DNA The Code of Life The Molecular Basis of Inheritance
DNA 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.
History Although we now accept the idea that DNA is responsible for our biological structure, But in the early 1800s 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.
History Charles Darwin is famous for challenging this view. In 1859 he published 'The Origin of Species‘ expressing that living things might appear to be designed, but were actually the result of natural selection. Darwin showed that living creatures evolve over several generations through a series of small changes.
History In the 1860s Darwin's ideas were supported when genetics was discovered by Gregor Mendel. He found that genes determine the characteristics a living thing will take. The genes are passed on to later generations, with a child taking genes from both its parents. The great mystery was where and how is this information stored?
History The main conclusions made by Mandle were: *SEGREGATION: Inherited traits are controlled by genes, which are in pairs. When sex cells are created one gene from each pair goes into the gamete. When two gametes fuse at fertilization, the offspring has two copies of each gene—one from each parent. *INDEPENDENT ASSORTMENT: The genes for different traits are sorted into gametes independently of other genes. So one inherited trait is not dependent on another. *DOMINANCE: Where there are two different forms of a gene are present in a pea plant, the one which is dominant is the one that is observed.
History 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. While he 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. They were made up of 20 different building blocks (amino acids), as opposed to the mere 4 building blocks of nucleic acids.
History Search for genetic material: In the early 1900s, Phoebus Levene, who also believed that proteins must be the chemicals of inheritance, studied the composition of nucleic acids. He discovered that DNA is a chain of nucleotides, with each nucleotide consisting of a deoxyribose sugar, a phosphate group and a nitrogenous base, of which there were four different types. He proposed that the four different types of nucleotide were repeated over and over in a specific order. This would make DNA a relatively simple repeat sequence – no wonder DNA wasn’t considered to be smart enough to code for all of life!
History Search for genetic material: 1928 Frederick Griffith: transforming principle
Search for genetic material: History It wasn’t until 1944 that Oswald Avery and his colleagues, who were studying the bacteria which causes pnuemonia, Pneumococcus, 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.
Search for genetic material: 1952 - Hershey-Chase Experiment History
History Search for genetic material: Classic experiments for evidence Griffith: transformation Hershey-Chase: DNA necessary to produce more virus Other supporting evidence DNA volume doubles before cells divide Chargaff: ratio of nucleotides A = T and G = C
The Discovery The 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.
The Discovery The 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 indicationthat genes might have a regular structure.
History Search for genetic material: James Watson joined the unit (its first biologist) and began by trying to crystallize myoglobin for Kendrew. The unsuccess of this left much time for discussion with Crick, whose office he was sharing, and the topic of DNA structure naturally arose – particularly how to determine it. They were inclined to follow the method of Pauling who had deduced the 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 too had a helical structure. The publishedx-ray patterns of DNA were not very clear, and so contact was made with King’s. Watson attended a DNA colloquium there in November 1951, at which Rosalind Franklin described her results.
History Search for genetic material: Watson brought back a less-than-accurate account to Cambridge, but with Crick produced a three-strandmodel structure only a week later. Invited to view this,Franklin pointed out that it was inconsistent with her results – it had thephosphate groups on the inside whereas her results showed they were on the outside,and the water content was too low. The work at Cambridge stopped abruptly for a bit.
History Search for genetic material: In July 1952, Erwin Chargaff visited the unit and told of his 1947 findings that the ratios of A/T and G/C were unity for a wide variety of DNAs. 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 of the pure B- form showing clear helical characteristics, plus the intense 10th layer line at 3.4A and a 20A equatorial reflection indicating the molecular diameter. Perutz also showed them a report on the work of the King’s group which gave the space group of the crystalline A-form as C2, from which Crick deduced that there were two chains running in opposite directions.
History Search for genetic material: Watson began pursuing the idea of hydrogen bonding 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 symmetry and with Chargaff’s results, and a paper was published in April 1953 in Nature accompanied by ones from the Wilkins and Franklin groups at King’s. 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”.
The 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 DNA Image produced by Rosalind Franklin
The Scientists Francis Crick was born in 1916. He went to London University and trained as a physicist. After the war he changed the direction of his research to molecular biology. James Watson was an American, born in 1928, so aged only 24 when the discovery was made. He went to Chicago University aged only 15 and had already worked on DNA.
The Nobel Prize Crick, Watson and Wilkins won the Nobel Prize for medicine in 1962. 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 recognised in the Nobel Award.
