17 Genes _ How they work Part I 09

17 Genes _ How they work Part I 09 - Biol 61- Genes &...

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Part I (Chapter 17) 2/27/09 From our study of Mendel’s work we can certainly see the connection between information carried by genes on chromosomes and the appearance of specific traits in an organism. But how to we get from one to the other? In other words, we know that DNA is the genetic material, but how is information actually contained or carried within the DNA, and how do we get at this information and turn it into all the proteins (and functional RNAs) that are needed for cells to function and for the growth and development of an organism? The first evidence for what a gene was likely to be came from work by Garrod, a physician studying the disease alkaptonuria. This is a defect in which a person builds up high levels of homogentisic acid in the urine, and this makes it turn black when exposed to air (just for your information, the acid is formed in the pathway used for breaking down an excess of the amino acid phenylalanine, and the defect is considered “largely harmless” from what I have read). Now alkaptonuria segregates like a recessive trait according to Mendel’s rules, and Garrod reasoned that people with alkaptonuria build up a lot of homogentisic acid because they lack an enzyme necessary to break down the homogentisic acid into smaller products in the urine. So he suggested that the trait (alkaptonuria) was due to the lack of an enzyme that was needed to break down homogentisic acid - he speculated that genes were responsible for making enzymes . George Beadle and Edward Tatum took this idea a step further. They looked for mutations affecting growth in the bread mold Neurospora . They were easily able to isolate many different kinds of mutants in this mold ( Neurospora is a haploid fungus with only one allele of each gene). By characterizing their mutant cells B & T tied together the function of a specific protein with an actual location on the DNA – they showed that effects on DNA (mutations) could lead to effects on the activity of an enzyme. From this they concluded that individual genes carry instructions for making individual enzymes, in other words they came up with the one gene-one polypeptide hypothesis . A better definition of a gene that we will use is: a gene is the specific sequence of DNA nucleotides that contains the information for making the primary structure of a polypeptide (or several related polypeptides) in the right place at the right time in the proper amounts in the cell or organism. As we will discuss later, sometimes a gene is just needed for the synthesis of just a polymer of RNA. So genes contain the information for building polypeptides. How do we get from DNA to protein? This is outlined by the Central Dogma of Biology, which explains the information flow from DNA to polypeptide (how the genetic material is turned into something useful). To sum it up simply, it states that specific information encoded by the sequence of nucleotides in the DNA of a gene is copied into a polymer of RNA by the process of transcription
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17 Genes _ How they work Part I 09 - Biol 61- Genes &...

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