Honors Biology The History Of DNA

Honors Biology

Quick Review  Chromosomes are made up of DNA and Proteins.  Genes are a segment of the DNA molecule that carries the instructions for producing a specific traits.

Honors Biology DNA or Proteins?  Prior to the 1950’s, scientists could not decide whether DNA or protein was the material responsible for heredity.  3 experiments would lead to the answer:  1. Frederick Griffith  2. Avery, McCarty & MacLeod  3. Hershey & Chase 1952)

Honors Biology Griffith’s Experiment FFrederick Griffith was working to find a cure for Streptococcus pneumonia (bacteria) TTest Subjects: lab mice HHe used different strains (types) of bacterium in this experiment. 1928

Honors Biology Strains of Bacteria Used  S Bacteria = Disease Causing (pathogenic)  Have protective outer coat that “hides” the bacteria from the body’s immune defenses.  R Bacteria = Do not cause disease (non- pathogenic)  Lacked the protective coat and thus the immune system can identify and destroy them.

Honors Biology The Experimental Trials Transformation? something in heat-killed bacteria could still transmit disease-causing properties live pathogenic strain of bacteria live non-pathogenic strain of bacteria mice diemice live heat-killed pathogenic bacteria mix heat-killed pathogenic & non-pathogenic bacteria mice livemice die A.B. C. D.

Honors Biology Griffith’s “Transforming Factor”  Why did the mice die that were given a mixture of heat-killed pathogenic bacteria and live non- pathogenic bacteria?  A substance had been passed from the dead bacteria to live bacteria = “Transforming Factor”

Honors Biology Griffith’s Results  After examining blood samples, he found that somehow the uncoated bacteria had TRANSFORMED into bacteria with coats.  Transformation is a change in the genetic makeup caused when cells take up foreign genetic material. BUT, what was it? DNA or Protein??

Honors Biology What Next?  Whatever the molecule, it had to have several properties in order to fit the bill:  It had to be duplicated whenever a cell divided, so it could be passed on unchanged.  It had to be in the form of an informational code  It had to be (mostly) stable and resistant to change

Honors Biology Avery, McCarty & MacLeod Oswald Avery Maclyn McCarty Colin MacLeod 1944

Honors Biology DNA causes Transformation  Avery, McCarty & MacLeod  purified both DNA & proteins from Streptococcus pneumonia bacteria  which will transform non-pathogenic bacteria?  Experiment 1:  injected protein into bacteria = no effect  injected DNA into bacteria = transformation!  Experiment 2:  added protein destroying enzymes to bacteria = transformation!  added DNA destroying enzymes to bacteria = no effect

Honors Biology Hershey & Chase Alfred HersheyMartha Chase

Honors Biology Confirmation of DNA  Hershey & Chase  “Blender” experiment using bacteriophages  viruses that infect bacteria  Half of the phages had their proteins tagged with radioactive sulfur ( 35 S)  The other half had their DNA tagged with radioactive phosphorus ( 32 P)  All phage were then allowed to infect bacteria! 1952 | 1969

Honors Biology

Protein coat labeled with 35 S DNA labeled with 32 P bacteriophages infect bacterial cells T2 bacteriophages are labeled with radioactive isotopes S vs. P bacterial cells are agitated to remove viral protein coats 35 S radioactivity found in the liquid 32 P radioactivity found in the bacterial cells Which radioactive marker is found inside the cell? Which molecule carries viral genetic info? Hershey & Chase

Honors Biology Blender experiment Results  Radioactive phage & bacteria in blender  35 S phage  radioactive proteins were in liquid  therefore protein did NOT enter bacteria  32 P phage  radioactive DNA was in pellet  therefore DNA did enter bacteria Taaa-Daaa!

Honors Biology Hershey & Chase’s Conclusion  Hershey & Chase Confirmed DNA is “transforming factor”  Proved that DNA AND NOT protein is the hereditary material in cells (i.e., genes are made up of DNA)

Honors Biology Wilkin’s and Franklin’s Photographs  In 1952, Maurice Wilkins and Rosalind Franklin, developed high quality X-ray diffraction photographs of strands of DNA.  These photos were key pieces of data used to determine the helical structure of DNA molecules.

Honors Biology Watson and Crick 1953 article in Nature

Honors Biology The Structure of DNA

Honors Biology Deoxyribonucleic Acid  DNA is the molecule of heredity.  DNA (Deoxyribonucleic Acid) is a double-stranded nucleic acid that determines an organism’s traits by controlling the production of proteins.  DNA holds the information for life!  Why do we study DNA??

Honors Biology DNA’s Size  DNA is an extremely long molecule  DNA from one of your cells would measure 6 feet tall!  All of the DNA in your body could stretch to the sun and back 400 times!  5 million strands of DNA can fit through the head of one needle! The earth is 93 million miles from the sun!