Watson & Crick What they deduced from: Franklin’s X-ray data Double helix Uniform width of 2 nm Bases stacked 0.34 nm apart Chargoff’s “rules” Adenine pairs with thymine Cytosine pairs with guanine
Watson & Crick What they came up with on their own: Bases face inward, phosphates and sugars outward Hydrogen bonding Hinted at semi-conservative model for replication
KEY PLAYERS Oswald Avery (1877-1955) Microbiologist Avery led the team that showed that DNA is the unit of Inheritance. One Nobel laureate has called the discovery "the historical platform of modern DNA research", and his work inspired Watson and Crick to seek DNA's structure.
KEY PLAYERS Erwin Chargaff (1905-2002) Chargaff discovered the pairing rules of DNA letters, noticing that A Matches to T and C to G. He later Criticized molecular biology, the discipline he helped invent, as "the practice of biochemistry without a licence",and once described Francis Crick as looking like "a faded racing tout".
KEY PLAYERS Francis Crick (1916- ) Crick trained and worked as a physicist, but switched to biology after the Second World War. After co-discovering the structure of DNA, he went on to crack the genetic code that translates DNA into protein. He now studies consciousness at California's Salk Institute.
KEY PLAYERS Rosalind Franklin (1920-58) 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 track towards the right structure. She went on to do pioneering work on the structures of viruses..
KEY PLAYERS Linus Pauling (1901-94) 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.
KEY PLAYERS James Watson (1928- ) Watson went to university in Chicago aged 15, and teamed up with Crick in Cambridge in late 1951. 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.
KEY PLAYERS Maurice Wilkins (1916- ) Like Crick, New Zealand-born Wilkins trained as a physicist, and was involved with the Manhattan project to build the nuclear bomb. Wilkins worked on X-ray crystallography of DNA with Franklin at King's College London, although their relationship was strained. He helped to verify Watson and Crick's model, and shared the 1962 Nobel with them.
Structure There are 4 different nucleotides in DNA AdenineThymine Adenine pairs with Thymine GuanineCytosine Guanine pairs with Cytosine
Structure AdenineThymine Adenine pairs with Thymine GuanineCytosine Guanine pairs with Cytosine
Structure Does DNA fit the requirements of a hereditary material? RequirementDNA component Has biologically useful information to make protein Genetic code: 3 bases code for 1 amino acid (protein) Must reproduce faithfully and transmit to offspring Complementary bases are faithful; found in germ cells Must be stable within a living organism Backbone is strong covalent bonds; hydrogen bonds Must be capable of incorporating stable changes Bases can change through known mechanisms
Protein Synthesis DNA carries the instructions for the production of proteins.A protein is composed of smaller molecules called amino acids, and the structure and function of the protein is determined by the sequence of its amino acids. The sequence of amino acids, in turn, is determined by the sequence of nucleotide bases in the DNA. A sequence of three nucleotide bases, called a triplet, is the genetic code word, or codon, that specifies a particular amino acid.
Protein Synthesis Protein synthesis begins with the separation of a DNA molecule into two strands. In a process called transcription, a section of the sense strand acts as a template, or pattern, to produce a new strand called messenger RNA (RNA). The RNA leaves the cell nucleus and attaches to the ribosomes, specialized cellular structures that are the sites of protein synthesis. Amino acids are carried to the ribosomes by another type of RNA, called transfer (RNA). In a process called translation, the amino acids are linked together in a particular sequence, dictated by the RNA, to form a protein.
Replication Before replication, the parent DNA molecule has 2 complementary strands First the 2 strands separate Each “old” strand serves as a template to determine the order of the nucleotides in the new strand Nucleotides are connected to form the backbone; now have 2 identical DNA molecules.
Replication Helicase unwinds the molecule Single-strand binding protein stabilized ssDNA Primase initiates the replication with RNA DNA polymerase extends the new DNA Second DNA polymerase removes the RNA DNA ligase joins all the fragments DNA Replication is simple, but it takes a large team of enzymes and proteins to carry out the process:
1971-Smith & Nathans 1971-Smith & Nathans Discovery of restriction endonucleases Hamilton Smith Discovered HindII in Haemophilus influenzae Daniel Nathans Used HindII to make first restriction map of SV40
1972 - Paul Berg Produces first recombinant DNA using EcoRI 1973 -Boyer, Cohen & Chang Transform E. coli with Recombinant plasmid
1977 - Genentech, Inc. Company founded by Herbert Boyer and Robert Swanson in 1976 Considered the advent of the Age of Biotechnology First human protein (somatostatin) produced from a transgenic bacterium. Walter Gilbert and Allan Maxam devise a method for sequencing DNA.
