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Learn key concepts of DNA structure, history, and significant experiments such as Griffith, Avery, Hershey-Chase, Chargaff's Rule, and Franklin's X-ray Crystallography. Explore how these findings shaped our understanding of genetics.
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AP Biology DNA History & Structure
Important concepts from previous units: • The basic unit of DNA or RNA is a nucleotide; composed of a Nitrogen base, 5 Carbon sugar, and phosphate. • DNA is like a “million dollar blueprint” having the genetic information for making proteins and enzymes. • Base pairing is always a pyrimidine (C, T, U) with a purine (A, G).
Frederick Griffith (in 1928) • He was a British Army doctor who was studying Pneumonia in the hopes of finding a cure. • He is given credit for the transformation experiment, even though this was not his original intent.
Mixture of heat-killed S cells and living R cells Heat-killed S cells (control) Living R cells (control) Living S cells (control) Frederick Griffith Experiment RESULTS Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells are found in blood sample
In the experiment, he took pathogenic (disease causing) bacteria and non-pathogenic bacteria and injected them into mice. The pathogenic bacteria killed the mice. The non-pathogenic did not kill the mice. • He then took some pathogenic bacteria and killed them by exposing them to heat. He took the dead bacteria and injected them into more mice. The mice did not die. • He then took some of the dead pathogenic bacteria and mixed them with the non-pathogenic bacteria. He then injected the mixture into some more mice. THEY DIED. • His reasoning was some “instructional agent” was exchanged between the dead pathogenic bacteria and the living non-pathogenic bacteria allowing them to “learn” a new trick. How to make the toxin (poison). So we say they were transformedfrom non-pathogenic into pathogenic bacteria.
Oswald Avery and associates (in 1944) • He retests Griffith’s experiment, but with the purpose to find out what the “instructional agent” was that led to the transformation of the non-pathogenic bacteria. • After the testing, he states that the transformation agent was DNA. • This statement sparks lots of controversy as DNA is too simple a molecule most scientists believe. It must be proteins, as they are very large complex molecules. So now the race is on to prove which was it, DNA or proteins.
Alfred Hershey and Martha Chase (in 1952) • They worked with the T2 Bacteriophage (a virus that infects bacteria) and E. Coli bacteria. • This becomes the Hershey-Chase Experiment.
They used radioactive Sulfur 35 to label the virus’s protein outer capsid in one container. (Remember, the amino acid Cysteine contains sulfur. The radioactivity allows them to follow where the proteins go by using a Geiger counter. A Geiger counter is used to measure radioactivity.) • They then used radioactive Phosphorus 32 to label the DNA inside the virus in a different container. (Remember, phosphorus is one piece of a nucleotide. They can also follow the DNA using the Geiger counter.)
The radioactive viruses where then exposed to bacteria. The viruses infected the bacteria. In the radioactive Sulfur container, the radioactive sulfur did NOT enter the bacteria. It remained outside the bacteria. When the viruses reproduced inside the bacteria, the reproduced viruses that came out of the dead bacteria were NOT radioactive. In the radioactive Phosphorus container, the radioactive phosphorus did enter the bacteria. When they reproduced inside the bacteria, the reproduced viruses that came out of the dead bacteria were radioactive from the phosphorus the possessed. • This proved with 100% accuracy, that DNA was the “transformation agent” and that this carries the information “blueprint” from one generation to the next.
Erwin Chargaff (in 1947) • He develops what becomes known as Chargaff’s Rule. • The rule states that, FOR ALL ORGANISMS, [A] = [T] and [C] = [G]. • This helps support the theme of Unity and Diversity. Unifying complementariness, as it always the same pairing of nucleotides. Diversity is in the percentages of each grouped nucleotide pairs between species. • For example: If you know a species has 32% Thymine; then there must ALSO be 32% Adenine. (32+32= 64%.) This means that there is 36% unaccounted for. (100- 64 = 36.) Since this 36% is BOTH Cytosine and Guanine, divide by 2 to find the percentage of each. (36÷ 2 = 18) There exists 18% Cytosine and 18% Guanine.
Chargaff’s Rule Adenine = Thymine (DNA) or Uracil (RNA) & Guanine = Cytosine If you know the % composition of 1, you can find the % composition of the other 3.
