DNA Structure and Function

Objectives

• Important experiments– 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– 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