure of DNA? The clues in Franklin’s X-ray pattern combined with Chargaff’s rules enabled Watson and Crick to build a model that explained the specific structure and properties of DNA. Franklin’s X-Rays In the 1950s, Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule. X-ray diffraction revealed an X-shaped pattern showing that the strands in DNA are twisted around each other like the coils of a spring (helix). The angle of the pattern suggested that there are two strands in the structure. Other clues suggest that the nitrogen bases are near the center of the DNA molecule. Chargaff’s Rules Erwin Chargaff discovered that the percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. The same thing is true for the other two nucleotides, guanine [G] and cytosine [C]. [A] = [T] and [G] = [C] The Work of Watson and Crick At the same time, James Watson, an American biologist, and Francis Crick, a British physicist, were also trying to understand the structure of DNA by building three-dimensional models. Early in 1953, Watson was shown a copy of Franklin’s X-ray pattern. The clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model in the shape of a double helix that explained the specific structure and properties of DNA. What does the double-helix model tell us about DNA? The double helix accounted for Franklin’s X-ray pattern and explains Chargaff’s rule of base pairing and how the two strands of DNA are held together. Antiparallel Strands In the double-helix model, the two strands of DNA are “antiparallel”—they run in opposite directions. This arrangement enables the nitrogenous bases on both strands to come into contact at the center of the molecule. It also allows each strand of the double helix to carry a sequence of nucleotides, arranged almost like letters in a four-letter alphabet. Hydrogen Bonding Watson and Crick discovered that hydrogen bonds could form between certain nitrogenous bases, providing just enough force to hold the two DNA strands together. Hydrogen bonds are relatively weak chemical forces that allow the two strands of the helix to separate. The ability of the two strands to separate is critical to DNA’s functions. Base Pairing Watson and Crick’s model showed that hydrogen bonds could create a nearly perfect fit between nitrogenous bases along the center of the molecule. These bonds would form only between certain base pairs—adenine with thymine (2 H-bonds) and guanine with cytosine (3 H-bonds). Watson and Crick realized that base pairing explained Chargaff’s rule. It gave a reason why [A] = [T] and [G] = [C]. For every adenine in a double-stranded DNA molecule, there had to be exactly one thymine. For each cytosine, there was one guanine.