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How Genes Work

How Genes Work. Chapter 15. What do genes do?. How do genes work?. Genes carry the instructions for making and maintaining an individual But how is this information translated into action? How does an organism’s genotype specify its phenotype?. RR=. Garrod.

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How Genes Work

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  1. How Genes Work Chapter 15

  2. What do genes do?

  3. How do genes work? • Genes carry the instructions for making and maintaining an individual • But how is this information translated into action? • How does an organism’s genotype specify its phenotype? RR=

  4. Garrod • Provided the first clue to gene function • studied alkaptonuria, a disease in which homogentisic acid is secreted in the urine. • Hypothesized that the metabolicpathway in which homogentisic acid is an intermediate must be blocked in alkaptonurics • Block due to lack of an enzyme that breaks down homogentisic acid, leading to its buildup.

  5. Garrod

  6. George Beadle and Edward Tatum • Developed the one-gene, one-enzyme hypothesis from Garrod’s work • Each gene carries the information for one protein or enzyme. • Did experiments using red bread mold Neurosporacrassa • Irradiated mold to create mutants • Tested if they could grow on minimal media • Adrian Srb and Norman Horowitz tested for arginine

  7. Adrian Srb and Norman Horowitz

  8. Adrian Srb and Norman Horowitz • Followed work by Krebs showing that cells from mammals synthesize arginine via a series of steps and that ornithine and citrulline act as intermediates • Enzymes are required to convert ornithine to citrulline and citrulline to arginine

  9. Adrian Srb and Norman Horowitz • Set out to link the biochemistry to genetics • Used a geneticscreen: • Technique for picking certain types of mutants out of many thousands of randomly generated mutants • Isolated knock-out mutants • inable to grow in the absence of arginine • Indicating that the biosynthetic pathway was not functioning properly

  10. Adrian Srb and Norman Horowitz • Grew the mutants on intermediates of arginine pathway to figure out where pathway was blocked • Mutants were able to grow in the presence of an intermediate in the pathway downstream, but not upstream, of the blocked step • Demonstrated that each type of mutant lacked the enzyme responsible for a certain step in the pathway

  11. The Gene • Crick proposed that the sequence of the bases in DNA is a kind of code • Information storage molecule • A particular stretch of DNA (a gene) contains the information to specify the amino acid sequence of only one protein.

  12. RNA • Francois Jacob and Jacques Monod proposed the Messenger RNA (mRNA) hypothesis • RNA is the intermediary between DNA and protein • Carries information from DNA to the site of protein synthesis

  13. RNA • RNA polymerase synthesizes mRNA by reading DNA sequences • Transcribes one strand of the DNA, making a complementary RNA copy

  14. The Genetic Code • George Gamow predicted that each word (codon) in the genetic code would be three nucleotides long • Minimum code length that could specify the 20 different amino acids found in proteins • This triplet code is redundant, with some amino acids being specified by more than one codon

  15. The Genetic Code

  16. The Genetic Code • FrancisCrick and colleagues established that the code is indeed based on three nucleotides for each amino acid • Used insertion and deletion mutations • Reading frame (sequence of codons) of a gene could be destroyed by mutation • Restored if the total number of deletions or additions were multiples of three

  17. The Genetic Code

  18. DNA to Protein

  19. DNA • Long-term information storage unit • Allows for the stable maintenance of the information and its passage from generation to generation • RNA carries the information from the DNA to the translation machinery (ribosomes) • RNA is ephemeral • Allows for preservation of the DNA • Proteins carry out cellular functions

  20. Genes ultimately code for proteins: • DNA is the hereditary material. • Genes consist of specific stretches of DNA. • The sequence of bases in DNA specifies the sequence of bases in an RNA molecule. • Groups of three bases within a sequence of mRNA specify the sequence of amino acids in a protein.

  21. Proteins • Most proteins function as enzymes • Motor proteins and contractile proteins move the cell itself or cellular cargo. • Peptide hormones carry signals from cell to cell. • Transport proteins conduct specific ions or molecules across the plasma membrane. • Antibodies and other immune system proteins recognize and destroy invading viruses and bacteria

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