Bài giảng Biology - Chapter 14: DNA: The Genetic Material

DNA: The Genetic MaterialChapter 141The Genetic MaterialFrederick Griffith, 1928studied Streptococcus pneumoniae, a pathogenic bacterium causing pneumoniathere are 2 strains of Streptococcus: - S strain is virulent - R strain is nonvirulentGriffith infected mice with these strains hoping to understand the difference between the strains2The Genetic MaterialGriffith’s results:- live S strain cells killed the mice- live R strain cells did not kill the mice- heat-killed S strain cells did not kill the mice- heat-killed S strain + live R strain cells killed the mice34The Genetic MaterialGriffith’s conclusion:- information specifying virulence passed from the dead S strain cells into the live R strain cells- Griffith called the transfer of this information transformation5The Genetic MaterialAvery, MacLeod, & McCarty, 1944repeated Griffith’s experiment using purified cell extracts and discovered: - removal of all protein from the transforming material did not destroy its ability to transform R strain cells - DNA-digesting enzymes destroyed all transforming ability - the transforming material is DNA6The Genetic MaterialHershey & Chase, 1952- investigated bacteriophages: viruses that infect bacteria- the bacteriophage was composed of only DNA and protein- they wanted to determine which of these molecules is the genetic material that is injected into the bacteria 7The Genetic Material- Bacteriophage DNA was labeled with radioactive phosphorus (32P)- Bacteriophage protein was labeled with radioactive sulfur (35S)- radioactive molecules were tracked - only the bacteriophage DNA (as indicated by the 32P) entered the bacteria and was used to produce more bacteriophage- conclusion: DNA is the genetic material89DNA StructureDNA is a nucleic acid.The building blocks of DNA are nucleotides, each composed of:a 5-carbon sugar called deoxyribosea phosphate group (PO4)a nitrogenous baseadenine, thymine, cytosine, guanine1011DNA StructureThe nucleotide structure consists ofthe nitrogenous base attached to the 1’ carbon of deoxyribosethe phosphate group attached to the 5’ carbon of deoxyribosea free hydroxyl group (-OH) at the 3’ carbon of deoxyribose1213DNA StructureNucleotides are connected to each other to form a long chainphosphodiester bond: bond between adjacent nucleotidesformed between the phosphate group of one nucleotide and the 3’ –OH of the next nucleotideThe chain of nucleotides has a 5’ to 3’ orientation.1415DNA StructureDetermining the 3-dimmensional structure of DNA involved the work of a few scientists:Erwin Chargaff determined that amount of adenine = amount of thymineamount of cytosine = amount of guanineThis is known as Chargaff’s Rules16DNA StructureRosalind Franklin and Maurice WilkinsFranklin performed X-ray diffraction studies to identify the 3-D structurediscovered that DNA is helicaldiscovered that the molecule has a diameter of 2nm and makes a complete turn of the helix every 3.4 nm17DNA StructureJames Watson and Francis Crick, 1953deduced the structure of DNA using evidence from Chargaff, Franklin, and othersproposed a double helix structure 18DNA StructureThe double helix consists of:2 sugar-phosphate backbonesnitrogenous bases toward the interior of the moleculebases form hydrogen bonds with complementary bases on the opposite sugar-phosphate backbone1920DNA StructureThe two strands of nucleotides are antiparallel to each otherone is oriented 5’ to 3’, the other 3’ to 5’The two strands wrap around each other to create the helical shape of the molecule.2122DNA ReplicationMatthew Meselson & Franklin Stahl, 1958investigated the process of DNA replicationconsidered 3 possible mechanisms:conservative modelsemiconservative modeldispersive model2324DNA ReplicationBacterial cells were grown in a heavy isotope of nitrogen, 15Nall the DNA incorporated 15N cells were switched to media containing lighter 14NDNA was extracted from the cells at various time intervals25DNA ReplicationThe DNA from different time points was analyzed for ratio of 15N to 14N it containedAfter 1 round of DNA replication, the DNA consisted of a 14N-15N hybrid moleculeAfter 2 rounds of replication, the DNA contained 2 types of molecules:half the DNA was 14N-15N hybridhalf the DNA was composed of 14N2627DNA ReplicationMeselson and Stahl concluded that the mechanism of DNA replication is the semiconservative model.Each strand of DNA acts as a template for the synthesis of a new strand.28DNA ReplicationDNA replication includes:initiation – replication begins at an origin of replicationelongation – new strands of DNA are synthesized by DNA polymerasetermination – replication is terminated differently in prokaryotes and eukaryotes29Prokaryotic DNA ReplicationThe chromosome of a prokaryote is a circular molecule of DNA.Replication begins at one origin of replication and proceeds in both directions around the chromosome.3031Prokaryotic DNA ReplicationThe double helix is unwound by the enzyme helicaseDNA polymerase III (pol III) is the main polymerase responsible for the majority of DNA synthesisDNA polymerase III adds nucleotides to the 3’ end of the daughter strand of DNA3233Prokaryotic DNA ReplicationDNA replication is semidiscontinuous.pol III can only add nucleotides to the 3’ end of the newly synthesized strandDNA strands are antiparallel to each otherleading strand is synthesized continuously (in the same direction as the replication fork)lagging strand is synthesized discontinuously creating Okazaki fragments3435Prokaryotic DNA ReplicationThe enzymes for DNA replication are contained within the replisome.The replisome consists ofthe primosome - composed of primase and helicase2 DNA polymerase III moleculesThe replication fork moves in 1 direction, synthesizing both strands simultaneously.3637Eukaryotic DNA ReplicationThe larger size and complex packaging of eukaryotic chromosomes means they must be replicated from multiple origins of replication.The enzymes of eukaryotic DNA replication are more complex than those of prokaryotic cells.38Eukaryotic DNA ReplicationSynthesizing the ends of the chromosomes is difficult because of the lack of a primer.With each round of DNA replication, the linear eukaryotic chromosome becomes shorter.3940Eukaryotic DNA Replicationtelomeres – repeated DNA sequence on the ends of eukaryotic chromosomesproduced by telomerasetelomerase contains an RNA region that is used as a template so a DNA primer can be produced4142DNA Repair- DNA-damaging agents- repair mechanisms- specific vs. nonspecific mechanisms43DNA RepairMistakes during DNA replication can lead to changes in the DNA sequence and DNA damage.DNA can also be damaged by chemical or physical agents called mutagens.Repair mechanisms may be used to correct these problems.44DNA RepairDNA repair mechanisms can be:specific – targeting a particular type of DNA damagephotorepair of thymine dimersnon-specific – able to repair many different kinds of DNA damageexcision repair to correct damaged or mismatched nitrogenous bases454647