2Role of the genetic material “A genetic material must carry out two jobs: duplicate itself and control the development of the rest of the cell in a specific way.”-Francis Crick
3DNA Deoxyribonucleic acid The information necessary to sustain and perpetuate life is found within a molecule. This is the genetic material that is passed from one generation to the next---a blue print for building living organisms.
4HistoryAlthough we now accept the idea that DNA is responsible for our biological structure, before the mid-1800’s it was unthinkable for the leading Scientists and Philosophers that a chemical molecule could hold enough information to build a human. They believed that plants and animals had been specifically designed by a creator.
5History Charles Darwin is famous for challenging this view. In 1859 he published ‘The Origin of Species‘ which expressed that living things appear to be designed, but may actually be the result of natural selection.Darwin showed that living creatures evolve over several generations through a series of small changes.
6HistoryIn the 1860s Darwin's ideas were finally supported when genetics was discovered by Gregor Mendel. He found that ‘Factors’ determine the characteristics a living thing will express. The genes are passed to later generations, with a child taking genes from both its parents. The great mystery was where and how would this information be stored?Czech monk
7Friedrich Miescher Swiss physician Search for genetic material:In 1870, a German scientist named Friedrich Miescher had isolated the chemicals found in the nucleus. These were proteins and nucleic acids.(His nuclei came from pus!)Structure was poorly understoodThought that NA is genetic material but further investigation showed that DNA contained only 4 kinds of monomers - seemed like it is too simple for such complicated role. They thought that NA is a structural material Names the substance nuclein because it seemed to come from cell nuclei
8HistoryWhile Miescher found these nucleic acids interesting, and spent a great deal of time studying their chemical composition, he wasn’t alone in believing that proteins were more likely to be the chemicals involved in inheritance, because of their immense variability.Proteins were made up of 20 different building blocks (amino acids), as opposed to the mere 4 building blocks of nucleic acids.
9Discovery of a “Transforming Principle” Frederick Griffith, in 1928- Pneumonia (Diplococcus pneumoniae) infects mice.- Mice develop pneumonia and die.Two types of bacteria:- S bacteria smooth coat – pneumonia- R bacteria rough coat - no pneumoniaCoat type is associated with virulence.English army medical officer
10History Search for genetic material: 1928 Frederick Griffith: transforming principle
11Frederick Griffith’s 1928 Experiment When mouse is injected with dead S strain with intact capsules, the mouselives – proving that the capsule alone does not kill the mouse.When living non-virulent R strain was mixed with deadvirulent cells with intact capsules, the mouse died and S cells were found inmouse.Some substance in the dead S cells transformed the R cells from nonvirulent to virulent. The unknown substance was called the “chemical transforming principle.” Griffith’s Experiment (1920s) • S (smooth) bacterial strain is deadly (virulent) because it has a protective capsule that resists the mouse immune system. • The R (rough) strain lacks the protective capsule, and is destroyed by mouse immune system.
12Griffith’s experiment identifying the “transforming principle” BacterialcoloniesRoughnonvirulent(strain R)+Roughnonvirulent(strain R)Heat-killedsmoothvirulent(strain S)Smoothvirulent(strain S)Heat-killedsmoothvirulent(strain S)InjectionResultsMouse healthyMouse diesMouse diesMouse healthyLive strain S bacteriain blood samplefrom dead mouse
14Discovery of DNA The extracts of heat-killed S bacteria cells contained protein, RNA and DNAWhich of these substances were essential fortransformation?How did they figure out which substance was essential for transformation?
15KEY PLAYERSOswald Avery ( )Microbiologist Avery led the teamthat showed that DNA is the unitof Inheritance. One Nobel laureatehas called the discovery "thehistorical platform of modern DNAresearch", and his work inspiredWatson and Crick to seek DNA'sstructure.
16Oswald Avery – American bacteriologist 1600s 1800s 1850s 1900s 1950s 2000sOswald Avery – American bacteriologist1943 – proved that DNA carries genesSRDNAExtracted DNA from smooth surface bacteria and introduced it to roughphysician and bacteriologist, best known for his discoveries in geneticsborn in Halifax, Nova Scotiaeducated at Columbia University's College of Physicians and Surgeonsfirst to show that the agent responsible for transferring genetic information was not a protein, as biochemists of the time believed, but deoxyribonucleic acid (DNA)Avery and his coworkers extracted a substance from a bacterium with a smooth surface and introduced it into a rough-surfaced bacterium. When the rough-surfaced bacteria transformed into the smooth-surfaced type, he knew the substance he had extracted contained the gene that coded for the smooth surface. Avery's team purified this substance and found it was pure DNA.the results were published in 1944, this paper led to more intensive studies of DNA, which eventually revealed it to be the agent of heredityS
17Discovery of DNA They decided to use the process of elimination Extracts were treated with eitherProteases (to destroy protein)RNase (to destroy RNA)DNase (to destroy DNA)Transformation was due exclusively to DNAThey decided to use the process of elimination to figure out which part or parts of the extract were important by destroying each one and seeing what the effect on transformation was.Extracts were treated with either proteases(to destroy protein), RNase (to destroy RNA),or DNase (to destroy DNA). The ability of eachof the treated extracts to transform R cells wasthen tested. Neither the proteases nor the RNasehad any effect, but the DNase rendered theextracts inactive - proving that transformationwas due exclusively to DNA. Also lipid andprotein extraction did not reduce transformation.Oswald Avery published this work in 1943, withColin MacLeod and Maclyn MacCarthy asco-authors.
