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Molecular Basis of Heredity Discovery of DNA Structure and Function of DNA Replication Transcription Translation Discovery of DNA: Frederick Griffith – 1928 Wanted to know how bacteria caused pneumonia Injected mice with disease-causing strain mice died Injected mice with harmless strain mice lived Injected mice with heat killed, disease-causing strain and harmless strain mixed together mice died Discovery of DNA: Frederick Griffith – 1928 Transformation: The heat-killed bacteria passed their disease-causing ability to the harmless strain One strain of bacteria was changed into another Oswald Avery – 1944 Repeated Griffith’s work Treated the heat-killed bacteria to enzymes that broke down everything but DNA bacteria still transformed Treated the heat-killed bacteria to enzyme that broke down DNA bacteria did not transform Discovered that DNA is the nucleic acid that stores and transmits genetic information Alfred Hershey & Martha Chase – 1952 Bacteriophage – “bacteria eater” (a virus that infects and kills bacteria) Placed a radioactive marker on phosphorus (DNA) and sulfur (protein) Only radioactive phosphorus was found in the bacteria Thus, the virus only injected DNA into the bacteria not protein DNA Structure Discovery Please review DNA discovery notes handed out in class. Portfolio worthy narrative account = Watson, Crick, Wilkens, and Franklin controversy newspaper article Function of DNA DNA carries information from one generation to the next DNA determines the heritable characteristics of organisms DNA is easily copied Structure of DNA Double helix Anti-parallel Complementary Sugar phosphate backbone Nitrogenous bases in the center held together with hydrogen bonds Chargaff’s Rule = A binds with T, C binds with G Structure of DNA Nucleotide = A phosphate group A deoxyribose sugar (5 carbon) A nitrogenous base • • • • Adenine Thymine Cytosine Guanine Chromosome Structure “supercoils” DNA Replication Overview Each strand of the double helix can be used as a template for a new strand of DNA “Semi-conservative” each new DNA molecules contains one new strand and one old strand Prokaryotes = replication is simple; typically one replication fork (circular DNA) Eukaryotes = replication is more complex; hundreds of replication forks The Cell Cycle DNA is replicated during the S phase of the cell cycle Replication Enzymes Gyrase – Unwinds the supercoils Helicase – Unwinds the double helix Single-strand Binding Proteins – stabilizes the DNA strands and keeps them apart Primase – Attaches the RNA primer to the parent DNA strand to begin replication Replication Enzymes (continued) DNA Polymerase (3 functions) – 1. 2. 3. Adds new nucleotides to the growing DNA strand Proofreads and makes repairs when needed Replaces RNA primer with DNA nucleotides Ligase – joins and bonds the DNA fragments together to form a complete double helix How Replication Occurs DNA is synthesized in the 5’ 3’ direction only!!! This means that new nucleotides are attached to the 3’ carbon of the deoxyribose molecule. Replication occurs in the nucleus! View the DNA Replication streaming video now and complete the replication activities How Replication Occurs Depending on how the replication fork opens : Continuous replication occurs on the leading strand (new strand is made continuously in the 5’ 3’ direction) Discontinuous replication occurs on the lagging strand (new strand is made in fragments called Okazaki fragments) DNA Replication VIDEO Watch DNA Replication streaming video from PBS. http://player.discoveryeducation.com/i ndex.cfm?guidAssetId=0CB6B02F092A-4035-98B66378AF13F567&blnFromSearch=1&p roductcode=US Telomeres Short repetitive sequence of DNA • ex. TTTAAGGG (guanine rich) Protect the ends of the chromosome from deterioration Over time there is loss of DNA at the 5’ end of the lagging strands • RNA primers cannot be replaced with DNA if there is no DNA after it for DNA polymerase to bind! Causes aging in somatic (body) cells! • Telomerase (enzyme that regenerates telomeres) only occurs in germ cells (sex cells) and malignant cells! Turn and Talk What is the consequence of losing telomeres on the 5’ end of the lagging strands of DNA molecules? What could happen if we could prevent that loss? Transcription and Translation Structure of RNA RNA Nucleotide = 5-carbon sugar (Ribose) Phosphate group Nitrogenous base • Adenine, cytosine, guanine, uracil • No thymine (only in DNA) Single stranded molecule Not a double helix like DNA Blueprint of DNA (DNA is the Master plan) Types of RNA 3 main types = Messenger RNA (mRNA) • Carries copy of DNA message to the ribosome to be made into a protein Transfer RNA (tRNA) • Transfers amino acids to the ribosome based on the mRNA coded message Ribosomal RNA (rRNA) • Reads the mRNA coded message like a decoder ring Transcription Overview Transcription begins in the nucleus and ends in the cytoplasm To make mRNA RNA polymerase Binds to DNA and uses one strand as a template for a molecule of mRNA How does it know where to bind? Promoters specific sequences in DNA that signal RNA polymerase to bind there (also tells when to stop) mRNA Editing mRNA must be edited before moving from the nucleus to the cytoplasm Introns – these intervening (non-coding) sequences must be cut out Exons – Coding sequences that encode for a specific protein No clear understanding why introns must be