The History and Discovery of the Structure and Role of DNA DNA Part I The History and Discovery of the Structure and Role of DNA

Nucleic Acids Nucleic acids store, transmit, and help express hereditary information The amino acid sequence of a polypeptide is programmed by a unit of inheritance called a gene Genes are made of DNA Ask questions to see what they already know about DNA structure. A good deal of this should be prior knowledge!

Two Types of Nucleic Acids There are two types of nucleic acids Deoxyribonucleic acid (DNA) Ribonucleic acid (RNA) DNA provides directions for its own replication DNA directs synthesis of messenger RNA (mRNA) and, through mRNA, controls protein synthesis Protein synthesis occurs on ribosomes Pause and let student examine both structures. WHERE exactly is the “deoxy” part of the name coming from?

Experiment of Hershey and Chase Alfred Hershey and Martha Chase demonstrated the genetic material is DNA by using viruses that infect bacteria. These viruses only stay on the outside of the cell when infecting the cells. Also viruses are composed of protein and DNA. It is known that the virus injects its genetic material into the bacterium which had to DNA or proteins. *** Students need to be familiar with this experiment.

Experiment of Hershey and Chase Trial 1 Replicate phages with radioactive sulfur (labels nucleotides) which is found in proteins and not DNA. These are now labeled phages for their radioactive proteins. Mix radioactively phages with bacteria. Allow them to infect cells. Put mixture in blender to separate infected cells from the viruses on the outside of the bacteria. Centrifuge the mixture to separate and layer viruses and infected bacteria. Determine if the shell of the virus is radioactive OR if the bacteria are radioactive. Conclusion If the bacteria are radioactive, then proteins are the molecules of heredity. The bacteria were not radioactive the empty viral coats were radioactive. Trial 2 Repeat the experiment above but this time replicate the viruses and with radioactive phosphorus. This labels the DNA and not the proteins. These viruses are now labeled phages for their radioactive DNA. When the experiment is complete and if the bacteria are radioactive, then DNA are the molecules of heredity. The bacteria were radioactive and the empty viral coats were not radioactive. Results In the second trial, the bacteria were radioactive. So it can be concluded that DNA is the molecule of heredity. This is a great experiment because the experiment took advantage of differences the structure of DNA and proteins. It demonstrated that DNA is the material that genes are made of and not protein.

Work of Rosalind Franklin Rosalind Franklin worked in a lab with Maurice Wilkins. She was investigating the structure of DNA using x-ray crystallography. She determined that the phosphate groups were on the outside of the double helix. She did not determine the base of DNA pairing of DNA or their role in the structure of DNA. Rosalind Franklin used x-ray crystallography to determine that DNA was double stranded, a helix, phosphates were on the outside and three distances, 2.0 nm, .34 nm, and 3.4 nm showed up in a pattern over and over again in the diffraction pattern.

Work of James Watson and Francis Crick Based on the rules of Chargaff and the information from the work of Franklin, James Watson and Francis Crick, determined the structure of DNA by making models. Determined that the sugar and phosphates were on the outside. 2. Determined that the nitrogenous bases were forming the rungs of the ladder. Watson and Crick determined that 2.0 nm was the distance from one strand to the other. .34 nm was the distance from one base pair to another and finally 3.4 nm determined that there were 10 bases to a complete twist in the helix. So with 2.0 nm from one strand to the other, it was determined that the a purine had had to be base paired with a pyrimidine. Next problem to investigate is why did adenine base pair with guanine and why did cytosine base pair with thymine? The answer had to do with the hydrogen bonding, as adenine base paired with thymine because they could form two hydrogen bonds and guanine base paired with cytosine because they could form three hydrogen bonds. Watson, Crick, and Wilken received the Nobel Prize in 1962. Unfortunately, Franklin died of cancer at age 38 in 1958. Watson, Crick, and Wilkens received the Nobel Prize in 1962.