19Search for genetic material: HistorySearch for genetic material:It wasn’t until 1944 that Oswald Avery and his colleagues, who were studying the bacteria which causes pnuemonia, discovered by process of elimination that bacteria contain nucleic acids, and that DNA is the chemical which carries genes.Despite the conclusive results of Avery’s experiments, the theory of nucleic acids being the genetic material was still not a popular one, but experiments Performed with viruses also showed that nucleic acids were the genetic material and this confirmed Avery’s work.
20Alfred Hershey and Martha Chase 1952 1600s 1800s 1850s 1900s 1950s 2000sAlfred Hershey and Martha Chase 1952used bacteriophage (a virus) to prove thatDNA was the hereditary materialthe bacteriophage was the ideal organism forsettling the debate between protein and DNA.
24DNA discovery Hershey-Chase 1952 connecting DNA and heredity.Side by side experiments are performed with separate bacteriophage (virus) cultures in which either the protein capsule is labeled with radioactive sulfur or the DNA core is labeled with radioactive phosphorus.The radioactively labeled phages are allowed to infect bacteria.Agitation in a blender dislodges phage particles from bacterial cells.Centrifugation concentrates cells, separating them from the phage particles left in the supernatant.Results:Radioactive sulfur is found predominantly in the supernatant.Radioactive phosphorus is found predominantly in the cell fraction, from which a new generation of infective phage can be isolated.Conclusion: The active component of the bacteriophage that transmits the infective characteristic is the DNA. There is a clear correlation between DNA and genetic information.
29History Search for genetic material: Classic experiments for evidence Griffith: transformation Hershey-Chase: DNA necessary to produce more virusOther supporting evidence DNA volume doubles before cells divide Chargaff: ratio of nucleotides A = T and G = C
30Erwin Chargaff – Austrian American biochemist 1600s 1800s 1850s 1900s 1950s 2000sErwin Chargaff – Austrian American biochemist# of Adenin units were EQUAL to the # of T G = C
31letters, noticing that A Matches to T, and C to KEY PLAYERSErwin Chargaff ( ) Chargaff discovered thepairing Rules of DNAletters, noticing that AMatches to T, and C toG. He later criticized molecularbiology, the discipline he helped invent, as "the practice of biochemistry without a license", and once described Francis Crick as looking like "a faded racing tout".
33James Watson – American ornithologist 1600s 1800s 1850s 1900s 1950s 2000s1953James Watson – American ornithologistFrancis Crick – British PhysicistComplementary double-helical structure of DNA
34The ScientistsJames Watson was an American, born in 1928, was only 24 when the discovery was made. He went to Chicago University at the age of 15.Francis Crick was born in He went to London University and trained as a physicist. After the war he changed the direction of his research to molecular biology.
35Watson went to university in Chicago at age 15, and KEY PLAYERSJames Watson ( )Watson went to universityin Chicago at age 15, andteamed up with Crick inCambridge in late After solving the double helix, he went on to work on viruses and RNA, another genetic information carrier. He also helped launch the human genome project, and is president of Cold Spring Harbor Laboratory in New York.
36switched to biology after KEY PLAYERSFrancis Crick ( ) Crick trained and workedas a physicist, butswitched to biology afterthe Second World War. After co discovering the structure of DNA, he went on to crack the genetic code that translates DNA into protein.At the time of his death he was studying consciousness at California's Salk Institute.
37The DiscoveryThe DNA molecule was discovered in 1951 by Francis Crick, James Watson and Maurice Wilkins using X-ray Diffraction.In Spring 1953, Francis Crick and James Watson, two scientists working at the Cavendish Laboratory in Cambridge, discovered the structure of the DNA a double helix, or inter-locking pair of spirals, joined by pairs of molecules.
39The DiscoveryThe seed that generated this was Watson’s presence at a conference in Naples in 1951, where an x-ray diffraction picture from DNA was shown by Maurice Wilkins from King’s College in London.This made a strong impression on Watson – the first indication that genes might have a regular structure.
