16 DNA Replication Ch.16 2009

16 DNA Replication Ch.16 2009 - Biol 61 DNA DNA...

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(Chapter 16) An organism has a phenotype , which is the physical appearance/physical characteristics of that organism. The phenotype of an organism is determined by its genotype , the alleles of the various genes that an organism carries on its chromosome or chromosomes. We now know that genes are the instructions for making the various polypeptides (and functional RNAs, which we’ll talk about for the next chapter) that are used in each cell of an organism. Now, of course, we know that each individual gene is comprised of a specific sequence of deoxyribonucleotides found along a portion of the DNA polymer that makes up a chromosome. But what evidence led us down the path to understanding what the “hereditary material” was in each cell and where it was located? You can read the first part of the chapter on your own to look at the experimental evidence for DNA as the genetic material. I will not be lecturing on these experiments, but they are described in my notes - read them over and understand them (you will see a couple of questions on them in the practice exams for Exam 2). In the 1930’s Joaquim Hammerling discovered that the hereditary material resided in the nucleus of a eukaryotic cell. The nucleus contains chromosomes, but they are made up of both nucleic acids and proteins, so what was the important part? Many people predicted that proteins were carrying the genetic information because they seemed so much more chemically complex than DNA. In the 1940’s Oswald Avery, Colin MacLeod and Maclyn McCarty showed that what was important was the DNA. They started with work carried out originally by Frederick Griffith. Griffith was conducting experiments with bacteria that can cause pneumonia – Streptococcus pneumoniae . He had found that if you inject live S. pneumoniae bacteria from the “smooth” strain (or S strain) into a mouse, the mouse would die. When he injected the mice with a mutant “rough” strain of live S. pneumoniae bacteria (called the R strain), the mice survived. (The R strain made “rough” looking, irregular colonies when grown on media in a petri dish, while the S strain made nice smooth round colonies. It turns out the R strain lacked the polysaccharide coat that made the S strain “smooth” and dangerous as a pathogen – something that causes a disease). If Griffith injected mice with killed (boiled!) S strain, they would live. However, when he mixed dead S bacteria with live R bacteria, he now found that the mice died. Griffith then found live S strain bacteria in the blood of the mice. Somehow, the dead S strain had “transformed” the live R strain into the dangerous, virulent S form. When boiled S strain extract was added to live R strain bacteria, the S strain extract was capable of producing a heritable change in the live R strain – it had “ transformed ” the phenotype and genotype of the R strain into an S strain. In 1944, Avery and pals wanted to understand what this “transforming principle” was.
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16 DNA Replication Ch.16 2009 - Biol 61 DNA DNA...

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