Central Dogma

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University of Massachusetts Amherst
Mark Huyler

Central Dogma Key concepts:  Most genes code for proteins  DNA is transcribed to messenger RNA (mRNA) by RNA polymerase and then mRNA is translated to proteins by ribosomes. In this way, genetic information is converted from DNA to RNA to proteins  Each amino acid in a protein is specified by a group of three bases (codon) in mRNA  Mutations are random changes in DNA, ranging in extent from single bases to large chromosomes regions. These changes may or may not produce changes in the phenotype Introduction:  While the work of early geneticists including Watson and Crick and others, illuminated the structure of DNA and genes, and the method of inheritance, biologists still do not understand how gene expression occurred o Gene expression is the process of translating the information in DNA into functioning molecules within the cell. What do genes do?  Early advances showed that genes carry the instructions for making and maintaining an individual  In order to infer what a particular gene does, George Beadle and Edward Tatum proposed damaging a gene, creating a mutant, and then observing the resulting effect on the mutant’s phenotype  Nonfunctioning alleles are now called knock-out, null, or loss-of-function alleles The One-Gene, One-Enzyme Hypothesis  Credited to George Beadle and Edward Tatum as a result of a paper they published in 1941  Damaged genes in the bread mold Neurospora crassa, and observed the defects in the particular gene resulted in the mold’s inability to produce specific proteins  The results of their experiments inspired their one- gene, one-enzyme hypothesis,  Srb and Horowitz further tested the one-gene, one- enzyme hypothesis by examining the production of the amino acid arginine by neurospora crassa  Arginine is produced via a metabolic pathway requiring the action of three different enzymes. Srb and Horowitz hypothesized that different genes  To test their hypothesis, Srb and Horowitx used radiation to create thousands of mutant individuals and then performed a genetic screen, which allowed them to select those mutants incapable of producing arginine  The results supported the one-gene, one-enzyme hypothesis: o Three distinct mutants were produced, each deficient in one of the three enzymes in the arginine metabolic pathway  Biologists finally understood what most genes do. They contain the instructions for making proteins  as long as you add the required enzyme then it will grow even if the pathway is ineffective Below is a set of results showing the growth of 3 strains of Neurospora on several media. The experimental design is similar to that used by Beadle and Tatum: 2 of the 3 strains are mutants for a metabolic pathway whose compounds include A and B. MM = minimal medium without additives. A “+” represents growth while a “–“ indicates no growth. Based on these data, select the correct statement. Medium Strain MM MM1A MM1B t409– + + t410 + + + r3 – + – a. Strain t409 is the nonmutant strain, and the last substance in the pathway is “B”. b. Strain t410 is the nonmutant strain, and the last substance in the pathway is “B”. c. Strain r3 is the nonmutant strain, and the last substance in the pathway is “B”. d. Strain t410 is the nonmutant strain, and the last substance in the pathway is “A”. *t410 is the nonmutant strain because everything grows regardless of additives. The last substance in the pathway is A because there was repeated growth (growth in t409/t410 and r3) The Central Dogma of Molecular Biology:  Francis Crick proposed that DNA is an information storage molecule, and that the sequence of bases in DNA is a kind of code in which different combinations of bases could specify the twenty amino acids  A particular stretch if DNA (gene) contains the information to specify the amino acid sequence of one protein  The information encoded in the base sequence of DNA is not directly translated into the amino acid sequence of proteins RNA – the intermediary between genes and proteins  Francois Jacob and Jaques Monod (1961) proposed that RNA molecules act as a link between genes, found in the cell’s nucleus, and the protein-manufacturing centers, located in the cytoplasm  Messenger RNA (mRNA) was found to carry information from DNA to the site of protein synthesis  The enzyme RNA polymerase synthesizes RNA according to the information provided by the sequence of bases in a particular stretch of DNA The central Dogma  The central dogma summarizes the flow of information in cells. It states that DNA codes for RNA, which codes for proteins: DNA RNA proteins  The sequence of bases in a particular stretch of DNA specifies the sequence of bases in an RNA molecule, which specifies the sequence of amino acids in a protein, in this way, genes ultimately code for proteins The roles of Transcription and Translation  DNA is transcribed to messenger RNA by RNA polymerase o Transcription is the process by which the hereditary information to DNA is copied to RNA  mRNA is then translated to protein o translation is the process wherein the language of nucleic acids, the order of the nucleotide bases, is converted to the language of proteins, the order of amino acids. DNA (information storage) Transcription mRNA (information carrier) Translation Proteins (active
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