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Chapter 11

BIOA01 Chapter 11

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Department
Biological Sciences
Course Code
BIOA01H3
Professor
Mark Fitzpatrick

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Biology Notes – Chapter 11 October 27, 2010 11.1 What is the Evidence that the Gene is DNA? - the presence of genes were associated with chromosomes by the 1920’s - chromosomes were made up of DNA and proteins - a new dye was developed that could bind specifically to DNA and turned red in direct proportion to the amount of DNA present in a cell - DNA was the genetic material because o It was in the right place – DNA was an impt component of the nucleus and chromosomes o Varied among species – own specific amount of DNA o It was present in the right amounts – amount of DNA in somatic cells was twice that in sex cells DNA from one type of bacterium genetically transforms another type - Frederick Griffith was studying the bacterium Streptococcus pneumonia (causes pneumonia in humans)  trying to determine a vaccine against it o Cells of the S strain produce colonies that looked smooth  protected from attack by a host’s immune system  cells cause pneumonia when injected into the mice (virulent) o Cells of the R strain produced colonies that looked rough  lacked protective capsule and did not cause pneumonia o Come mice with heat killed S pneumocci did not produce infection but the ones with a mixture of R bacteria and heat killed S bacteria, the mice died with pneumonia  When he looked at blood from these mice he found living bacteria with characteristics of virulent S strain  Some of the R strain had been transformed into the virulent S strain, because they were in the presence of the dead bacteria  Therefore when you put R strain and heat killed S strained together it yields the same transformation  A chemical transforming principle from the dead S strain can cause a heritable change in the affected R cells The transforming principle is DNA - Oswald Avery was the one that found the transforming principle - Treated the samples to contain this transforming principle in ways to destroy different molecules like proteins, nucleic acids, carbs and lipids o If DNA is destroyed there is loss in the transforming activity  destroying any other molecules did not result in the same effect o Was not always believed  DNA was not as genetically variant like protein could be  Did not know if bacteria even contained DNA Viral replication experiments confirmed that DNA is the genetic material - Hershey – Chase experiment o Used to determine whether DNA or proteins were the genetic material o Bacteriophage virus consists of a DNA core packed inside a protein coat o When this virus attacks a bacteria and enters the bacterial cell o The cell then burst 20 mins later releasing dozens of viruses  the virus is able to replicate itself in the bacteria o Wanted to figure whether the DNA component or the protein component of the virus enters the cell - Proteins contain more sulphur  not present in DNA  grew bacteriophage in the presence sulphur - DNA is rich with phosphorus  not present in DNA  grew another batch of bacteriophage in the presence of phosphorus - The P- bacteriophage infected one bacteria, while the S-bacteriophage infected another bacteria - The then agitated both bacteria’s in a kitchen blender, to get rid of parts of the virus that had not penetrated the bacteria - They separated the bacteria from the rest of the material in a centrifuge  causes solutes or particles to separate and form a gradient according to their density - The lighter remains of the virus ( the ones not penetrated by the bacteria) were captured in the fluid while the heavier bacterial cells fell to the bottom - The fluid contained more sulphur whereas the phosphorus had stayed with the bacteria o Suggests that DNA had been transferred to the bacteria and was responsible for redirecting the genetic program of the bacterial cell Eukaryotic cells can be genetically transformed by DNA - Wanted to see whether or DNA was the genetic material in more complex eukaryotes - Can be seen using transfection o Use a gene marker, a gene whose presence in the recipient cell confers an observable phenotype o Researches usually use a nutritional or antibiotic resistance marker that permits the growth of genes in transformed cells but not in nontransformed cells  any cell can be transfected  results in a transgenic cell o Thymidine example in textbook 11.2 What is the Structure of DNA? The chemical composition of DNA was known - DNA was a polymer of nucleotides - Each nucleotide consists of a sugar deoxyribose, a phosphate group and nitrogen containing base - The difference between the nucleotides are the bases o Adenine, cytosine, guanine and thymine - Erwin Chargoff found that DNA from different species exhibit certain regularities - The amount of A always equals the amount of T - The amount of G always equals the amount of C - The total abundance of (A+G) = the total abundance of (T + C) Watson and Crick described the double helix - Watson and Crick tried to combine all that they learned so far about DNA structure into a single coherent model - They were convinced that DNA had a helical structure , measurements suggested that there are two polynucleotide chains in the molecule - In 1953, built a model out of tin that established the general structure of DNA \ Four Key features define DNA structure - It is a double stranded helix of uniform diameter - It is right handed - It is antiparallel (two strands run in opposite direction) - The outer edges of the nitrogenous bases are exposed in the major and minor grooves The Helix - The sugar-phosphate backbone are the outside of the helix and the nitrogenous bases point toward the centre - The two chains are held together by hydrogen bonds - Adenine pairs with thymine forming two hydrogen bonds - Guanine pairs with cytosine by forming three hydrogen bonds - Complementary base pairing = each pair is made up of one purine and one pyrimidine - The stacking of the base pairs in the centre is stabilized by hydrophobic interactions = more stability Antiparallel Strands - What does it mean by antiparallel? - The phosphate group connect to the 3 carbon of one deoxyribose molecule and the 5 carbon of the next = linking successive sugars together - At one end of the chain is a free 5 phosphate group and at the other end is a free 3 phosphate group - In a double helix, the 5 end of one strand is paired with the 3 end of the other strand Base exposure at the grooves - There are major and minor grooves in the double helix, the outer edges of the flat hydrogen-bonded base pairs are accessible for potential hydrogen bonding o Ex. A protein could come and bind on to it The double helical structure of DNA is essential to its function - The genetic material stores an organisms genetic info - The genetic material is susceptible to mutation - The genetic material is precisely replicated in the cell divison cycle - The genetic material is expressed as the phenotype o Provide the information required to make proteins, and proteins provide much of the phenotype for an organism 11.3 How is DNA replicated? Three modes of DNA replication appeared possible - The prediction that DNA contained the information required for its own replication was founded by Arthur Kornberg - DNA can be synthesized in a test tube containing three substances o The substrates dATP, dCTP, dGTP, Dttp o DNA polymerase o DNA which serves as a template to guide the incoming nucleotides - Three possible replication patterns o Semiconservative – parent strand serves as a template for the new strand, the two DNA molecules each have one old strand and one new one o Conservative – original double helix serves as a template but does not contribute to the new helix o Dispersive – fragments of original DNA serve as template forming a two new molecules that contains old and new parts at random Mesalson and Stahl demonstrated that DNA replication is semiconservative - Developed an experiment that involved the use of a heavy isotope of nitrogen o This isotope makes molecules containing it more dense then ones identical to it o One culture was grown in a medium with N15 and all the bacteria was heavy o One culture was grown in the medium containing N14 and all the DNA in these bacteria was “light” o Both were centrifuged and formed two separate DNA bands with different densities o Then grew another culture in the N15 medium and tr
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