DNA Discovering the Molecule of Inheritance

QUESTION: People had long realized that offspring tend to resemble their parents. Question for scientists: “What is the hereditary molecule that gets passed from parent to offspring?”

WHAT SCIENTISTS KNEW: 1. DNA Components: - Through testing on extracted DNA samples, scientists understood the basic ingredients of DNA but NOT the structure. - made of ______ different types of nucleotides 2. Protein Components - Large complex molecule made of 20 different amino acids (much variability)

DNA Nucleotides Scientists at this point understood the chemical components of the DNA molecule (But not the Structure!) 3 Basic Parts  5-carbon sugar (deoxyribose)  Phosphate group  Nitrogenous Base (4 kinds) Adenine Guanine Cytosine Thymine

BIG DEBATE PROTEIN OR DNA????? Uh oh…sounds like the fixin’s for a science fight…..I hear snapping…..

Frederick Griffith 1928 British Medical Officer Discovers that genetic information can be transferred from heat- killed bacteria cells to live ones. Pneumococcus (which infects mice), Pneumococcus  S (smooth)  lethal  R (rough) strain  non-lethal Experiment  S strain were killed by heat, and their remains were added to R strain bacteria.  formerly harmless R strain now was able to kill its host.  CONCLUSIONS? This phenomenon, called transformation, provides the first evidence that the genetic material is a heat-stable chemical.

Oswald Avery 1944  Used two different enzymes One digested DNA One digested Proteins  Using different enzymes how can we modify Griffith’s experiment?

Alfred Hershey and Martha Chase Studies viruses,  Bacteriophage – infects bacteria Viruses composed of  a nucleic acid (DNA or RNA) core  protein coat (capsid) Viruses  Inject hereditary information (DNA) into host cell  host cell create many more virus particles.

HERSHEY CHASE EXPERIMENT Used radioisotope labels P32 and S35 Phosphorus(P32) is a component of __________________ Sulfur (S35) is a components of ____________. HMMM…….What could we do with this?????

Results Observed that the molecule that enters the bacterial cell, causing the infection was ……………. !!!!!!There is a clear correlation between DNA and genetic information.!!!!!!!

DNA Structure Now what? Scientists realized that the DNA molecule needed to do 3 things  Carry information generation to generation  Code had to be translatable into proteins  Easily copied during cell division

When you do a puzzle, what strategies do you use? We knew the parts  Sugar, phosphate group, nitrogenous bases (A, T, C, G) SHAPE???????

Chargaff’s Data!

Erwin Chargaff 1949 Discovery:  amount of adenine equals the amount of thymine  amount of guanine equals the amount of cytosine  CHARGAFF’S LAW!

What did scientists know? DNA molecule of heredity DNA made of nucleotides 4 different types of nucleotides  Difference involves which nitrogenous base it has. Chargaff’s rule; amount of A=T, amount of C=G Big Question: Now we know the pieces….how does all this go together?

Rosalind Franklin ( ) Early 1950’s King’s College, London, England Using X-ray photography and crystalized DNA, took pictures of the molecule. Hinted that the DNA molecule is two strands wrapped around each other.

James Watson & Francis Crick Working on developing model of DNA molecule  Figured out the “ladder” 1953, shown one of Franklin’s X-ray picture indicating shape of molecule “twisted ladder” Published paper in 1953 of DNA being a double helix with two strands being wound around each other

Double Helix Each DNA strand has a Sugar- Phosphate backbone Nitrogenous bases face inward towards each other Hydrogen bonds that develop between bases on adjoining strand keep helix together

Nitrogenous Bases Purines  Adenine and Guanine Pyrimadines  Thymine and Cytosine

This gives us 4 different types of nucleotides!

Base Pairing Chargaff’s rule,  A bonds to T  G bonds to C. Weak hydrogen bonds connect the two bases.

NOW WHAT? How does a simple molecule made of four pieces code for a whole organism? DNA  Protein 