Bacterial Genetics PDF

MB-251-MJ Bacterial Genetics Ms. Vedanti Prakash Satonkar MES’ Abasaheb Garware College,Pune-04  1909: The Word Gene Coined 1909: The Word Gene Coined Danish botanist Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity. He also made the dist...

MB-251-MJ Bacterial Genetics Ms. Vedanti Prakash Satonkar MES’ Abasaheb Garware College,Pune-04  1909: The Word Gene Coined 1909: The Word Gene Coined Danish botanist Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity. He also made the distinction between the outward appearance of an individual (phenotype) and its genetic traits (genotype). Four years earlier, William Bateson, an early geneticist and a proponent of Mendel's ideas, had used the word genetics in a letter; he felt the need for a new term to describe the study of heredity and inherited variations. But the term didn't start spreading until Wilhelm Johannsen suggested that the Mendelian factors of inheritance be called genes. The proposed word traced from the Greek word genos, meaning "birth". The word spawned others, like genome  1911: Fruit Flies Illuminate the Chromosome Theory 1911: Fruit Flies Illuminate the Chromosome Theory Using fruit flies as a model organism, Thomas Hunt Morgan and his group at Columbia University showed that genes, strung on chromosomes, are the units of heredity. Morgan and his students made many important contributions to genetics. His students, who included such important geneticists as Alfred Sturtevant, Hermann Muller and Calvin Bridges, studied the fruit fly Drosophila melanogaster. They showed that chromosomes carry genes, discovered genetic linkage - the fact that genes are arrayed on linear chromosomes - and described chromosome recombination. In 1933, Morgan received the Nobel Prize in Physiology or Medicine for helping establish the chromosome theory of inheritance.  1940's 1941: One Gene, One Enzyme 1943: X-ray Diffraction of DNA 1944: DNA is "Transforming Principle" 1944: Jumping Genes  1941: One Gene, One Enzyme 1941: One Gene, One Enzyme George Beadle and Edward Tatum, through experiments on the red bread mold Neurospora crassa, showed that genes act by regulating distinct chemical events - affirming the "one gene, one enzyme" hypothes George Beadle had spent two years in T. H. Morgan's lab at Caltech, studying genetics using fruit flies as a model organism. In 1941, he and Edward Tatum turn to an even simpler model for studying genetics. In its normal, or "wild", state, the mold Neurospora crassa can grow on a medium containing just sugar, a small amount of biotin, and inorganic salts. When the mold is exposed to X-ray radiation, mutations arise in occasional cells. Some of the mutations affect the mold's ability to form organic compounds from simpler building blocks. For example, some lose the ability to assemble particular amino acids. To thrive, those strains need to have the particular amino acids supplied in their nutrient medium or, sometimes, they can make do with precursor compounds that the cells can convert into the required amino acids  1943: X-ray Diffraction of DNA 1943: X-ray Diffraction of DNA William Astbury, a British scientist, obtained the first X-ray diffraction pattern of DNA. X-ray diffraction patterns of crystallized molecules can reveal their structures with atomic precision. Astbury obtained X-ray diffraction patterns of uncrystallized DNA He extracted DNA from cells, then dipped a needle into the viscous DNA solution and dragged out a strand containing many molecules lined up roughly parallel to each other. The X-ray diffraction patterns off this strand revealed that DNA must have a regular periodic structure. He suggested that the nucleotide bases are stacked on top of each other  Simple observation shows that a lot of variation exists between individuals of a given species. Individual humans vary in eye color, height, skin color, and hair color, even though all humans belong to the species Homo sapiens. The differences between individuals within and among species are mainly the result of differences in the DNA sequences that constitute the genes in their genomes. The genetic information coded in DNA is largely responsible for determining the structure, function, and development of the cell and the organism. In the next several chapters, we explore the molecular structure and function of genetic material—both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)—and examine the molecular mechanisms by which genetic information is transmitted from generation to generation.  Long before DNA and RNA were known to carry genetic information, scientists realized that living organisms contain some substance—a genetic material—that is responsible for the characteristics that are passed on from parent to child. Geneticists knew that the material responsible for hereditary information must have three key characteristics: 1. It must contain, in a stable form, the information about an organism’s cell structure, function, development, and reproduction. 2. It must replicate accurately, so that progeny cells have the same genetic information as the parental cell. 3. It must be capable of change. Without change, organisms would be incapable of variation and adaptation, and evolution could not occur.  