1978 David Botstein discovers RFLP analysis 1980 U.S. Supreme Court rules that life forms can be patented Kary Mullis develops PCR. Sells patent for $300M in 1991
1981 First transgenic mice produced 1982 The USFDA approves sale of genetically engineered human insulin 1983 An automated DNA sequencer is developed A screening test for Huntington’s disease is developed using restriction fragment length markers.
1984 Alec Jeffreys introduces technique for DNA fingerprinting to identify individuals 1985 Genetically engineered plants resistant to insects, viruses, and bacteria are field tested for the first time The NIH approves guidelines for performing experiments in gene therapy on humans
1987 invention of YACs (yeast artificial chromosomes) as expression vectors for large proteins 1989 National Center for Human Genome Research created to map and sequence all human DNA by 2005.
1990 UCSF and Stanford issued their 100th recombinant DNA patent and earning $40 million from the licenses by 1991. BRCA-1 discovered First gene therapy attempted on a four-year-old girl with an inherited immune deficiency disorder.
1992 U.S. Army begins "genetic dog tag" program 1994 The Flavr Savr tomato gains FDA approval The first linkage map of the human genome appears 1995 The first full gene sequence of a living organism is completed for Hemophilus influenzae. O.J. Simpson found not guilty despite DNA evidence
1996 The yeast genome, containing approximately 6,000 genes and fourteen million nucleotides, is sequenced. 1997 Dolly cloned from the cell of an adult ewe DNA microarray technology developed
1997 The genome of the bacterium E. coli, a classic model organism for studying microbiological and molecular genetic mechanisms, and a natural symbiont in the human digestive tract, is completely sequenced, revealing about 4,600 genes among about four and one-half million nucleotides.
1998 The genome of a nematode worm Caenorhabditis elegans, a key model organism for investigating genetic regulation of development, is sequenced, revealing approximately 18,000 genes among some 100 million nucleotides of DNA sequence.
1999 1,274 biotechnology companies in the United States At least 300 biotechnology drug products and vaccines currently in human clinical trials Human Genome Project is on time and under budget, the complete human genome map expected in five years or less
1999 Jesse Gelsinger, an eighteen year-old with a genetic disorder affecting liver metabolism, dies from an immune reaction to a gene therapy treatment. This tragic event slows gene therapy applications and results in greater scrutiny and caution toward the growing number of gene therapy research trials.
1999 The first complete sequence of a human chromosome (number 22) is completed by the public genome project and is published. This step indicates that the genome project is proceeding ahead of schedule, and also shows a surprisingly small number of genes (about 300) relative to the anticipated 100,000 or so for all twenty-four human chromosomes (twenty-two chromosomes called autosomes shared equally by males and females, plus the X-chromosome which is paired in females but occurs in a single copy in males, plus the Y- chromosome that is unique to males).
2000 Celera sequences the genome of the fruitfly (Drosophila melanogaster), identifying approximately 13,000 genes among 170 million nucleotides. First plant genome sequenced (Arabidopsis thaliana) from the mustard family. The Arabidopsis genome consists of about 100 million nucleotides, and approximately 20,000 genes, indicating that at the molecular genetic level, plant and animal genomes are about equally complex.
2000 "Golden rice," a genetically engineered strain of rice manufactures its own vitamin A. Golden rice is created by Ingo Potrykus, plant geneticist, and his colleagues to help alleviate severe health problems in many areas of the world caused by vitamin A deficiency.
2001 In mid-February, the journal SCIENCE publishes an analysis of the Celera Human Genome Project, and the journal NATURE publishes an analysis of the public Human Genome Project. Both revealed a surprisingly small number of human genes, estimated jointly at about 30,000 to 35,000, barely more than a worm, fruitfly, or plant. Both show that only about 2 percent of our DNA actually codes for amino acid sequences of proteins, and both identify many sequences of unknown function and variable length present in multiple copies making up approximately half the genome.
Extraction Each human cell has enough DNA to code for all the traits in the human body. If the DNA in one cell was stretched out, how long would it be? Do the math! There are 6 X 10 9 base pairs/cell Each base pair is 0.34 X 10 -9 meters long A human body has approximately 75 trillion cells. If the distance to the sun is 150 X 10 9 meters, how many round trips could your DNA make? Answer: 2 meters Answer: 500 trips