Alfred Hershey and Martha Chase (1952)
Experiment – used radioactive proteins and radioactive DNA in bacteriophages Bacteriophage – a virus that infects bacteria
Conclusion – confirmed that DNA was genetic material
http://www.youtube.com/watch?v=3QJ4CjFsflA
Hershey and Chase
Rosalind Franklin (1952)
Experiment – used x-ray diffraction to see structure of DNA X-ray diffraction—powerful x-
rays shown through a concentrated DNA sample
Worked with Maurice Wilkins
Conclusion – DNA is a helix
What is X-ray Diffraction?
James Watson &Francis Crick (1953)
Experiment – Built many 3-D models of DNA; Used Franklin, Wilkins, Pauling’s work
Conclusion – DNA is a double helix, a twisted ladder. Each nucleotide made of sugar, phosphate, and base.
“Sides”: sugar and phosphates “Rungs”: bases Bases are held together by hydrogen
attraction http://www.youtube.com/watch?v=TZUun93_V18
Watson and Crick
Hydrogen bonds
Nucleotide
Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Erwin Chargaff (1947)
Experiment – Analyzed the amounts of the four bases in different organism’s DNA
Conclusion –base-pairing rules adenine = thymine (A-T) cytosine = guanine (G-C)
Erwin Chargaff
Percentage of Bases in Four Organisms
Source of DNA A T G CSource of DNA A T G C
Streptococcus 29.8 31.6 20.5 18.0
Yeast 31.3 32.9 18.7 17.1
Herring 27.8 27.5 22.2 22.6
Human 30.9 29.4 19.9 19.8
Streptococcus 29.8 31.6 20.5 18.0
Yeast 31.3 32.9 18.7 17.1
Herring 27.8 27.5 22.2 22.6
Human 30.9 29.4 19.9 19.8
Nucleotides: Page 291
Adenine Guanine
Cytosine Thymine
Phosphate group Deoxyribose
DNA Structureand
DNA Replication
DNA Structure
DNA stands for Deoxyribonucleic Acid
Shape: Double helix
Monomers: Nucleotides Nucleotide has 3 parts: Sugar (deoxyribose),
phosphate, and a nitrogenous base Bases are held together by hydrogen attractions.
Why are hydrogen attractions important in DNA’s structure?
Four Nitrogenous Bases
Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
Purines (double ring) – Adenine and Guanine
Pyrimidines (single ring) – Cytosine and Thymine
Chargaff’s rule: Adenine pairs with thymine. A –T T – A Guanine pairs with cytosine. G – C C – G
Nucleotides: Page 291
Purines Pyrimidines
Adenine Guanine
Cytosine Thymine
Phosphate group Sugar:
Deoxyribose
So if every living thing has the same 4 bases, how are we so different?
It’s the SEQUENCE!! The order of those 4 nitrogen bases determines whether you are a human or a dog!
It also determines traits of that organism (why you look different than your shoulder partner or sibling.)
A Closer Look at DNA: Page 294
Hydrogen bonds
Nucleotide
Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
A closer look at DNA:How DNA condenses
Chromosome
Supercoils
ChromatinHistones
DNA
double
helix
Centromere
What is DNA Replication?
The process of making copies of DNA during S phase of interphase in the nucleusMakes sure that a daughter cell will have a
complete set of chromosomes
Original strand of DNA serves as a template for 2 new strands Template: a pattern; an original model
Each new strand will be identical to the original strand because of Chargaff’s rule.
How does DNA replication work?
1) Helicase unwinds the double helix, forming a “replication fork,” where replication takes place
2) DNA polymerase bonds the necessary nucleotides to complement the parent/original DNA strand to complete 2 new DNA molecules
3) DNA polymerase also “proofreads” the new DNA strand to make sure it is an exact copy of the original DNA
http://www.youtube.com/watch?v=zdDkiRw1PdU
1. Helicase unwinds/unzi
ps DNA strand
2. DNA Polymerase complements the
original DNA strand using
Chargaff’s Rules A-T C-G
3.. DNA Polymerase
proofreads the exact copy
Parent
New
ParentNew
DNA http://www.youtube.com/watch?v=z685FFqmrpo
Growth
Growth
Replication fork
DNA polymerase
New strand
Original strand DNA
polymerase
Nitrogenous basesReplication fork
Original strand
New strand