The Swiss biochemist Friedrich Miescher is credited with the discovery, in 1869, of nucleic acid. He isolated a substance from white blood cells of pus in used bandages during the Crimean War. At first he believed the substance to be protein; but chemical tests indicated that it contained carbon, hydrogen, oxygen, nitrogen, and phosphorus, the last of which was not known to be a component of proteins. Searching for the same substance in other sources, Miescher found it in the nucleus of all the samples he studied—and, therefore, he called it nuclein. At the time, its function was unknown, and its exact location in the cell was unknown  In the early 1900s, experiments showed that chromosomes—the threadlike structures found in nuclei— are carriers of hereditary information. Chemical analysis over the next 40 years revealed that chromosomes are composed of protein and nucleic acids, which by this time were known to include DNA and RNA. At first, many scientists believed that the protein in the chromosomes must be the genetic material. They reasoned that proteins have a great capacity for storing information because they were composed of 20 different amino acids. (Note: Twenty amino acids were known at the time. A twenty-first amino acid was identified in the 1970s, and a twenty-second was identified in 2002.) By contrast, DNA, with its four nucleotides, was thought to be too simple a molecule to account for the variation found in living organisms. However, beginning in the late 1920s, a series of experiments led to the definitive identification of DNA as genetic material Frankel– Conrat and Singer experiment (TMV virus)  Harshey and Chase experiment In 1953, Alfred D. Hershey and Martha Chase published a paper that provided more evidence that DNA was the genetic material. They were studying a bacteriophage called T2. Bacteriophages (also called phages) are viruses that attack bacteria. Like all viruses, the T2 phage must reproduce within a living cell. T2 reproduces by invading an Escherichia coli (E. coli) cell and using the bacterium’s molecular machinery to make more viruses (Figure 2.5). Initially the progeny viruses are assembled inside the bacterium; but eventually the host cell ruptures, releasing 100–200 progeny phages. The suspension of released progeny phages is called a phage lysate.  The in which a phage infects a bacterial cell and produces progeny phages that are released from the broken-open bacterium is known as the lytic cycle Hershey and Chase knew that T2 consisted of only DNA and protein, and their working hypothesis was that the DNA was the genetic material. T2 phages are very simply put together. They have an outer shell that surrounds their genetic material. When they infect a bacterium, they inject their genetic material inside the host cell but leave their outer shell on the surface of the bacterium. Once the genetic material has been injected into the host cell, the empty outer shell that is left is sometimes referred to as a phage ghost  To prove that the phage genetic material was made up of DNA and not protein, Hershey and Chase grew cells of E. coli in media containing either a radioactive isotope of phosphorus or a radioactive isotope of sulfur (Figure 2.6a). They used these isotopes because DNA contains phosphorus but no sulfur, and protein contains sulfur but no phosphorus. The E. coli took (35S) (32P) up whichever isotope was provided and incorporated the into all the nucleic acids made inside the cell or incorporated the into all the proteins made inside the cell. Any phage inside the bacteria would use its host bacterium’s nucleic acids and proteins to construct progeny phages. Hershey and Chase then infected the bacteria with T2 and collected the progeny phages.  At this point, the researchers had two batches of T2, one with DNA labeled radioactively with and the other with protein labeled with. Next, they infected two cultures of E. coli with one or other of the two types of radioactively labeled T2 (Figure 2.6b). When the infecting phage was 32P-labeled, most of the radioactivity was found within the bacteria soon after infection. Very little was found in the phage ghosts released from the cell surface after the cells were agitated in a kitchen blender. After completion of the lytic cycle, some of the was found in the progeny phages. In contrast, after E. coli were infected with 35S -labeled T2, almost none of the radioactivity appeared within the cell or in the progeny phage particles, while most of the radioactivity was in the phage ghosts.  Hershey and Chase reasoned that, because it was DNA and not protein that entered the cell—as evidenced by the presence of and the absence of 35S inside the bacterial cells immediately after the phage had begun the infection process by injecting their genetic material inside their host cells—DNA must be the material responsible for the function and reproduction of phage T2. That is, DNA must be the genetic material of phage T2. This was also consistent with the finding that but not was found in the progeny phages, because the phage genetic material inside the host cells would be partially repackaged in the progeny phages being assembled during the infection process. Only genetic material (DNA) is passed from parent to offspring in phage reproduction. Structural materials (the proteins) are not. Alfred Hershey shared the 1969 Nobel Prize in Physiology or Medicine for his “discoveries concerning the genetic structure of viruses.