DNA Discovery PDF

DNA 1  STRUCTURE Deoxyribonucleic Acid Macromolecule: Polynucleotide or Nucleic Acid The genetic material of the cell The instructions on how to make proteins Structure: Made of nucleotides Nitrogenous bases Pentose (5 Carbon Carbohydrate) Deox...

DNA 1  STRUCTURE Deoxyribonucleic Acid Macromolecule: Polynucleotide or Nucleic Acid The genetic material of the cell The instructions on how to make proteins Structure: Made of nucleotides Nitrogenous bases Pentose (5 Carbon Carbohydrate) Deoxyribose Phosphate group Double-helix: 2 strands Strands run anti-parallel: opposite directions Complementary base pairings: specific bases will always pair with each other through Hydrogen-bonds. Each nucleotide is linked by phosphodiester bonds. 2  FUNCTIONS Stores genetic information (where?) Eukaryotes = Nucleus vs. Prokaryotes = Nucleoid Copies genetic information (when & process?) S-phase of Interphase and Replication Gene expression: controls which proteins are made Universal molecule of heredity: transmitted to next generation DNA analysis has practical applications such as: Identifying criminals (forensics) Identifying pathogens: (Ex. newly discovered organism. What part of genome is harmful?) Tracing disease outbreaks: (Ex. Covid PCR testing to identify infected individuals) Identifying defective cells: (Ex. Cancer in order to come up with therapies) Tracing genealogy Intraspecies: within the species. Each person’s DNA is unique and is possible to detect differences among individuals within a species on the basis of these features Interspecies: between different species. How closely related are organisms 3  ARRANGEMENT & PACKAGING Genome: a cell’s full genetic information DNA is far too large to fit inside of the cell unraveled, so it must be supercoiled, wrapped tightly The structure is maintained by an enzyme called gyrase. DNA is arranged inside of cells in the form of chromosomes, which are passed down to offspring in asexual (1 parent) or sexual reproduction. Sexual reproduction involves 2 parents which produce sex cells called gametes, which contain 1 (haploid) out of the 2 homologous chromosomes passed down by the parents to the offspring in heredity. Chromosomes have genes inside, which code for proteins. Eukaryotes have multiple linear chromosomes. Prokaryotes have a single circular chromosome. 4  ARRANGEMENT & PACKAGING DNA is supercoiled by a protein known as histones. The histone-DNA complex is known as nucleosomes. The structure is maintained by an enzyme called gyrase. A nucleosome is tied to the next one with the help of linker DNA. This forms a “beads on a string” structure. (Draw on board) Multiple nucleosomes are known as chromatin. Euchromatin: region of DNA loosely compacted. Contains genes which are transcribed to make proteins. Heterochromatin: region of DNA tightly compacted. Contains genes which are not expressed. 5  TIMELINE Our modern understanding of DNA is obtained from various scientists: 1668 Redi’s Non-spontaneous generation 1670 Leeuwenhoek’s 1st Microscope 1839 Schwann’ & Schleiden’s Cell Theory 1859 Darwin’s Origin of Species 1861 Pasteur’s Germ Theory 1863 Mendel’s Pea Plant Experiment 1902 Sutton’s and Boveri’s Chromosomal Theory 1905 & 1967 Merechkowski’s and Margulis’s EndoSymbiotic Theory 1928 Griffith’s Transformation Experiment 1944 Avery’s Degradation Experiment 1950 Chargaff ’s Complementary Base Pairing 1952 Miller & Urey’s Chemical Evolution Experiment 1952 Hershey & Chase Bacteriophage Experiment 1952 Franklin’s X-Ray Crystallography 1953 Watson & Crick’s Double-Helix Model 1958 Messelson & Stahl Semi-Conservative Replication Experiment 6  WALTER SUTTON & THEODOR BOVERI Mendel concluded that each parent contributed 1 of 2 paired unit factors to each offspring Paired unit factors of heredity are transmitted faithfully from generation to generation by dissociation (gametogenesis) and re-association of paired factors during fertilization. After crossing peas with contrasting traits and finding recessive trait resurfacing in F2, Mendel deduced hereditary factors must be inherited as discrete units. This finding contradicted belief at that time that parental traits were blended in offspring and originals disappeared. In 1902, Sutton and Boveri independently identified chromosomes as these “unit factors” carriers of genetic material in their Chromosomal Theory of Inheritance: During meiosis, homologous chromosome pairs migrate as discrete structures independent of other chromosome pairs Sorting of chromosomes from each homologous pair appear to be random Each parent synthesizes gametes that contain only ½ chromosomal complement Male & female gametes differ in size and morphology, but have same number of chromosomes, suggesting equal genetic contributions from each parent. Gametic chromosomes combine during fertilization to produce offspring with same # of chromosomes as parents. 7  FREDERICK GRIFFITH Bacteriologist Griffith was studying Streptococcus pneumonia There are 2 strains: variants of the same specie The disease-causing bacteria was called “S” for smooth The harmless bacteria was called “R” for rough Experiment: 1. He injected mice with the S-strain and they died 2. He injected mice with R-strain and they remained healthy 3. He heated S-strain bacteria Injected it into mice They remained healthy 4. He heated S-strain bacteria Mixed it with the R-strain bacteria Injected it into the mice Mice died. 8  TRANSFORMATION When he biopsy the dead mice’s lungs he found the harmful bacteria. How did this happened if they were killed by the heat? Even though the harmful bacteria (S) was dead they were able to pass “something” to the harmless bacteria (R). The “something” was DNA. However, Griffith did not know this at the time. Transformation: process in which external DNA taken up by cell, changes its morphology & physiology His experiment showed that hereditary information could be transferred from 1 cell to another horizontally rather than descent. 