honors biology 2006-2007 the history of dna honors biology 2006-2007
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The History Of DNA
Quick ReviewChromosomes are made up of DNA and Proteins.
Genes are a segment of the DNA molecule that carries the instructions for producing a specific traits.
DNA or Proteins?Prior to the 1950s, 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 & Chase1952)
Griffiths ExperimentFrederick Griffith was working to find a cure for Streptococcus pneumonia (bacteria)Test Subjects: lab miceHe used different strains (types) of bacterium in this experiment.
Strains of Bacteria UsedS Bacteria = Disease Causing (pathogenic)Have protective outer coat that hides the bacteria from the bodys immune defenses.
R Bacteria = Do not cause disease (non-pathogenic)Lacked the protective coat and thus the immune system can identify and destroy them.
The Experimental TrialsTransformation?something in heat-killed bacteria could still transmit disease-causing propertieslive pathogenicstrain of bacterialive non-pathogenicstrain of bacteriamice diemice liveheat-killed pathogenic bacteriamix heat-killed pathogenic & non-pathogenicbacteriamice livemice dieA.B.C.D.
Griffiths Transforming FactorWhy 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
Griffiths ResultsAfter 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??
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
Avery, McCarty & MacLeodOswald AveryMaclyn McCartyColin MacLeod1944
DNA causes TransformationAvery, McCarty & MacLeodpurified both DNA & proteins from Streptococcus pneumonia bacteriawhich will transform non-pathogenic bacteria?Experiment 1:injected protein into bacteria = no effectinjected DNA into bacteria = transformation!
Experiment 2:added protein destroying enzymes to bacteria = transformation!added DNA destroying enzymes to bacteria = no effect
Hershey & ChaseAlfred HersheyMartha Chase
Confirmation of DNAHershey & ChaseBlender experiment using bacteriophagesviruses that infect bacteriaHalf of the phages had their proteins tagged with radioactive sulfur (35S)The other half had their DNA tagged with radioactive phosphorus (32P)All phage were then allowed to infect bacteria!1952 | 1969
Hershey & ChaseProtein coat labeledwith 35SDNA labeled with 32Pbacteriophages infectbacterial cellsT2 bacteriophagesare labeled withradioactive isotopesS vs. Pbacterial cells are agitatedto remove viral protein coats35S radioactivityfound in the liquid32P radioactivity found in the bacterial cellsWhich radioactive marker is found inside the cell?Which molecule carries viral genetic info?
Blender experiment ResultsRadioactive phage & bacteria in blender35S phageradioactive proteins were in liquidtherefore protein did NOT enter bacteria32P phageradioactive DNA was in pellettherefore DNA did enter bacteria
Hershey & Chases ConclusionHershey & 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)
Wilkins and Franklins PhotographsIn 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.
Watson and Crick1953 article in Nature
The Structure of DNA
Deoxyribonucleic AcidDNA is the molecule of heredity.
DNA (Deoxyribonucleic Acid) is a double-stranded nucleic acid that determines an organisms traits by controlling the production of proteins.
DNA holds the information for life!
Why do we study DNA??
DNAs SizeDNA 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!
DNAs Helical StructureWatson & 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
Subunits of DNAWhen 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
Nucleotides ContinuedSugar = 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.
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.phosphatedeoxyribosebases
Four nitrogenous basesDNA has four different nitrogenous bases: Adenine (A) Thymine (T) Cytosine (C) Guanine (G)
Two Classes of DNA BasesPyrimidines are single ring bases.ThymineCytosine (Pyrimidins have a y)
Purines are double ring bases.AdenineGuanine
CCCC N NO N
Chargaffs RuleThe 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
Base pairing in DNAPairingAdenine and Thymine always join togetherTwo Hydrogen Bonds
Cytosine and Guanine always join together3 Hydrogen Bonds
Adenine-Thymine and Guanine-Cytosine are called Complimentary Base Pairs.
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!
Nucleotide SequenceNucleotide 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.
Copying DNAEach 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 wasnt copied before cell division, new cells would only have half the DNA of their parents.
DNA ReplicationBase pairing allows each strand to serve as a pattern (template) for a new strandSemi-Conservatice Model2 molecules are formed.Each has an original strand and one new strand.
DNA Replication Large team of enzymes coordinates replication:
DNA Helicase - unwinds the DNA so that it can be copied
DNA Polymerase(s) - Brings in the nucleotide building blocks to make the complimentary strand AND proofreads the finished strands.
DNA Ligase - Acts as glue cementing new pieces of DNA together to form a continuous strand.
1st step: UnwindingUnwind & unzip DNAhelicase enzymeunwinds DNA helixstabilized by single-stranded binding proteinsThe areas where the double helix separate are called replication forks.
single-stranded binding proteins
2nd step - Adding BasesDNA Polymerases bring in new nucleotides that match up to template strands
Results of DNA ReplicationDNA 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!
Checking for ErrorsSometimes 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.
- Fast & accurate!It takes E. coli