Chapter 11 Nucleic Acid Structure and DNA Replication PDF

CHAPTER 11 NUCLEIC ACID STRUCTURE AND DNA REPLICATION 1  Genetic material Genetic material must be able to:. Contain the information necessary to construct an entire organism. Pass from parent to offspring and from cell to cell during cell division....

CHAPTER 11 NUCLEIC ACID STRUCTURE AND DNA REPLICATION 1  Genetic material Genetic material must be able to:. Contain the information necessary to construct an entire organism. Pass from parent to offspring and from cell to cell during cell division. Be accurately copied. Account for the known variation within and between species. Late 1800s scientists postulated a biochemical basis. Researchers became convinced chromosomes carry genetic information. 1920s to 1940s expected the protein portion of chromosomes to be the genetic material Genetic Material – DNA or Protein? What did scientists do to figure out if it is DNA or Protein? 2 Genetic Material: DNA or Protein?The answer! The Griffith transformation Experiment: Injection into mice of the pneumoniae bacteria Streptococus pneumoniae Experiment 1: 2 strains were injected: S strain: produce Shiny and smooth colonies. S strain is encapsulated R Strain: produce Rough appearance colonies Observations: S strain injected to mice: mice died (virulent), Virulence due to the capsule R strain injected to mice: mice did not die (non-virulent)  Experiment 2: Observations: Living S strain were recovered from body Substance necessary for virulence passed from dead S strain to living R strain Bacteria R strain bacteria was transformed Conclusion: Genetic material from the heat-killed type S bacteria had been transferred to the living type R bacteria. The transforming substance passed was genetic material. This trait gave them the capsule and was passed on to their offspring  Levels of DNA structure 1. Nucleotides are the building blocks of DNA (and RNA). 2. Nucleotides polymerizes to form the strand of DNA (or RNA) 3. Two strands form a double helix. 4. In living cells, DNA is associated with an array of different proteins to form chromosomes. 5. A genome is the complete complement of an organism’s genetic material. 5 Nucleotides 3 components: - Phosphate group - Pentose sugar - Nitrogenous base DNA 3 components: Phosphate group Pentose sugar: Deoxyribose Nitrogenous base: Purines Adenine (A), guanine (G) Pyrimidines Cytosine (C), thymine (T), RNA 3 components: Phosphate group Pentose sugar: Ribose Nitrogenous base Purines Adenine (A), guanine (G) Pyrimidines Cytosine (C), uracil (U) 6 Conventional numbering system. Sugar carbons 1’ to 5’. Base attached to 1’. Phosphate attached to 5’ DNA strands. Nucleotides covalently bonded. Phosphodiester bond: phosphate group links 2 sugars. Phosphates and sugars from backbone. Bases project from backbone. Directionality- 5’ to 3’. 5’ – TACG – 3’ 7 DNA is: Double stranded Helical Sugar-phosphate backbone Bases on the inside Stabilized by hydrogen bonding Base pairs with specific pairing AT/GC or Chargoff’s rule A pairs with T G pairs with C 10 base pairs per turn 2 DNA strands are complementary 5’ – GCGGATTT – 3’ 3’ – CGCCTAAA – 5’ 2 strands are antiparallel One strand 5’ to 3’ Other stand 3’ to 5’ 8  Space-filling model shows grooves  Major groove Where proteins bind  Minor groove 9  DNA Replication 3 different models for DNA replication proposed in late 1950s: - Semiconservative - Conservative - Dispersive Newly made strands are daughter strands Original strands are parental strands 11  DNA Replication During replication 2 parental strands separate and serve as template strands New nucleotides must obey the AT/GC rule End result 2 new double helices with same base sequence as original 12  DNA Replication - Origin of replication Site of start point for replication - Bidirectional replication Replication proceeds outward in opposite directions -Bacteria have a single origin -Eukaryotes require multiple origins - Origin of replication provides an opening called a replication bubble that forms two replication forks DNA replication occurs near the fork 13  DNA Replication - Synthesis begins with a primer and proceeds 5’ to 3’ - Leading strand made in direction fork is moving. Synthesized as one long continuous molecule - Lagging strand made as Okazaki fragments that have to be connected later 14 https://www.youtube.com/watch?v=TNKWgcFPHqw  DNA Replication DNA helicase Binds to DNA and travels 5’ to 3’ using ATP to separate strand and move fork forward DNA topoisomerase Relives additional coiling ahead of replication fork Single-strand binding proteins Keep parental strands open to act as templates DNA polymerase Covalently links nucleotides Deoxynuceloside triphosphates Free nucleotides with 3 phosphate groups Breaking covalent bond to release pyrophosphate (2 phosphate groups) provides energy to connect adjacent nucleotides 15  DNA Replication DNA polymerase has 2 enzymatic features to explain leading and lagging strands 1. DNA polymerase unable to begin DNA synthesis on a bare template strand - DNA primase must make a short RNA primer - RNA primer will be removed and replaced with DNA later 2. DNA polymerase can only work 5’ to 3’ In the leading strand - DNA primase makes one RNA primer - DNA polymerase attaches nucleotides in a 5’ to 3’ direction as it slides forward In the lagging strand - DNA synthesized 5’ to 3’ but in a direction away from the fork - Okazaki fragments made as a short RNA primer made by DNA primase at the 5’ end and then DNA laid down by DNA polymerase - RNA primers will be removed by DNA polymerase and filled in with DNA - DNA ligase will join adjacent DNA fragments 16  The family of DNA polymerases 3 important issues for DNA polymerase are speed, fidelity, and completeness Nearly all living species have more than 1 type of DNA polymerase Genomes of most species have several DNA polymerase genes due to gene duplication E. coli has 5 DNA polymerases:. DNA polymerase III with multiple subunits responsible for majority of replication. DNA polymerase I has a single subunit whose job is to rapidly remove RNA primers and fill in DNA. DNA polymerases II, IV and V are involved in DNA repair and replicating damaged DNA - DNA polymerases I and III stall at DNA damage -DNA polymerases II, IV and V don’t stall but go slower and make sure replication is complete Humans have 12 or more DNA polymerases. Designated with Greek letters. DNA polymerase α has its own built in primase subunit. DNA polymerase δ and ε extend DNA at a faster rate. DNA polymerase γ replicates mitochondrial DNA. When DNA polymerases α, δ or ε encounter abnormalities they may be unable to replicate. Lesion-replicating polymerases may be able to synthesize complementary strands to the damaged area https://www.youtube.com/watch? v=TNKWgcFPHqw  Specialized form of DNA replication only in eukaryotes in the telomeres Telomeres are a series of repeat sequences within DNA and special proteins Telomere at 3’ does not have a complementary strand and is called a 3’ overhang DNA polymerase cannot copy the tip of the DNA strand with a 3’ end No place for upstream primer to be made If this replication problem were not solved, linear chromosomes would become progressively shorter 19  Telomeres and aging Body cells have a predetermined life span Skin sample grown in a dish will double a finite number of times Infants, about 80 times Older person, 10 to 20 times Senescent cells have lost the capacity to divide Progressive shortening of telomeres correlated with cellular senescence Telomerase present in germ-line cells and in rapidly dividing somatic cells Telomerase function reduces with age Inserting a highly active telomerase gene into cells in the lab causes them to continue to divide 20  Telomeres and cancer When cells become cancerous they divide uncontrollably In 90% of all types of human cancers, telomerase is found at high levels Prevents telomere shortening and may play a role in continued growth of cancer cells Mechanism unknown 21