13.1: DNA is the Genetic Material Oliver Dorfner, Emily Miller, Chrissy Carrucci, Charlie Derbyshire, Logan McDonagh

The Search for the Genetic Material T.H Morgan demonstrated that genes were part of chromosomes. The next question was, “What made up this genetic material?” They believed it was either protein or DNA. The opinion then was that it was more likely to be proteins, as it is a very diverse class of macromolecule with specificity of function. Not much was known about nucleic acids, and they were believed to be too uniform to account for all the specific differences in genetic material. The appropriate experimental organisms to determine the role of DNA in heredity was originally bacteria and the viruses that infect them, as they are much simpler than pea plants, fruit flies, or humans.

Evidence That DNA Can Transform Bacteria 1928- Frederick Griffith was trying to develop vaccine against pneumonia Studying Streptococcus pneumoniae- bacterium that causes pneumonia in mammals Had two strains- one pathogenic and one nonpathogenic Surprised to find that when the pathogenic bacteria was killed with heat and then mixed the cell remains with the living nonpathogenic strain of bacteria, some of the living cells became pathogenic Some unknown chemical component of the dead pathogenic cells caused the heritable change Called phenomenon transformation- now defined as change in genotype and phenotype due to the assimilation of external DNA by a cell Later on Oswald Avery and others identified the transforming substance as DNA Scientists remained skeptical- many viewing proteins as better candidates for the genetic material Biologists not convinced not convinced that genes of bacteria woudl be similar in composition and function to those of more complex organisms Major reason for continued doubt- little known and DNA

Evidence That Viral DNA Can Program Cells These viruses are known as bacteriophages (bacteria eaters), or phages for short. Additional evidence that DNA can program cells came from studies of viruses that infect bacteria. Viruses are less complex than cells, usually consisting of little more than DNA or RNA enclosed by a protective coat. To produce more viruses, a virus must infect a cell and take over its metabolic machinery. Phages have been widely used as tools by researchers in molecular genetics, most notably by Alfred Hershey and Martha Chase in 1952. . In this experiment, they concluded that DNA is the genetic material of a phage known as T2.

Hershey-Chase Experiment

Additional Evidence that DNA is the Genetic Material Erwin Chargaff discovered that nitrogenous bases differed between different species This diversity could explain differences in species, suggesting DNA as the genetic material Chargaff also noticed that there was a regularity in the frequencies of the bases Chargaff discovered that Adenine and Thymine had almost the same frequencies, while Cytosine and Guanine also had the almost same frequencies. These two discoveries are known as Chargaff’s Rules: The DNA base composition differs between different species Within a species, The number of A and T bases are roughly equal and the number of G and C bases are roughly equal.

DNA Molecular Makeup

Building a Structural Model of DNA Now the challenge was to determine how DNA structure could account for its role in inheritance The arrangement of covalent bonds had already been established, so the next step was for scientists to focus on the three-dimensional shape of DNA. Linus Pauling, Maurice Wilkins, Rosalind Franklin, James Watson and Francis Crick worked on problem Watson saw an X-ray diffraction image of DNA by Rosalind Franklin - he was familiar with type of x-ray diffraction pattern that helical molecules produce- after examining photo confirmed that DNA was helical in shape Other data obtained by Franklin suggested that the presence of 2 strands is a double helix

Building A Structural Model of DNA Watson and Crick began buildings models of a double helix that would conform to the X-ray measurements and what was known about the chemistry of DNA, including Chargaff’s rule They investigated Franklin’s conclusion that the sugar-phosphate backbones were located on the outside of the DNA molecule. This proposed arrangement placed the negatively charged phosphate groups facing aqueous surroundings, while hydrophobic nitrogenous bases were hidden in the interior. Building upon this, Watson created an antiparallel model in which the subunits run in opposite directions (5’-3’ in one direction, 3’-5’ in the next) The DNA can be seen like a ladder; the rungs as the pairs of nitrogenous bases and the sides as the sugar-phosphate backbones, then the ladder is twisted to form the double helix Franklin also produced data from X-rays that indicated that the Helix makes one full turn every 3.4 nm; there are base pairs every 0.34 nm, so there would be 10 ‘rungs’ in every full turn

Building a Structural Model of DNA (cont.) The nitrogenous bases are paired in specific combinations; adenine (A) with thymine (T) and guanine (G) with cytosine (C) They also concluded that each bases has chemical groups that can form two hydrogen bonds with its appropriate partner; Adenine forms two hydrogen bonds with thymine and guanine forms three hydrogen bonds with cytosine. Originally, it was thought that like bases would pair together (Ex. Purine and purine) but this was disproved by the discrepancy in ‘rung’ size that would form Purines (A and G) are about twice as wide as pyrimidines (C and T); a pyrimidine combination would result in a pair that is too narrow to amount to the 2nm diameter of the double helix and a purine combination would be too wide