removed Only the mature (“edited”) mRNA moves to the cytoplasm The Genetic Code Proteins are made using amino acids joined together by peptide bonds 20 different amino acids The code consists of 4 letters: A, U, C, and G (RNA bases) The genetic code is read 3 letters at a time mRNA “Codon” = 3 bases (AUG) tRNA “Anti-codon” = 3 complimentary bases (UAC) 64 possible 3-base codons (some amino acids have more than one codon that codes for it) Each amino acid has an amino group, a carboxyl group, and an R-group. The R-group gives the amino acid it’s unique personality!!! The peptide bond forms between the amino group of one amino acid and the carboxyl group of another! The Genetic Code (continued) Start codon (for all proteins) = AUG methionine Several stop codons (do not code for an amino acid…allows for release of the protein from the ribosomal complex) UGA, UAA, UAG Translation (or protein synthesis) Overview mRNA serves as instructions for the protein to be made (made during transcription) Translation begins when an mRNA molecule attaches to the ribosomal complex and begins with the 1st codon (AUG) tRNA (the “anticodon”) transfers the corresponding amino acid to the ribosome. As each codon is read tRNA brings the corresponding amino acids to the ribosome The amino acids are bonded to each other via a peptide bond Once a stop codon is reached the protein molecule is released Ribosomal complex Mutations Mutations Changes in the genetic material 2 Types: Gene Mutation Chromosome Mutation Gene Mutations Point mutations Occurs at a single point in the DNA sequence Could change one of the amino acids Example: AAA TTT (normal) AAC TTT (mutation) Frameshift mutations Addition or deletion of a nucleotide in the DNA Changes the “reading frame” of the code Consequences more serious Example: AAA TTT (normal) AAT TT (mutation) Chromosomal Mutations Involves changes in the number or structure of chromosomes. Deletion Duplication Inversion Translocation Gene Regulation Genes are not always “on” Genes are regulated to turn “on” and “off” In Prokaryotes: The Lac Operon (a series of genes that work together) breaks down lactose if present into galactose and glucose. These genes are turned off by repressors and are only turned on by the presence of lactose. Eukaryotic Gene Regulation Genes are controlled individually Have regulatory sequences that are much more complex than prokaryotic gene regulation Why are they more complex? Cell specialization!!! • Each cell has DNA for the whole organism’s functioning, however, only liver cells need to produce liver proteins (etc.) Regulation and Development Differentiation Cells become specialized in structure and function Hox genes Controls the differentiation of cells and tissues in the embryo (controls the “body plan”) • Example: Mouse eye gene inserted into the “knee” of a fly gene fly grew an eye on its leg!!! Genes have descended from a common ancestor Genetic Engineering Selective Breeding Humans take advantage of naturally occurring genetic variations Hybridization Select desired traits to pass on to the next generation (domestic animals) Cross dissimilar individuals to bring out the best of both organisms (“Hybrid vigor”) Inbreeding Maintains the desired characteristics of a line of organisms (although not without risk) Example: Dog breeds Increasing Variation Breeders can increase variation in a population by inducing mutations Radiation and chemicals Many mutations are harmful to the organism New Kinds of Bacteria Development of useful strains of bacteria (digestion of oil) New Kinds of Plants Produces polyploid (multiple sets of chromosomes) individuals in plants, larger and stronger than diploid individuals (fatal in animals) Manipulating DNA Different techniques are used to: Extract DNA from cells Cut DNA into smaller pieces Identify the sequence of bases in a DNA molecule Make unlimited copies of DNA Tools of Molecular Biology Makes changes in the DNA code of a living organism DNA Extraction (SLE A1: banana DNA extraction lab) Cutting DNA DNA is separated from the rest of the cell using a simple chemical procedure Restriction enzymes cuts specific sequences of nucleotides Separating DNA Gel electrophoresis a DNA sample is placed at one end of a porous gel and an electric current is applied making the DNA fragments separate according to size • Large fragments move more slowly than short fragments Using the DNA sequence Reading the sequence Cutting and Pasting Creates a series of dye-tagged copies from which the order tells the exact sequence of DNA Recombinant DNA DNA molecules produced by combining DNA from different sources (DNA synthesizers) Making copies Polymerase Chain Reaction (PCR) makes several copies of the same gene by repeated heating and cooling Applications of Genetic Engineering Transgenic Organisms (contains genes from other organisms) Transgenic bacteria • Useful for health (bacteria can be transformed to create human insulin and other forms of proteins) and industry (raw materials for plastics and synthetic fibers) Transgenic animals • Used to study genes (example: mice with human immune systems) and improve the food supply Transgenic plants • Important part of our food supply (25% corn and 52% soybeans have been modified) Cloning A member of a population of genetically identical cells. Easy to do with microorganisms/Hard with multicellular organisms • There are ethical concerns too!