DNA Structure and Function

Objectives Important experiments Franklin and Wilkins Griffith Hershey and Chase Franklin and Wilkins Watson and Crick model Structure of DNA

Griffith: a “transforming principle” 1928 Investigated two forms of bacteria by injecting them into mice “S form” have a smooth coating “R form” have a rough coating Only the live S type is virulent (kills mice) Mice injected with a combination of heat-killed S bacteria and live R bacteria also died

Griffith’s Experiment “Transforming principle”– Griffith found live S bacteria in blood samples of the dead mice Some material must have transferred from dead S bacteria to live R bacteria

Hershey and Chase 1952 Conclusive evidence for DNA as the genetic material Studied bacteriophages (viruses that take over bacteria cells) Phage structure: DNA molecule Protein coat This could help answer the DNA vs. protein debate!

Hershey and Chase Protein has sulfur but not much phosphorous DNA has phosphorous but not sulfur Hershey and Chase grew phages in cultures with radioactive sulfur or phosphorous isotopes Experiment 1 Bacteria infected with phages that had radioactive sulfur in their protein Used a blender to separate bacteria from parts of the phages that remained outside the bacteria No radioactivity found in the bacteria

Hershey and Chase Experiment 2 Bacteria infected with phages grown in radioactive phosphorous Used a blender to separate bacteria from parts of phage that remained outside bacteria Radioactivity present inside bacteria These results showed that bacteriophages’ DNA had entered bacteria but protein had not! Finally convinced scientists that DNA and not protein is the genetic material Also called the “blender experiment”

Hershey and Chase

DNA Structure Franklin and Wilkins Watson and Crick X-ray crystallography photographs of DNA Watson and Crick Early 1950s Described the structure of DNA Double helix model- two strands twisted together, antiparallel

DNA Structure DNA molecule is a polymer made of many nucleic acid monomers, called nucleotides Nucleotide has three parts: Phosphate group Deoxyribose (sugar) Nitrogen-containing base Adenine Guanine Cytosine Thymine

One DNA Strand The backbone of the molecule is alternating phosphates and deoxyribose sugar The teeth are nitrogenous bases.

Four Nitrogenous Bases Purines (double ring) Adenine (A) Guanine (G) Pyrimidines (single ring) Thymine (T) Cytosine (C)

Chargaff’s Rule Ernest Chargaff analyzed DNA of different organisms Proportions of different bases varied BUT in ALL organisms, the amount of adenine (A) always equaled the amount of thymine (T) The amount of cytosine (C) always equaled the amount of guanine (G) Why would this happen?

Base Pairing A always pairs with T (2 hydrogen bonds) C always pairs with G (3 hydrogen bonds)

Antiparallel strands DNA strands are complementary Fit together (one purine and one pyrimidine) Opposite of each other For example, if one strand’s bases are ATCTT, what will the other strand be? TAGAA