Lecture8dna

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Published on October 15, 2007

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Lecture 8 BIOL L100 Indiana University Southeast David Partin, Instructor:  Lecture 8 BIOL L100 Indiana University Southeast David Partin, Instructor Today’s Agenda:  Today’s Agenda Announcements/Questions Lecture 8: DNA Structure & Function Video clips One Minute Paper DNA Modeling Lab & lab report Introduction to Case Study #2 Chapter 13::  Chapter 13: DNA Structure and Function Part I: A Scientific Mystery:  Part I: A Scientific Mystery Investigators::  Investigators: Gregor Mendel-1866 Friedrich Miescher-1869 Walter Sutton-1902 Thomas Hunt Morgan-1910 Frederick Griffith-1931 Oswald Avery-1944 Erwin Chargaff-1940s Alfred Hershey/Martha Chase-1952 Rosalind Franklin-1953 James Watson/Francis Crick-1953 Gregor Mendel-1866:  Gregor Mendel-1866 Proposed the basic laws of genetics from his experiments using pea plants. Dominant & recessive genes Independent assortment Probability of inheriting traits Friedrich Miescher-1869:  Friedrich Miescher-1869 Swiss chemist Isolated the nucleus from a cell Discovered a chemical he named “nuclein” Many chemists continued his work Found that nuclein was rich in phosphorus and contained no sulfur Found that nuclein contained an acidic substance they termed “nucleic acid” Found 2 types of nucleic acids: DNA & RNA Walter Sutton-1902:  Walter Sutton-1902 Suggested that “genes” are located on chromosomes. Noticed that “genes” are inherited in the same fashion as chromosomes. Thomas Hunt Morgan-1910 Stated that each “gene” had a locus on a particular chromosome. Used Drosophila melanogaster (fruit flies) for his studies. Hit the PAUSE button…:  Hit the PAUSE button… At this point, the idea of “genes” was an accepted notion. It was a very abstract idea: gene = trait that can be passed on. DNA had been discovered. Chromosomes had been discovered. Nobody had yet made the connection between genes & DNA. BIG questions remained…:  BIG questions remained… What IS the genetic material? What molecular substance is a “gene” made of? Scientists agreed that—no matter what substance genes were made of—this substance must be: 1. able to store information 2. stable so that it can be copied and passed on 3. able to undergo rare changes called mutations in order for evolution to occur Frederick Griffith-1931:  Frederick Griffith-1931 Oswald Avery & associates-1944:  Oswald Avery & associates-1944 Griffith’s experiment led Avery to design his own experiments to identify the “transforming agent.” Avery’s evidence showed: 1. DNA from S strain bacteria caused R strain bacteria to be transformed. 2. Enzymes that degrade proteins and RNA did not prevent transformation. 3. Enzymes that digest DNA did prevent transformation. 4. The DNA segment that transformed the bacteria contained about 1600 nucleotides—enough for genetic variability to be possible. So Avery had proven that the genetic material was DNA, right?:  So Avery had proven that the genetic material was DNA, right? Nope. Some folks still didn’t buy it. The skeptics replied that maybe DNA was just used to activate protein-based genes. In the 1950s, bacteriophages were beginning to be used as scientific tools. Scientists of the day were still asking: What does the virus inject into the bacteria? That’s where we’ll find the answer to our question: Is protein or DNA the genetic material?? Alfred Hershey/Martha Chase-1952:  Alfred Hershey/Martha Chase-1952 Step 1 Slide16:  Alfred Hershey/Martha Chase-1952 Step 2 Hershey & Chase’s conclusion?:  Hershey & Chase’s conclusion? DNA—not protein—is the genetic material! DNA transmits all the genetic information needed to produce new viruses. Part II: Case Closed! Well, not quite…:  Part II: Case Closed! Well, not quite… After almost 100 years, scientists had finally proven that DNA is the material that genes are made of. But they only had a foggy idea of what DNA really was. What was its structure? Since Friedrich Mieschner discovered “nuclein” in 1869, chemists had been working to figure out its chemical composition.:  Since Friedrich Mieschner discovered “nuclein” in 1869, chemists had been working to figure out its chemical composition. = 5 carbon sugar Chemists knew DNA was a polymer made of nucleotide monomers. They also knew that each nucleotide consisted of: 1. a phosphate group 2. a 5-carbon sugar 3. One