Ch16WordLectureOutline

Ch16WordLectureOutline - CHAPTER 16 THE MOLECULAR BASIS OF...

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CHAPTER 16 THE MOLECULAR BASIS OF INHERITANCE Introduction In April 1953, James Watson and Francis Crick shook the scientific world with an elegant double-helical model for the structure of deoxyribonucleic acid or DNA. Your genetic endowment is the DNA you inherited from your parents. Nucleic acids are unique in their ability to direct their own replication. The resemblance of offspring to their parents depends on the precise replication of DNA and its transmission from one generation to the next. A. DNA as the Genetic Material 1. The search for genetic material led to DNA Once T.H. Morgan’s group showed that genes are located on chromosomes, the two constituents of chromosomes - proteins and DNA - were the candidates for the genetic material. Until the 1940s, the great heterogeneity and specificity of function of proteins seemed to indicate that proteins were the genetic material. However, this was not consistent with experiments with microorganisms, like bacteria and viruses. The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928. He studied Streptococcus pneumoniae , a bacterium that causes pneumonia in mammals. One strain, the R strain, was harmless. The other strain, the S strain, was pathogenic. In an experiment Griffith mixed heat-killed S strain with live R strain
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bacteria and injected this into a mouse. The mouse died and he recovered the pathogenic strain from the mouse’s blood. Griffith called this phenomenon transformation , a change in genotype and phenotype due to the assimilation of a foreign substance (now known to be DNA) by a cell. For the next 14 years scientists tried to identify the transforming substance. Finally in 1944, Oswald Avery, Maclyn McCarty and Colin MacLeod announced that the transforming substance was DNA. Still, many biologists were skeptical. In part, this reflected a belief that the genes of bacteria could not be similar in composition and function to those of more complex organisms. Further evidence that DNA was the genetic material was derived from studies that tracked the infection of bacteria by viruses. Viruses consist of a DNA (sometimes RNA) enclosed by a protective coat of protein. To replicate, a virus infects a host cell and takes over the cell’s metabolic machinery. Viruses that specifically attack bacteria are called bacteriophages or just phages . In 1952, Alfred Hershey and Martha Chase showed that DNA was the genetic material of the phage T2. The T2 phage, consisting almost entirely of DNA and protein, attacks Escherichia coli ( E. coli ), a common intestinal bacteria of mammals. This phage can quickly turn an
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Ch16WordLectureOutline - CHAPTER 16 THE MOLECULAR BASIS OF...

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