DNA – The Scientists and Their Discoveries

Frederick Griffith (1928) Experiment – worked with bacteria and injected into mice Conclusion – transforming material is genetic material Transformation – when 1 cell is changed to another cell permanently

Frederick Griffith Disease-causing bacteria (smooth colonies) Harmless bacteria (rough colonies) Heat-killed, disease- causing bacteria (smooth colonies) Control (no growth) Heat-killed, disease-causing bacteria (smooth colonies) Harmless bacteria (rough colonies) Dies of pneumoniaLives Live, disease-causing bacteria (smooth colonies) Dies of pneumonia

Oswald Avery (1944 ) Experiment – to determine which molecule in a cell transformed  expanded on Griffith’s experiment Conclusion – DNA stores and transmits genetic information throughout generations

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

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 

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 DNAATGC Streptococcus Yeast Herring Human Streptococcus Yeast Herring Human

Nucleotides: Page 291 AdenineGuanine Cytosine Thymine Phosphate group Deoxyribose

DNA Structure and 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 P y rimidines (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 PurinesPyrimidines AdenineGuanine 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 Chromatin Histones DNA double helix Centromere

What is DNA Replication? The process of making copies of DNA during S phase of interphase in the nucleus Makes 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

DNA Replication Facts: DNA polymerase can only work in one direction so:  2 new strands of DNA are made in opposite directions  One strand is made in sequence of nucleotides  The other strand is what we call the “lagging strand” because nucleotides are added in fragments

1. Helicase unwinds/unzip s 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 Parent New

DNA Growth Replication fork DNA polymerase New strand Original strand DNA polymerase Nitrogenous bases Replication fork Original strand New strand