A.  DNA Experiments
     1.  Frederick Griffith - 1928 - studied mice infected with pneumonia caused by two different bacterial strains
          a.  Virulent - contained DNA coding for production of a capsule to protect bacteria from mouse's immune system; therefore, the mouse would die
          b.  Nonvirulent - did not contain DNA coding for capsule production; therefore, the mouse would survive
          c.  Griffith's Experiment - injected mice with four different treatments of the above bacterial strains
               1) Living, virulent bacteria ---- Death of Mouse

               2) Living, nonvirulent bacteria --- Survival of Mouse

                               
               3) Heat-killed, virulent bacteria --- Survival of Mouse
                                   

               4) Heat-killed, virulent + living, nonvirulent --- Death of Mouse

                   

          d.  Griffith's Conclusion - there was a transfer of genetic information between bacterial cells (bacterial transformation)

     2.  Oswald Avery - 1944 - showed that Griffith's bacterial transformation only occurred with DNA and not protein
          a.  Avery's Conclusion - DNA is the genetic material
          b.  Scientific Dissent - the scientific community did not accept Avery's conclusion

     3.  Hershey & Chase - 1952 - worked with bacteriophages (viruses, made only of DNA and protein, that infect bacteria)
          a.  Hypothesis - bacteriophages inject their DNA into bacteria in order to produce new bacteriophages; therefore, DNA is the genetic material
          b.  Experiment - used radioactive labeling of bacteriophage
               1)  Labeled Phosphorus - DNA contains Phosphorus; therefore, since this label was found inside the cell, DNA moves into bacteria during viral infection
               2)  Labeled Sulfur - Proteins contain Sulfur; therefore, since this label was found outside the cell, proteins do not move into bacteria during viral infection
       
          c.  Conclusion  - Since only the radioactively labeled Phosphorus entered the bacterial cell, DNA must be the genetic material.

B.  Structure & Organization of DNA
     1.  Nucleotides - subunits (monomers) of DNA
          a.  Deoxyribose - five carbon sugar
          b.  Phosphate - one, two or three phosphate groups
          c.  Nitrogenous Base - one of four possible molecules
               1)  Adenine - purine base - two rings
               2)  Thymine - pyrimidine base - one ring
               3)  Guanine - purine base - two rings
               4)  Cytosine - pyrimidine base - one ring
          d.  Covalent Bonds - joins the nucleotides together between the sugars and the phosphate groups

                       

     2.  Chargaff's Rule - A = T and C = G in regards to amount

     3.  Rosalind Franklin - used x-ray diffraction to determine that DNA was double-stranded and helical in shape

                       

     4.  Watson & Crick - finalized the overall structure of DNA
           a)  Double-stranded
           b)  Double Helix
           c)  Sides of Ladder = sugars + phosphates
           d)  Rungs of Ladder = complementary base pairs

                       

     5.  Antiparallel Orientation - the two strands of DNA run in opposite directions
          a)  3' end - hydroxyl group of deoxyribose on the #3 carbon
          b)  5' end - phosphate group of deoxyribose on the #5 carbon

   

    6.  Prokaryotes vs. Eukaryotes
          a)  Prokaryotes - one large circular strand of double-helical DNA

          b)  Eukaryotes - multiple chromosomes each containing double-stranded, double-helical DNA

C.  DNA Replication


      1.  Base Pairing - nucleotide sequence of one DNA strand provides the information to create a second strand
           a)  A pairs with T
           b)  C pairs with G
           c)  template - strand used to direct DNA replication
           d)  example:
                template = A  A  T  C  G  A  C  C  T  A  G
                copy =       T  T  A  G  C  T  G  G  A  T  C



      2.  DNA Polymerase - enzyme that joins DNA nucleotides together during replication
           a)  makes very few errors
           b)  proofreading enzymes - check new DNA strand for complementary base pairing just in case DNA Polymerase makes an error

      3.  Prokaryotes vs. Eukaryotes
           a)  Prokaryotes - one origin of replication that proceeds bidirectionally around the circular chromosome

                       

           b)  Eukaryotes - multiple origins on chromosomes; allows for faster replication of a much larger DNA amount

