1. Discovery of DNA and Function
1.1. James Watson and Francis Crick
1.1.1. Watson and Crick took a crucial conceptual step, suggesting the molecule was made of two chains of nucleotides, each in a helix as Franklin had found, but one going up and the other going down. Crick had just learned of Chargaff's findings about base pairs in the summer of 1952. He added that to the model, so that matching base pairs interlocked in the middle of the double helix to keep the distance between the chains constant.
1.1.1.1. Watson and Crick showed that each strand of the DNA molecule was a template for the other. During cell division the two strands separate and on each strand a new "other half" is built, just like the one before. This way DNA can reproduce itself without changing its structure -- except for occasional errors, or mutations.
2. A Gene Is Made from DNA
2.1. Oswald Avery
2.1.1. In 1944, he and his coworkers discovered that DNA carries a cell’s genetic material and can be altered through transformation. He did not receive a Nobel Prize, but his research led to understanding the genetic code. He died on February 20, 1955.
2.1.2. He is best known for his discovery that deoxyribonucleic acid (DNA) serves as genetic material.
3. DNA Is Needed For Viral Replication
3.1. Hershey and Chase
3.1.1. Alfred Hershey and Martha Chase suggest that only DNA is needed for viral replication. Using radioactive isotopes 35S to track protein and 32P to track DNA, they show that progeny T2 bacteriophage isolated from lysed bacterial cells have the labeled nucleic acid. Further, most of the labeled protein doesn’t enter the cells but remains attached to the bacterial cell membrane.
3.1.2. When bacteriophages containing 32P (radioactive), were allowed to infect nonradioactive bacteria, all the infected cells became radioactive and, in fact, much of the radioactivity was passed on to the next generation of bacteriophages.
3.1.2.1. However, when the bacteria were infected with bacteriophages labeled with 35S, and then the virus coats removed (by whirling them in an electric blender), practically no radioactivity could be detected in the infected cells.
4. Chargaff's Rule
4.1. RULE 1
4.1.1. Human DNA is 30.9% A and 29.4% T, 19.9% G and 19.8% C. The rule constitutes the basis of base pairs in the DNA double helix: A always pairs with T, and G always pairs with C. He also demonstrated that the number of purines (A+G) always approximates the number of pyrimidines (T+C), an obvious consequence of the base-pairing nature of the DNA double helix.
4.1.2. Chargaff determined that in DNA, the amount of one base, a purine, always approximately equals the amount of a particular second base, a pyrimidine. Specifically, that in any double-stranded DNA the number of guanine units equals approximately the the number of cytosine units and the number of adenine units equals approximately the number of thymine units.
4.2. RULE 2
4.2.1. In 1947 Chargaff showed that the composition of DNA, in terms of the relative amounts of the A, C, G and T bases, varied from one species to another. This molecular diversity added evidence that DNA could be the genetic material.
5. "The Secret of Life"
5.1. Rosland Franklin
5.1.1. Born in 1920 in London, England, Rosalind Franklin earned a Ph.D. in physical chemistry from Cambridge University. She learned crystallography and X-ray diffraction, techniques that she applied to DNA fibers. One of her photographs provided key insights into DNA structure. Other scientists used it as the basis for their DNA model and took credit for the discovery. Franklin died of ovarian cancer in 1958, at age 37.
5.1.2. He taught her X-ray diffraction, which would largely play into her discovery of "the secret of life"—the structure of DNA. In addition, Franklin pioneered the use of X-rays to create images of crystalized solids in analyzing complex, unorganized matter, not just single crystals.
6. DNA Carries Information Within the Cell
6.1. Frederick Griffith
6.1.1. The paper showed that a nonpathogenic strain of the bacterium Streptococcus pneumonaie could be induced to take on the disease-causing characteristics of a different strain, a finding which formed the foundation of the transforming principle.
6.1.2. Griffith's paper drew substantial attention, and by the time of his death in a 1941 Axis bombing of London, further research inspired by his work had led to progress against puerperal fever, scarlet fever, surgical sepsis, and infections from wounds. The ultimate importance of his paper, however, was not truly understood until a decade after his death, as further research by Oswald Avery and others concluded that the transforming agent described by Griffith is deoxyribonucleic acid (DNA), and that DNA carries information within the cell.
7. The FATHER of Genetics
7.1. Founded the science of genetics.
7.1.1. Identified many of the rules of heredity. These rules determine how traits are passed through generations of living things.
7.1.2. Saw that living things pass traits to the next generation by something which remains unchanged in successive generations of an organism – we now call this ‘something’ genes.
7.1.3. Realized that traits could skip a generation – seemingly lost traits could appear again in another generation – he called these recessive traits.
7.1.4. Identified recessive and dominant traits which pass from parents to offspring.
7.1.5. Established, momentously, that traits pass from parents to their offspring in a mathematically predictable way.
7.2. Mendel’s work only made a big impact in 1900, 16 years after his death, and 34 years after he first published it.
8. Leeuwenhoek
8.1. Antony van Leeuwenhoek learned to grind lenses, made simple microscopes, and began observing with them. He seems to have been inspired to take up microscopy by having seen a copy of Robert Hooke's illustrated book Micrographia, which depicted Hooke's own observations with the microscope and was very popular.
8.1.1. Schleiden
8.1.1.1. he wrote “Contributions to Phytogenesis” (1838), in which he stated that the different parts of the plant organism are composed of cells or derivatives of cells. Schleiden became the first to formulate what was then an informal belief as a principle of biology equal in importance to the atomic theory of chemistry. He also recognized the importance of the cell nucleus, discovered in 1831 by the Scottish botanist Robert Brown, and sensed its connection with cell division. Schleiden was one of the first German biologists to accept Darwin’s theory of evolution. He became professor of botany at Dorpat, Russia, in 1863.
8.1.1.2. Schwann
8.1.1.2.1. In 1836, while investigating digestive processes, he isolated a substance responsible for digestion in the stomach and named it pepsin, the first enzyme prepared from animal tissue. At Liège he investigated muscular contraction and nerve structure, discovering the striated muscle in the upper esophagus and the myelin sheath covering peripheral axons, now known as Schwann cells. He coined the term metabolism for the chemical changes that take place in living tissue, identified the role played by microorganisms in putrefaction, and formulated the basic principles of embryology by observing that the egg is a single cell that eventually develops into a complete organism.