DNA Synthesis and the Gene PDF

Ch. 15 DNA and The Gene Discovery of DNA In 1953, a double-helical model for the structure of deoxyribonucleic acid, or DNA, was proposed. How do we know what DNA looks like? – DNA, the substance of inheritance, is the most celebrated molecule of our time. Hereditary information is encoded in DNA an...

Ch. 15 DNA and The Gene Discovery of DNA In 1953, a double-helical model for the structure of deoxyribonucleic acid, or DNA, was proposed. How do we know what DNA looks like? – DNA, the substance of inheritance, is the most celebrated molecule of our time. Hereditary information is encoded in DNA and reproduced in all cells of the body. – This DNA program directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits. How do we know that DNA is the substance of inheritance? Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres DNA as Genetic Material What two classes of large biological molecules are found in chromatin? DNA and proteins How do we know which of these two types of molecules stores information? DNA as Genetic Material Most biologists hypothesized that genes were made of proteins. oMainly because little was known about DNA. Why might some biologists have thought that proteins would be better candidates? Hint: How many amino acids are found in proteins? How many nucleotides are found in DNA? DNA as Genetic Material Hershey and Chase studied how the T2 virus infects the bacterium Escherichia coli. T2 infection of E. coli begins when: o The virus injects its genes into the cell. o These genes direct production of new virus particles. The virus’s coat, or capsid, is left on the outside of the cell. Does protein or DNA enter the cell? DNA as Genetic Material Hershey and Chase studied how the T2 virus infects the bacterium Escherichia coli. T2 infection of E. coli begins when: o The virus injects its genes into the cell. o These genes direct production of new virus particles. The virus’s coat, or capsid, is left on the outside of the cell. Does protein or DNA enter the cell? DNA as Genetic Material Hershey and Chase studied how the T2 virus infects the bacterium Escherichia coli. T2 infection of E. coli begins when: o The virus injects its genes into the cell. o These genes direct production of new virus particles. The virus’s coat, or capsid, is left on the outside of the cell. Does protein or DNA enter the cell? DNA as Genetic Material What elements are found in nucleic acids? C, H, O, N, and P What elements are found in proteins? C, H, O, N, and sometimes S Hershey-Chase Experiment Hershey and Chase grew the virus in the presence of one of two radioactive isotopes: 32P, which is incorporated into DNA, or 35S, which is incorporated into proteins. Labeled viruses were used to infect E. coli cells. Only the radioactive DNA was found inside the cells. This evidence supported the hypothesis that genes must be composed of DNA. Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres Structure of DNA DNA Replication and Repair Based on what you know about the structure of DNA, suggest a mechanism for replication. Which structural features of DNA make this mechanism possible? Base Pairing to a Template Strand Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication. oIn DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules. Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres Discovering DNA Replication In 1958, it was not known how DNA replication takes place. There were three common hypotheses for how replication occurs: o The semiconservative model suggests that, after DNA replication, the new DNA duplex consists of one old strand (parental) and one new strand (daughter). o The conservative model suggests that, after DNA replication, the new DNA duplex consists of two newly-synthesized daughter strands, leaving the parental duplex intact. o The dispersive model suggests that, after DNA replication, the new DNA duplex consists of a mix of new and old segments along each replicated molecule. Discovering DNA Replication To determine which model was correct, Matthew Meselson and Franklin Stahl synthesized DNA in medium containing different isotopes of nitrogen. oThey labeled the nucleotides of the old strands with a heavy isotope of nitrogen (15N), while any new nucleotides were labeled with a lighter isotope (14N). Discovering DNA Replication Discovering DNA Replication Which hypothesis does the data support? Explain. Eukaryotic DNA Replication Meselson and Stahl’s experiment provided support for semiconservative replication in bacteria. But, DNA replication in eukaryotes could be different. Experiments with fluorescently labeled DNA led to the discovery that eukaryotic cells also replicate semiconservatively. Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres DNA Replication The copying of DNA is remarkable in its speed and accuracy. oMore than a dozen enzymes and other proteins participate in DNA replication. DNA Replication Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble”. oA eukaryotic chromosome may have hundreds or even thousands of origins of replication. § Replication proceeds in both directions from each origin, until the entire molecule is copied. DNA Replication Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble”. oA eukaryotic chromosome may have hundreds or even thousands of origins of replication. § Replication proceeds in both directions from each origin, until the entire molecule is copied. Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres DNA Replication At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating. DNA Helicase is an enzyme that separates the two strands of the double helix at the replication forks. Single-strand DNA-binding protein binds to and stabilizes single-stranded DNA until it can be used as a template. Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands. DNA Replication Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork. DNA polymerases require a primer and a DNA template strand. oIn E. coli, the two main DNA polymerases are: § DNA polymerase III (DNA pol III) § DNA polymerase I (DNA pol I) DNA Replication DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3¢ end of a polynucleotide. What is meant by the 3’ and 5’ ends of a polynucleotide? DNA Replication The initial nucleotide strand is a short RNA primer. An enzyme called primase can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template. Primase is one type of RNA polymerase. The RNA primer is short (5–10 nucleotides long), and the 3¢ end serves as the starting point for the new DNA strand. DNA Replication Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres Video – DNA replication https://www.youtube.com/watch?v=TNKWgcFPHqw Review What are the three components of a nucleotide? Nitrogenous base, pentose sugar, phosphate group What is a nucleoside? Nitrogenous base, pentose sugar Give an example of a nucleoside triphosphate. Adenosine triphosphate DNA Replication Each nucleotide that is added to a growing DNA strand starts as a nucleoside triphosphate. oe.g., dATP supplies adenine to DNA. § dATP has deoxyribose while ATP has ribose. What is the difference between ATP and dATP? Name three other nucleoside triphosphates. H DNA Replication As each monomer of dATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate. o Subsequent hydrolysis of pyrophosphate is a coupled exergonic reaction that helps drive the polymerization reaction. Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres DNA Replication The antiparallel structure of the double helix affects replication. DNA polymerases add nucleotides only to the free 3¢ end of a growing strand; therefore, a new DNA strand can elongate only in the 5¢ to 3¢ direction. Along one template strand of DNA, the DNA polymerase III synthesizes a leading strand continuously, moving toward the replication fork. DNA Replication To elongate the other new strand, called the lagging strand, DNA polymerase III must work in the direction away from the replication fork. oThe lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase. Overview of DNA Replication Overview of DNA Replication Lecture Outline 1. DNA Function and Structure a) DNA as Genetic Material b) Structure of DNA c) Discovering DNA Replication 2. DNA Replication a) b) c) d) Replication Proteins Adding a Nucleotide Leading vs. Lagging Strands Telomeres Replicating the Ends of DNA Molecules Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes. oThe usual replication machinery provides no way to complete the 5¢ ends, so repeated rounds of replication produce shorter DNA molecules. Replicating the Ends of DNA Molecules A single-stranded DNA is left at the end of the lagging strand. The single-stranded DNA is eventually degraded, which would shorten the chromosome by 50 to 100 nucleotides each time replication occurs. Over time, linear chromosomes would vanish. Telomeres Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called telomeres. oTelomeres do not contain any genes. oTTAGGG repeated between 1500 and 3000 times. Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules. Telomeres If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce. An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells. The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions. There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist. Video – Telomere Replication https://www.youtube.com/watch?v=2NS0jBPurWQ Video - What are telomeres? https://www.youtube.com/watch?v=U0fRAr-ZHCo