Discovery of DNA and its 3D Structure

What is hereditary material (believed to by scientists in the early 1900s)?

Scientists believed that proteins, not nucleic acids, because proteins were made up of 20 amino acids (more variation), while nucleic acids were composed of just 4 bases (fewer variations).

How long did it take scientists to be convinced that DNA was the make up of hereditary material?

50 years

What does DNA stand for?

Deoxyribonucleic acid

1869: Friedrich Miescher

• Swiss biochemist

• Investigated the contents of pus cell nuclei

• Pus cells widely available from infectious patients

• Found that nuclei contained a significant amount of material that was not protein

• Called it nuclein

1900s: Phoebus Levene

• Russian-American biochemist

• Chemically analyzed “nuclein” and renamed it deoxyribose nucleic acid (later became deoxyribonucleic acid)

• Also isolated RNA (ribose nucleic acid —> ribonucleic acid

1919: Levene (cont.)

• correctly described the make-up of nucleotides with 1 of 4 nitrogen bases, a sugar molecule, and phosphate group

• proposed that nucleic acids had a tetranucleotide structure

  • polymer was made up of a repeating sequence of 4 nucleotides

• this was too simple, so they went back to the idea that protein was the hereditary material

1928: Frederick Griffith

• trying to develop a vaccine against pneumonia

• conducted the mice experiment

  • injected mice with R-strain S. pneumoniae and they lived (non-pathogenic)

  • injected mice with S-strain pneumoniae and they died (pathogenic)

  • S-strain could be made non-pathogenic through heat (denatured)

  • heat-killed S-strain and R-strain together, killed the mice

• Called this transformation, where something from the heat-killed pathogenic bacteria transformed the living non-pathogenic bacteria (transforming factor/principle)

After Griffith’s untimely death, who continued his work?

• Oswald Avery - a Canadian-American microbiologist

• continued research on S-strain pneumoniae

• him and research group identified the molecules in S pneumoniae that cause the transformation

• tested material from the nucleus:

  • DNA

  • RNA

  • protein

1944: Oswald Avery, Maclyn McCarty, Collin MacLeod

• found that DNA was being used in the transformation process —> not protein

• used enzymes to destroy each type of macromolecule before injecting into bacteria (to see what would happen)

• When DNA was destroyed, scientists were unable to cause transformation

• They were not ready to publish their findings

1952: Alfred Hershey & Martha Chase

• blender experiment

• used a virus that infects bacteria (bacteriophage T2) to prove that DNA theory

• T2 virus has 2 parts: DNA and a protein coat

• no one knew for sure which part became injected into the host cell and helped with the creation of new viruses

• radiolabelled viral proteins with S-35 and viral DNA with P-32

• radioisotopes were very unstable and easy to measure the radioactive decay

• DNA contains phosphorus but not sulfur and protein contains sulfur but not phosphorus

• no viruses were allowed to infect the bacterial hosts

  • no S-35 was found, but P-32 was, proving that DNA contained genetic info

1949: Erwin Chargaff

• biochemist

• showed there is variation in the composition of nucleotides, disproving Levene’s theory of tetranucleotides

• Chargaff’s Rule

  • # of adenine = # of thymine

  • # of cytosine = # of guanine

1953: Rosalind Franklin

• produced x-ray diffraction picture of DNA —> showed the shape was a double helix

• noticed repeating patterns that occurred every 3.4 nm and 0.34 nm

• Maurice Wilkens (co-worker) “borrowed” the info and passed it on to Francis Crick and James Watson

1954: Francis Crick & James Watson

• came up with the double helix structure of DNA used today

• used Chargaff’s info and concluded that purines always bonded to pyrimidines (complementary base pairing)

• because of the pairing rule, the width of DNA helix will be constant

• H-bonds hold the structure together (individually weak —> together strong)

• proposed the helix turns clockwise and every 10 nucleotides (3.4 nm) it makes 1 full rotation

• the space between the nucleotides is 0.34 nm

• the phosphate group is attached to the 5th carbon through a phosphodiester linkage

• the nitrogenous base is attached to the 1st carbon through a glycosyl bond

• DNA strands run anti-parallel

  • 1 strand runs 5’ (phosphate end) to 3’ (hydroxyl group end)

  • the complementary strand runs 3’ to 5’