Induction of mutation in neurospora crassa and yeast

Induction of mutation in neurospora crassa and yeast

• 1.
  Induction of mutationin Neurospora crassa
• 2.
  Why bread moldis great for experiments  Neurospora crassa was one of the first eukaryotic microbes to be adopted by geneticists as a model organism.  Beadle had planned to work with (fruit fly) Drosophila . However, as he got more interested in the connection between genes and metabolism, he realized that Neurospora might give him a better way to answer the questions he was curious about.  why neurospora bcz Neurospora had a fast and convenient life cycle, one with both haploid and diploid phases that made it easy to do genetic experiments.
• 3.
  Mutation analysis  Garrod’s"inborn errors of metabolism’’ gAEruhd  Sir Archibald Garrod, an English medical doctor working during 20th century, and was the first to draw a connection between genes and biochemistry in the human body.  Garrod worked with patients who had metabolic diseases and saw that these diseases often ran in families. So He focused on patients who has alkaptonuria.  This is a non-fatal disorder where a person’s urine turns black because they cannot break down a molecule called alkapton (which, in normal people without the disorder, gets broken down into other, colorless molecules).  the nature of a gene was not fully understood at the time, by Garrod or anyone else . He is considered as "the father of chemical genetics" and he was , the first to have linked genes with the enzymes that carry out metabolic reactions.
• 4.
  Beadle and Tatum:Connecting genes to enzymes  Garrod's ideas went largely unnoticed in his own time . George Beadle and Edward Tatum, carried out a series of experiments on Garrod's work .  Beadle and Tatum worked with a simple organism: common bread mold, or Neurospora crassa. Using Neurospora, they were able to show a clear connection between genes and metabolic enzymes. There are 2 medium which they have used – minimal medium and complete medium  The cells could grow on minimal medium, a nutrient source with just sugar, salts, and one vitamin (biotin). Neurospora cells can survive on this medium while many other organisms (such as humans!) cannot.  That’s because Neurospora has biochemical Pathways that turn sugar, salts, and biotin into all the other building blocks needed by cells (such as amino acids and vitamins).
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   Neurospora cellswill also grow on complete medium, which contains a full set of amino acids and vitamins.  If genes were connected to biochemical enzymes, Beadle and Tatum reasoned that it should be possible to induce mutations, or changes in genes, that "broke" specific enzymes (and thus, specific pathways) needed for growth on minimal medium.  A Neurospora line with such mutation would grow normally on complete medium, but would lose the ability to survive on minimal medium.  To look for mutants like this, Beadle and Tatum exposed Neurospora spores to radiation (x-ray, UV, or neutron) to make new mutations.  After a few genetic steps, they took descendants of the irradiated spores and grew them individually in test tubes containing complete medium.
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   Once eachspore had established a growing colony, a small piece of the colony was transferred into another tube containing minimal medium.  Most colonies grew on either complete or minimal medium. However, a few colonies grew normally on complete medium, but couldn't grow at all on minimal medium.  On minimal medium, each mutant would die because it could not make an particular essential molecule out of the minimal nutrients.  Complete medium would allow the mutant it to live by providing the missing molecule, along with a variety of others.
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   To figureout which metabolic pathway was "broken" in each mutant, Beadle and Tatum performed , two-step experiment.  First, they grew each mutant on minimal medium supplemented with either the full set of amino acids or the full set of vitamins (or sugar’s).  If a mutant grew on the vitamin medium but not the amino acid medium, it must be unable to make one or more vitamins.
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   Beadle andTatum further pinpointed the "broken" pathway in each mutant through a second round of tests. if a mutant grew on minimal medium containing all 20 amino acids, they might next test it in 20 different vials, each containing minimal medium plus just one of the 20amino acids.  If the mutant grew in one of these vials, Beadle and Tatum knew that the amino acid in that vial must be the end product of the pathway disrupted in the mutant.  In this way, Beadle and Tatum linked many nutritional mutants to specific amino acid and vitamin biosynthetic pathway.
• 9.
  "One gene-one enzyme" Their work produced a revolution in the study of genetics and showed that individual genes were indeed connected to specific enzymes.  "One gene-one enzyme’’ -The initially discovered link between genes and enzymes was called the “one gene-one enzyme” hypothesis.  For their work on Neurospora biochemical mutants they concluded that each gene controls a single specific biochemical reaction.  Genes control chemical reactions by controlling the production of specific enzymes that catalyze these reactions.  One gene one enzyme hypothesis simply states that each gene specifies a single specific enzyme.
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  Reference :  https://www.khanacademy.org/science/biology/gene-expression- central-dogma/central-dogma-transcription/a/one-gene-one- enzyme-hypothesis https://www.mun.ca/biology/scarr/Gr09-01.html  http://nlhawksapbiology.weebly.com/chapter-17.html  http://www.biologydiscussion.com/genetics/use-of-neurospora-in- understanding-genetical-principle/12083