9  OSWALD AVERY Avery, MacLeod and McCarty were interested in exploring what was the material that was transferred in between both strains (S and R) of Streptococcus pneumonia bacteria in Griffith’s transformation experiment. They extracted a mixture of various molecules from the heat-killed S strain and broke it up into 4 test tubes. They exposed each test tube to different enzymes that would degrade a diff substance : Test Tube #1: Lipid-degrading enzyme 1 2 3 4 Test Tube #3: RNA-degrading enzyme Test Tube #2: Protein-degrading enzyme Test Tube #4: DNA-degrading enzyme Then, they injected it each mixture into a different mice. 10  OSWALD AVERY Test Tube #1: Lipid-degrading enzyme 1 2 3 4 Test Tube #3: RNA-degrading enzyme Test Tube #2: Protein-degrading enzyme Test Tube #4: DNA-degrading enzyme Mice injected solution of test tube #1 died. Mice injected solution of test tube #2 died. Mice injected solution of test tube #3 died. Mice injected solution of test tube #4 remained healthy. It was concluded that DNA was the molecule of heredity, which contained genes. 11  BACTERIOPHAGE Hershey and Chase were studying bacteriophage, a virus that infects bacteria Viruses have a simple structure: Protein coat called capsid Nucleic acid core containing DNA as genetic material. They infects host bacterial cell by attaching to a receptor at the surface It then injects its nucleic acid inside the cell Phage DNA makes multiple copies of itself using host machinery Eventually the sheer amount of viral particles lyses (burst) the host cell Large number of phages are released to start the cycle again with other cells from the multi-cellular organism. 12  ALFRED HERSHEY & MARTHA CHASE They designed an experiment to find out if the protein coat or the DNA core is the one that entered the bacterial cell. They grew the viruses in 2 batches, each with a different radioactive isotope: Phosphorus-32 vs. Sulfur-35 (Why?) Radioactive isotopes: markers with fluorescent tags to identify each structure separately DNA contains Phosphorous and no Sulfur. Each batch was allowed to infect the host separately. 13  ALFRED HERSHEY & MARTHA CHASE Each solution was placed inside of a blender, which caused the phage coat to be detached from the bacterial cells. The phages and host cells are now suspended separately in the solution Next, they used a centrifuge to aggregate/cluster the heavier bacterial cells into a clump called a pellet, which is deposited at the bottom of the tube. Radioactivity was detected in the pellet tagged with Phosphorus This experiment confirmed Avery’s findings that: DNA was the genetic material. 14  ERWIN CHARGAFF In 1950 Chargaff examined the content of DNA in different species. Percentages Of Bases In Five Organisms Organism Adenine Thymine Guanine Cytosine Streptococcus 29.8% 31.6% 20.5% 18.0% E. Coli 24.7% 23.6% 26.0% 25.7% Yeast 31.3% 32.9% 18.7% 17.1% Herring 27.8% 27.5% 22.2% 22.6% Human #1 30.9% 29.4% 19.9% 19.8% Human #2 30.8% 29.5% 19.8% 19.9% He found that the amount of nitrogenous base adenine was very similar to nitrogenous base thymine He found that the amount of nitrogenous base guanine was very similar to nitrogenous base cytosine Chargaff ’s Rule states that the nitrogenous bases mentioned above pair complementary 15  ERWIN CHARGAFF Percentages Of Bases In Five Organisms Organism Adenine Thymine Guanine Cytosine Streptococcus 29.8% 31.6% 20.5% 18.0% E. Coli 24.7% 23.6% 26.0% 25.7% Yeast 31.3% 32.9% 18.7% 17.1% Herring 27.8% 27.5% 22.2% 22.6% Human #1 30.9% 29.4% 19.9% 19.8% Human #2 30.8% 29.5% 19.8% 19.9% In addition, he found that nitrogenous bases were not found in equal quantities in each organism. As a result, this variation from organism to organism lay the foundation to later find that: The more DNA species share the closer related the organisms are to each other. The more DNA individuals within a specie share, the more closely related they are to each other. The difference in DNA accounts for the variation found in a specie. 16  ROSALIND FRANKLIN Franklin studied the structure of DNA through a technique called X-ray diffraction. She stretched the DNA fibers in a thin glass tube so that the strands were parallel. DNA is invisible to the naked eye. However, just like bacteria, when there is a lot of it, it can be seen as a mass. She aimed the X-ray beam at the DNA sample and recorded the scattered pattern. X-Ray photograph number 51: X-shape patterns shows strands are twisted around each other creating shape known as helix. Nitrogenous bases are also shown to be stacked at regular intervals near the center of the molecule. 17  JAMES WATSON & FRANCIS CRICK Watson and Crick were attempting to build a 3-dimensional model of DNA. They looked at Chargaff ’s research of complementary base pairings. Watson saw a copy of Franklin’s X-ray Photo 51 and then it clicked. Using cardboard and wire, along with the information from both Chargaff and Franklin, they were able to finally build their model. The model incorporated the following: Double-helix: DNA is made of 2 strands which twist around each other. Antiparallel: the strands of DNA run in opposite directions. Complementary: Nitrogenous base pairs Adenine always pairs with Thymine. Guanine always pairs with Cytosine. 18 MATTHEW MESSELSON & FRANKLIN STAHL During Interphase (S), before the cell divides, DNA must be copied in a process known as replication. Conservative: the parental DNA remains together and the newly formed daughter strands are also together. Semi-conservative: each of the 2 parental DNA strands act as a template for new DNA to be synthesized. After replication, each double-stranded DNA includes one parental or old strand and one new strand. Dispersive: both copies of DNA have double-stranded segments of parental DNA and newly synthesized DNA. Designed an experiment using radioactive Nitrogen in order to find out which of the 3 models was correct. DNA Replications is carried out through the semi-conservative model. 19