40KEY PLAYERSLinus Pauling ( ) The titan of twentieth-century chemistry, Pauling led the way in working out the structure of big biological molecules, and Watson and Crick saw him as their main competitor. In early 1953, working without the benefit of X-ray pictures, he published a paper suggesting that DNA was a triple helix.
41James Watson shared an office with Crick, and the topic of DNA structure naturally arose – particularly how to determine it. They were inclined to follow the methods of Pauling who had designed a helical structure by building a model consistent with the x-ray patterns from fibrous proteins.Like proteins, DNA was built from similar units – the bases adenine(A), thymine(T), guanine(G) and cytosine(C), and so it seemed likely that DNA also had a helical structure. The published x-ray patterns of DNA were not very clear, so contact was made with King’s.Watson attended a DNA colloquium there in November 1951, at which Rosalind Franklin described her results.
42Rosalind Franklin- English Chemist 1600s 1800s 1850s 1900s 1950s 2000s1920 – 1958Rosalind Franklin- English Chemistthe most beautiful X-ray photographs of any substance ever taken(1952) crucial contributions to the solution of the structure of DNAX-ray diffraction images double stranded and helical
44Of the four DNA researchers, only Rosalind Franklin had any degrees in chemistry. She was born into a prominent London banking family, where all the children—girls and boys—were encouraged to develop their individual aptitudes.She attended St. Paul’s Girls School, one of the few schools in London where girls were taught science. Then she proceeded to Newnham College, one of the women’s colleges at Cambridge University.She completed her degree in 1941 in the middle of World War II and undertook graduate work at Cambridge with Ronald Norrish, a future Nobel Prize winner.
45She resigned her research scholarship in just one year to contribute to the war effort at the British Coal Utilization Research Association. There she performed fundamental investigations on the properties of coal and graphite.She returned briefly to Cambridge, where she presented a dissertation based on this work and was granted a Ph.D. in physical chemistry.After the war, through a French friend, she gained an appointment at the Laboratoire Centrale des Services Chimiques de l’Etat in Paris, where she was introduced to the technique of X-ray crystallography and rapidly became a respected authority in this field.In 1951 she returned to England to King’s College London, where her charge was to upgrade the X-ray crystallographic laboratory there for work with DNA.
46towards the right structure. She went on to do pioneering work KEY PLAYERSRosalind Franklin ( ) Franklin, trained as a chemist, was expert in deducing the structure of molecules by firing X-rays through them. Her images of DNA – disclosed without her knowledge - put Watson and Crick on the tracktowards the rightstructure. She went onto do pioneering workon the structures ofviruses.
47The Evidence Search for genetic material: James Watson and Francis Crick used this photo with other evidence to describe the structure of DNA.X-ray diffraction photo of DNAImage produced by Rosalind Franklin
48In July 1952, Erwin Chargaff visited Watson and Crick and told of his 1947 findings that the ratios of A/T and G/C were statistically equal for a wide variety of DNA’s. Crick became convinced that base pairing was the key to the structure.Prompted by receiving a flawed manuscript on DNA structure from Pauling, Watson again visited King’s and Wilkins showed him a DNA x-ray pattern taken by Franklin showing clear helical characteristics.
49OOPS!!!Watson brought back a less-than-accurate account to Cambridge, but Crick produced a three-strand model structure only a week later.Invited to view this,Franklin pointed out that it was inconsistent with her results – it had the phosphate groups on the inside whereas her results showed they were on the outside,and the water content was too low.
50Watson & Crick What they deduced from: Franklin’s X-ray data • Double helix• Uniform width of 2 nm• Bases stacked 0.34 nm apartChargoff’s “rules”• Adenine pairs with thymine• Cytosine pairs with guanine
51Watson & Crick What they came up with on their own: • Bases face inward, phosphatesand sugars outward• Hydrogen bonding• Hinted at semi-conservativemodel for replication
52HistoryWatson began pursuing the idea of hydrogen bonding by using cardboard cutouts of the four bases. He found that (A+T) and (G+C) could be bonded together to form pairs with very similar shapes. On this basis, a model was built consistent with the Franklin’s symmetry and Chargaff’s results, and Watson & Crick published in April 1953 in Nature accompanied by ones from the Wilkins and Franklin groups. Watson and Crick’s paper ends with the oft-quoted line “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material”.
54The Nobel PrizeCrick, Watson and Wilkins won the Nobel Prize for medicine in Maurice Wilkins was at King's College, London and was an expert in X-ray photography. His colleague, Rosalind Franklin, did brilliant work developing the technique to photograph a single strand of DNA. She received little recognition for this at the time and died tragically of cancer in 1958, so could not be recognized in the Nobel Award.