Molecular Structure of DNA and RNA

Molecular Structure of DNA and RNA - Molecular Structure of...

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1 Molecular Structure of DNA and RNA Identification of DNA as the Genetic Material To fulfill its role, the genetic material must meet several criteria. Information: It must contain the information necessary to make an entire organism. Transmission: It must be passed from parent to offspring. Replication: It must be copied to be passed from parent to offspring. Variation: It must be capable of changes to account for the known phenotypic variation in each species. The data of many geneticists, including Mendel, were consistent with these four properties; however, the chemical nature of the genetic material cannot be identified solely by genetic crosses. Experiments with Pneumococcus suggested that DNA is the genetic material. Frederick Griffith Experiments with the bacterium Streptococcus pneumoniae. Streptococcus pneumoniae comes in two strains. S Smooth Secrete a polysaccharide capsule (protects bacterium from the immune system of animals). Produce smooth colonies on solid media. R Rough Unable to secrete a capsule. Produce colonies with a rough appearance. In addition, the capsules of two smooth strains can differ significantly in their chemical composition (see fig. 9.1). Rare mutations can convert a smooth strain into a rough strain, and vice versa; however, mutations do not change the type of the strain.
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2 In 1928, Griffith conducted experiments using two strains of Streptococcus pneumoniae : type IIIS and type IIR (see fig. 9.2). Inject mouse with live type IIIS bacteria. Mouse died. Type IIIS bacteria recovered from the mouse’s blood. Inject mouse with live type IIR bacteria. Mouse survived. No living bacteria isolated from the mouse’s blood. Inject mouse with heat-killed type IIIS bacteria. Mouse survived. No living bacteria isolated from the mouse’s blood. Inject mouse with live type IIR + heat-killed type IIIS cells. Mouse died. Type IIIS bacteria recovered from the mouse’s blood. Griffith concluded that something from the dead type IIIS was transforming type IIR into type IIIS; he called this process transformation. The substance that allowed this to happen was termed the transformation principle; Griffith did not know what it was. The nature of the transforming principle was determined using experimental approaches that incorporated various biochemical techniques. The Experiments of Avery, MacLeod and McCarty. Avery, MacLeod and McCarty realized that Griffith’s observations could be used to identify the genetic material. They did their experiments in the 1940s; at that time, it was known that DNA, RNA, proteins and carbohydrates are major constituents of living cells. They prepared cell extracts from type IIIS cells containing each of these macromolecules.
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3 Only the extract that contained purified DNA was able to convert type IIR into type IIIS. Avery
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This note was uploaded on 11/12/2010 for the course BIO 49180 taught by Professor Bierner during the Fall '10 term at University of Texas.

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Molecular Structure of DNA and RNA - Molecular Structure of...

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