Honors Biology DNA’s Helical Structure  Watson & Crick discovered DNA is made up of 2 chains of nucleotides joined together by their nitrogen bases.  The 2 strands are twisted together, forming a Double-Helix.  Like a spiral staircase, twisted ladder, or zipper

Honors Biology Subunits of DNA  When we group the deoxyribose sugar, phosphate group and nitrogen base together into a subunit it is called a Nucleotide.  One strand of DNA has many millions of nucleotides. nucleotide

Honors Biology Nucleotides Continued Phosphate C C O O C C O -P O O O Nitrogenous base Deoxyribose C  Sugar = Deoxyribose.  The Phosphate Group = Phosphorus atom surrounded by 4 oxygen atoms.  The Nitrogen Bases are carbon ring structures that contain one or more atoms of nitrogen.

Honors Biology What is DNA made of?  The backbone of the molecule is alternating phosphate groups and deoxyribose sugars.  The “rungs” of the DNA “ladder” are nitrogenous bases. phosphate deoxyribose bases

Honors Biology Four nitrogenous bases  DNA has four different nitrogenous bases:  Adenine ( A)  Thymine ( T)  Cytosine ( C)  Guanine ( G)

Honors Biology Two Classes of DNA Bases  Pyrimidines are single ring bases.  Thymine  Cytosine (Pyrimidins have a “y”) (Pyrimidins have a “y”)  Purines are double ring bases.  Adenine  Guanine C C C C N N O N C C C C N N N N N C

Honors Biology Chargaff’s Rule  The amount of Adenine is always equal to the amount of Thymine.  Adenine and Thymine have a 1:1 ratio  The amount of Guanine is always equal to the amount of Cytosine.  Guanine and Cytosine have a 1:1 ratio

Honors Biology Base pairing in DNA  Pairing  Adenine and Thymine always join together  Two Hydrogen Bonds  Cytosine and Guanine always join together  3 Hydrogen Bonds  Adenine-Thymine and Guanine- Cytosine are called Complimentary Base Pairs.

Honors Biology How do the strands stick?  Hydrogen Bonds form between the nitrogenous bases.  Weak, but there are millions and millions of them in a single molecule of DNA.  Remember hydrogen bonds can form between hydrogen and any electronegative atom! hydrogen bonds

Honors Biology Nucleotide Sequence  Nucleotide Sequence is extremely important.  Differences in organisms are due to the difference in sequence of the nucleotides along a strand of DNA.  Nucleotide sequence forms the unique genetic information of an organism.

Honors Biology DNA Replication

Honors Biology Copying DNA  Each cell in an organism has an exact copy of the DNA that was in the fertilized egg (Mitosis)  The DNA in the chromosome is copied in a process called DNA Replication.  If DNA wasn’t copied before cell division, new cells would only have half the DNA of their parents.

Honors Biology DNA Replication  Base pairing allows each strand to serve as a pattern (template) for a new strand  Semi-Conservatice Model  2 molecules are formed.  Each has an original strand and one new strand.

Honors Biology let ’ s meet the team … DNA Replication  Large team of enzymes coordinates replication: 1.DNA Helicase - unwinds the DNA so that it can be copied 2.DNA Polymerase(s) - Brings in the nucleotide building blocks to make the complimentary strand AND “proofreads” the finished strands. 3.DNA Ligase - Acts as “glue” cementing new pieces of DNA together to form a continuous strand.

Honors Biology st step: Unwinding single-stranded binding proteins  Unwind & unzip DNA  helicase enzyme  unwinds DNA helix  stabilized by single-stranded binding proteins  The areas where the double helix separate are called replication forks.

Honors Biology Replication fork direction of replication

Honors Biology  DNA Polymerases bring in new nucleotides that match up to template strands Now, that ’ s a compliment! 2nd step - Adding Bases

Honors Biology Results of DNA Replication  DNA Replication results in the formation of 2 DNA molecules, each identical to the original DNA molecule.  Each new molecule has one strand from the original DNA molecule and one new strand.  Semi-conservative!

Honors Biology Checking for Errors  Sometimes errors occur when the wrong nucleotide is added to the new strand.  Luckily, DNA Polymerase has a “proofreading function.”  DNA Polymerase can backtrack and remove incorrect nucleotides.

Honors Biology Fast & accurate!  It takes E. coli <1 hour to copy 5 million base pairs in its single chromosome  divide to form 2 identical daughter cells  Human cell copies its 6 billion bases & divide into daughter cells in only few hours  remarkably accurate  only ~1 error per 100 million bases  ~30 errors per cell cycle