Chargaff’s Snub Chargaff is quoted as saying, “I told them all I knew. If they had heard before about the pairing rules, they concealed it. But as they did not seem to know much about anything, I was not unduly surprised. I mentioned our early attempts to explain the complementarity relationships by the assumption that, in the nucleic acid chain, adenylic was always next to thymidylic acid and cytidylic next to guanylic acid...I believe that the double-stranded model of DNA came about as a consequence of our conversation.*”

Chargaff's Rule -> A+G=C+T=50% Experiment Chargaff Chargaff's Rule -> A+G=C+T=50% Percentage of Various Nucleotides in Genome Organisms A T G C Humans 30.9 29.4 19.9 19.5 Wheat 27.3 27.1 22.7 22.8 Sarcina lutea 13.4 12.4 37.1 T7 26.3 26 23.8 23.9 Erwin Chargaff (1911-2005) was a Austrian biochemist that immigrated to the U.S. during W.W. II. He was a professor at Columbia University. Using paper chromatography and using a ultraviolet spectrophotometer, Chargaff was able to demonstrate that in a given organism the number of adenine nucleotides was approximately the same as the number of thyamine nucleotides and that the number of cytosine nucleotides was approximately the same as the number of guanine nucleotides. This is called Chargaff’s first rule. His second rule was based on the observation that these percentages were unique for various species. Students are responsible for this information. Based on the observations above, two rules can be deduced A+G=C+T=50%. The percentages of the nucleotide vary for different species

Structure of a Nucleotide DNA is the longest molecule found in the cell, yet its structure is quite simple. The human cell contains 5-6 feet of DNA in every cell autosomal cell. The basic building blocks of nucleic acids are the nucleotides. There are 3 billion base pairs or 6 billion nucleotides in a human cell. Have the students compare and contrast the differences between purines and pyrimidines and point out the differences. Purines have two rings whereas pyrimidines only have one ring. Adenine and guanine are purines and cytosine, thymine, and uracil are pyrimidines Have the students compare and contrast the differences between ribose and deoxyribose. Deoxyribose is missing an atom of oxygen. Be sure to go over how the carbons are numbered and emphasize that # 3 and # 5 are important and termed 3’ or three prime and 5’ or five prime. The significance of the various carbons are listed below: Carbon #1 is where the nitrogenous base is attached. Carbon #2 is what differentiates between ribose (C5H10O5) which has a hydroxyl group attached (OH) and deoxyribose (C5H10O4) has only hydrogen (H). Hence the name deoxyribose makes sense. Carbon #3 is where the next nucleotide will attach. Carbon #5 is the phosphate group is attached.

Determining the Nitrogen Base Pairing Based on the work Franklin’s x-ray crystallography, Watson and Crick found the bonding; two purines are too wide and would overlap. two pyrimidines are too far apart to form the hydrogen bonds. a purine and a pyrimidine however, are just right! Also due to the hydrogen bonding, A and T forms two hydrogen bonds and C and G forms three hydrogen bonds. That is the only way they hydrogen bonds will fit.

Sides of the Ladder DNA is a double stranded and analogous to a ladder. The sides of the ladder are composed of alternating sugars (deoxyribose) and phosphate groups that run antiparallel to one another. On the left side (next card) the first carbon found on the strand is #5 and moving on down the last carbon is carbon # 3. This side is said to be 5'-3'. The opposite side is upside down compared to the other side. The right hand side, the first carbon found on the strand is #3 and moving on down the last carbon is carbon # 5. This side is said to be 3'-5'.

Hydrogen Bonding and Nitrogenous Bases The nitrogenous bases form the rungs of the ladder. Thymine will base pair with adenine on the opposite side, which is a pyrimidine base paired with a purine. This will form 2 hydrogen bonds. Hydrogen bonds are weak but millions of them together will keep the two strands together.

Hydrogen Bonding and Nitrogenous Bases Guanine will base pair with cytosine on the opposite side. This is a pyrimidine base paired with a purine. This will form 3 hydrogen bonds instead of 2 hydrogen bonds.

Hydrogen Bonding and Nitrogenous Bases This will continue for billions of base pairings forming a molecule of DNA.

Hydrogen Bonding and Nitrogenous Bases The DNA molecule once formed will make a double helix. There are 10 base pairs in one complete turn of the DNA molecule.

Could Prove Useful The “type of linkage” is not nearly as important as the rest of this summary. Student still have to be able to read and interpret test questions!