CHAPTER 16 How Genes Work 3 9 2014 gene expression o o process of translating information in DNA into functioning molecules within cell genes contain instructions for making maintaining an individual nonfunctioning alleles knock out null loss of function alleles George Beadle Edward Tatum wanted to discover the importance of genes by making them defective one gene one enyzme hypothesis each gene contains information needed to make an enzyme o o wanted to knock out a gene by damaging it and then observing phenotype of mutant damaged genes in bread mold Neurospora crassa defects in genes made the mold unable to produce specific proteins Crick proposed the Central Dogma of Molecular Biology o DNA is an information storage molecule w a code of base sequences in DNA o o o different combinations of bases specify 20 amino acids genes contain information to specify amino acid sequence of one protein information encoded in base sequence of DNA not directly translated into amino acid sequence of proteins Francois Jacob Jacques Monod proposed o RNA molecules are the link btwn genes found in cell s nucleus protein manufacturing centers in cytoplasm mRNA messenger RNA carries information from DNA to site of protein synthesis enzyme RNA polymerase synthesizes RNA according to information provided by sequence of bases in particular stretch of DNA Central Dogma summarizes flow of information in cells genes ultimately code for proteins o DNA codes for RNA RNA codes for proteins o sequence of bases in stretch of DNA specifies sequence of bases in RNA molecule which specifies sequence of amino acids in a protein genotype determined by sequence of bases in DNA phenotype product of proteins and organism produces alleles of the same gene differ in DNA sequence o o o proteins produced by different alleles of same gene differ in amino acid sequence o work forward or backward in central dogma helps to predict codons amino acid sequence encoded by particular DNA sequence approximate mRNA and DNA sequence that would code for a particular sequence of amino acids Exceptions to Central Dogma o many genes code for RNA molecules that do not function as mRNAs and are not translated into proteins other RNAs perform important functions in cell o sometimes information flows in opposite direction RNA DNA o some viral genes composed of RNA and use reverse transcriptase viral polymerase to synthesize DNA version of virus s RNA genes 3 9 2014 transcription the process where hereditary information in DNA is copied to RNA o DNA is transcribed to messenger RNA by RNA polymerase translation the process where order of nucleotide bases converted to order of amino acids o MRNA is translated to protein template strand vs non template coding strand genetic code contains rules that specify relationship btwn sequence of nucleotide bases in DNA or RNA contains the corresponding sequence of amino acids in a protein redundant all amino acids except 2 encoded by more than one codon unambiguous one codon never codes for more than one amino acid universal all codons specify same amino acids in all organisms with a few minor exceptions conservative first 2 bases usually identical when multiple codons specify the same amino acid o o o o o o o o George Gamow predicted that each word in the genetic code contains three bases 20 amino acids only 4 RNA bases 3 base code triplet code is the least that could specify enough amino acids could code for 43 64 diff amino acids 3 base code provides more than enough messages to code for all 20 amino acids triplet code is redundant some amino acids specified by more than one triplet code triplet code aka codon group of 3 bases that specifies a particular amino acids start codon AUG start of protein encoding sequence of mRNA stop codons UGA UAA UAG end of protein coding sequence Crick Brenner found that the reading frame sequence of codons of a gene could be destroyed by mutation and then restored if the total number of deletions or additions were multiples of 3 3 9 2014 mutation o o any permanent change in organism s DNA modification in cell s information archive change in genotype ability to create new alleles categories of mutations most are neutral slightly deleterious beneficial mutations increase fitness of organism neutral mutations do not affect organism s fitness silent mutations deleterious mutations decrease fitness of organism o point mutations occur when DNA polymerase inserts wrong base into newly synthesized DNA strand single base change result in change in DNA base sequence occur if the DNA polymerase proofreading and mismatch repair systems fail missense replacement mutations changes in the amino acid sequence of encoded protein silent mutations do not change amino acid sequence of gene product o chromosome level mutations larger in scale result from addition deletion of chromosome from individual s karyotype visualized via karyotype of cell polyploidy increase in of each type of chromosome aneuploidy addition or deletion of a chromosome inversions sections of a chromosome break and rotate before rejoining translocation when a broken section of one chromosome becomes attached to another
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