Rosalind Franklin (in the 1950’s) • She performed X-ray Crystallography on DNA. This picture was extremely important in helping Watson and Crick develop their model of DNA. • The picture indicates the Double Helix structure of DNA (The picture would be from the view of looking down a strand of DNA. It would be similar to looking down a paper towel cardboard tube.) • The picture also indicates that the Nitrogen Bases (the X in the center) point inward and are equal lengths in binding, because it is always one Pyrimidine (C and T) and one Purine (A and G). • The large areas around the “X” are the sugar phosphate backbone of DNA.
James Watson and Francis Crick (in 1953) • They constructed the first accurate model of DNA. • They used Chargaff’s work and Franklin’s work to fill in the gaps that they could not figure out. • The Double Helix backbone is composed of Phosphorus and the 5 Carbon sugar Deoxyribose. (It would be like the side supports on a ladder.) • The “rungs or steps of the ladder” would be the Purine base + Pyrimidine Bases. (A=T and C=G) • Hydrogen Bonds hold the two sides together and it is twisted into the Double Helix shape (It looks like a twisted ladder.) Remember, Hydrogen bonds are weak bonds. We will want to “open up” the DNA during DNA replication AND Protein Synthesis.
AP Biology DNA Structure & Replication
Important concepts from previous units: • Monomers of Nucleic Acids are called Nucleotides; Polymers are DNA or RNA. • Nucleotides are linked together by a covalentPhosphodiester bond. • The sequence of nucleotides determines what protein or enzyme is made (expressed).
DNA Replication • The process of making of a complete copy of an entire length of DNA. (Applies to all Chromosomes.) • This occurs duringthe S-Phase of the Cell Cycle for Mitosis or Meiosis. • In bacteria, it is referred to as Circular or Theta replication. (Symbol for Greek letter Theta is: Θ.) • In other organisms that possess chromosomes, it is referred to as Linear Replication.
ThetaReplication in Prokaryotes Replication fork Origin of replication Termination of replication
It is easy to do for cells because the two sides are complimentary (A with T and C with G always.) • The Semi-conservative Model best explains the process of DNA replication. • It shows one original DNA side serving as a template (guide) for making the other DNA side. • Easy as A = T and C = G. • The replication work is being done in opposite directions, but on both sides at the same time. • In humans, it takes just a few hours to copy over 6 Billion nucleotides in our cells thanks to ENZYMES!
. The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.
. The first step in replication is separation of the two DNA strands. The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.
Semi Conservative process of DNA Replication The first step in replication is separation of the two DNA strands. Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand. The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.
Origins Of Replication (Starting points) • These are specific nucleotide sequences encoded in the DNA strands that act as “starting points”. • The enzyme helicaseunwinds the DNA double helix to create a Replication Bubble (This provides “space” to do the actual building work of making the new complimentary side of the new DNA molecule by other enzymes.) • The ends of the bubbles are called Replication Forks. There is one on each end of the bubble. • Work is happening on both sides of the forks and both sides of the bubbles. • Many bubbles can be on the same DNA strand. (This speeds up the process of replication.)
DNA Replication Elongation • Elongation of the new DNA complimentary side will require the enzyme DNA Polymerase III. (This enzyme performs the addition of new nucleotidesto the new DNA complimentary side and also acts as a proofreader to help prevent errors in construction from occurring. (Look at the name and see the function. Remember, “polymers” means “many units” or “many monomers”. In this case, the monomers are called nucleotides. The ending “ase” tells you it is an enzyme.) • The enzyme works at a rate of about 500 nucleotides being added per second.
DNA Nucleosides are brought to the enzyme from the cytoplasm of a cell. (Nucleosides were “created” from broken down DNA strands found in the cells or particles of food during the process of digestion. • A nucleoside has three phosphates to supply the bonding process with energy. (Remember, to create a bond requires “free” energy.) • The nucleoside will lose two phosphates in the bonding (attachment) process to the new DNA. • Lose of phosphates makes it a nucleotide. • This saves ATP for other cellular processes.
The two sides of the Double Helix are said to be Anti-parallel. (This means that the DNA information runs in different directions.) • DNA is ALWAYS READ AND MADE 5’ 3’. (REMEMBER THIS IMPORTANT FACT!) • The 5’ Carbon of the sugar (Deoxyribose or Ribose) has a phosphate attached to it. • The 1’ Carbon of the sugar has the Nitrogen Base attached to it. • The 3’ Carbon of the sugar has an open bond. (This is the connector site for the next nucleoside.)
Helicase is the GREEN “blob”Helicase enzyme causes the Double Helix to unwind.