      4.  Detailed Explanation
           a)  Steps of DNA Replication
                1)  DNA Helicase - unwind and unzip DNA double helix
                2)  Replication Fork - y-shaped structure of DNA formed after the action of DNA Helicase


                3)  DNA Polymerase - adds DNA nucleotides to the replicated strand by forming Phosphodiester Bonds between the sugar of one nucleotide and the phosphate of another nucleotide

                       

                4)  Restrictions of DNA Polymerase:
                     a.  Can only add nucleotides to an already growing strand of DNA
                          1.  RNA Primer - short segment of RNA that begins the new strand of replicated DNA
                          2.  RNA Primase - enzyme that catalyzes the production of RNA Primer

                     b.  Can only add nucleotides in the 5' to 3' direction
                          1.  Leading Strand - replicated continuously by DNA Polymerase's 5' to 3' movement down the strand; the template for the leading strand is the original 3' to 5' DNA strand


                          2.  Lagging Strand - replicated discontinuously
                               a)  Okazaki fragments - segments of nucleotides that must be added in chunks by DNA Polymerase moving in the 5' to 3' direction; the template for the lagging strand is the original 5' to 3' DNA strand
                               b)    DNA Ligase - enzyme that joins together Okazaki fragment to form a continuous replicated strand of DNA



            b)  Semiconservative DNA Replication - both replicated DNA molecules contain one strand from the original DNA molecule and one strand of newly replicated DNA
                 1)  Other Options
                      a.  Conservative Replication - after replication, one DNA molecule contains two strands from the original DNA and the other DNA molecule contains two strands from the newly replicated DNA
                      b.  Dispersive Replication - both replicated DNA molecules contain pieces of the original DNA and pieces of the newly replicated DNA

           

                2)  Meselson & Stahl Experiment - used heavy and light forms of nitrogen in the presence of growing bacteria (which also means DNA replication) and found that both types were present in the offspring; therefore, replication must be semiconservative

D.  DNA Damage
                             1.  Causes - many different substances can cause DNA damage
            a)  Ultraviolet light (sun, tanning beds, etc.)
            b)  X-rays (doctor, dentist, etc.)
            c)  Extreme heat (burn victims in fires)

      2.  Repair Enzymes - use undamaged template strand to repair damaged DNA through complementary base pairing

      3.  Mutations - inheritable changes in DNA molecules
           a)  Chromosome Level Mutations
                1)  Attachment
                2)  Loss of entire chromosome
                3)  Polyploidy - extra chromosomes
           b)  Nucleotide Level Mutations
                1)  Substitution
                2)  Insertion
                3)  Deletion
                4)  Inversion (part of DNA inserted backwards)
           c)  Mutagen - any substance that causes mutation
                1)  Examples - radiation, nicotine, heroine, etc.
                2)  Results - somatic AND sex cells can be affected
           d)  Benefit - increases genetic variation; thus, provides the raw material for evolution
           e)  Harms - most mutations are harmful if not deadly; genetic engineering is currently trying to determine causes and cures for most mutations affecting humans

E.  Structure of Eukaryotic Chromosomes 
      1.  Structure - organized into chromosomes which are made of DNA and proteins

      2.  Components:
            a)  Histones - chromosomal proteins that protect DNA from breaking and tangling by winding DNA around themselves
            b)  Nucleosomes - structure composed of DNA wrapped around clusters of histones to allow for tight packaging of DNA into chromosome
                   

F.  Genome Organization
      1.  Components - total of all genes in a cell or organism
           a)  Transposable Gene (Jumping gene) - segment of DNA that can copy and insert itself into another area of a genome; involved in antibiotic resistance in bacteria
           b)  Repetitive DNA - multiple copies of genes coding for proteins which are used in large amounts by the organism

      2.  Human Genome Project - U.S. Government funded project whose goal is to sequence the entire genome of human beings

** For more information on the HGP, please click on the link below.
                             
                        

G.  The Central Dogma

                  

To test your knowledge about DNA, click on the DNA Questions Link at the top of this page.  After you answer the questions, be sure to check your responses by clicking on the DNA Answers Link.

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