of 4 nitrogen-containing bases Nucleotides are classified into 2 groups based on structure::  Nucleotides are classified into 2 groups based on structure: 1. Purines Adenine, Guanine 2. Pyrimidines Cytosine, Thymine, Uracil (RNA only) Mnemonics (brain ticklers for studying) Slide21:  Study tip: small name = BIG BASE STRUCTURE (2 RINGS) Slide22:  Study tip: BIG NAME = small base structure (one ring) Let’s take a break…:  Let’s take a break… Thanks for your attention! When we return: DNA structure replication Erwin Chargaff-1940s:  Erwin Chargaff-1940s Conclusions of Chargaff’s experiments came to be known as “Chargaff’s Rules”: The amount of A, T, G and C in DNA varies from species to species. In each species, the amount of A = T and the amount of G = C. Slide25:  Deductive reasoning concluded that A pairs with T and G pairs with C. These rules were presented before scientists knew the structure of DNA. The rules would only be useful in the next decade, when the structure was more certain. Rosalind Franklin-1953:  Rosalind Franklin-1953 This was the first indication that DNA was composed of a double helix structure. James Watson & Francis Crick-1953:  James Watson & Francis Crick-1953 Constructed a model for DNA structure Awarded the Nobel Prize in 1962 Rosalind Franklin’s work was essential to the Watson and Crick model. She died of cancer due to overexposure to x-rays. Her lab partner, Maurice Wilkins was also awarded the Nobel Prize along with Watson & Crick because of his contributions to Ms. Franklin’s work. Watson & Crick’s model of DNA structure:  Watson & Crick’s model of DNA structure Complimentary base pairing: always purine paired with pyrimidine. The 2 strands of DNA are held together by the hydrogen bonds between the complimentary bases. Watson & Crick’s model of DNA structure:  Watson & Crick’s model of DNA structure Space-filling model That wraps up one mystery!:  That wraps up one mystery! But in science, new information almost always raises new questions… Now we know “what”… but how?:  Now we know “what”… but how? What is the genetic material? DNA! How is it passed on to the next generation? How is it copied? Slide32:  1 3 2 During DNA replication (or copying), the parent DNA molecule unwinds and unzips. Then each old strand serves as a template for a new strand. Quiz yourself: In which phase of the cell cycle does DNA replication occur? Slide33:  In 1958, Matthew Meselson and Franklin Stahl discovered that DNA replication is semiconservative. Each new double helix contains a new strand and an old strand. Slide34:  Prokaryotes have a single, circular chromosome. 5’ vs. 3’:  5’ vs. 3’ Carbons in the deoxyribose sugar are numbered for identification. Slide36:  DNA replication proceeds from 5’ end to 3’ end. Slide37:  *Unwinds the DNA double helix, so DNA polymerase can get better access to the template strands. *attaches new nucleotides Direction of replication is always 5’ to 3’. Okazaki fragments are short pieces of DNA on the new strand, specifically the “lagging strand”. They will later be joined by an enzyme called DNA ligase into a complete strand. Slide38:  DNA Polymerase can ONLY add a nucleotide to the 3’end of an existing nucleotide. Why? Because of that, it cannot start the replication process. The process is started by RNA polymerase, which lays down a small RNA fragment called a “primer”. DNA Polymerase can then add a nucleotide to the RNA primer. The RNA primer on the lagging strand will later be replaced with DNA by DNA polymerase as it proofreads the strand. The RNA primer on the leading strand cannot be replaced with DNA when the primer is removed, so the small segment at the end is not replicated. It is lost. Remember telomeres? (50 times) Thanks for your attention! Any questions?:  Thanks for your attention! Any questions? Take a short break if you need one!:  Take a short break if you need one! What’s next? One-Minute Paper DNA video clip Watch Jurassic Park movie clip DNA Modeling Lab Lab summary Introduction to Case Study #2 Overview of today’s lab:  Overview of today’s lab You will use colored beads to build a model of the DNA double helix. Follow the instructions in your lab manual to use this DNA “molecule” to demonstrate replication. Let’s look at your lab manual together. Preparation for Lecture 9:  Preparation for Lecture 9 Skim chapters 14-16 Keep up with Oncourse!

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