2Central Dogma of Molecular Biology! Scientists call this the:DNADNACentral Dogma of Molecular Biology!RNARNAProteinProtein
3How do we know that all of our genetic information comes from DNA? What type of experiment would you design to determine that DNA is the source of all genetic information?
4The smooth colonies must carry the disease! Griffith’s Experiment with Pneumonia and the accidental discovery of TransformationFrederick Griffiths was a bacteriologist studying pneumoniaHe discovered two types of bacteria:Smooth coloniesRough coloniesCONCLUSION:The smooth colonies must carry the disease!
5Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation When heat was applied to the deadly smooth type…And injected into a mouse…The mouse lived!
6Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation Griffith injected the heat-killed type and the non-deadly rough type of bacteria.The bacteria “transformed” itself from the heated non-deadly type to the deadly type.
7Griffith’s Experiment did not prove that DNA was responsible for transformation How would you design an experiment to prove that DNA was responsible for transformation?
9To the Heat-Killed Smooth Type, added enzymes that destroyed… Avery, McCarty, and MacLeod Added the non-deadly Rough Type of Bacteria to the Heat-Killed Smooth TypeTo the Heat-Killed Smooth Type, added enzymes that destroyed…CarbohydratesLipidsProteinsRNADNA
10DNA was the transforming factor! S-Type Carbohydrates DestroyedS-Type Lipids DestroyedS-Type Proteins DestroyedS-Type RNA DestroyedS-Type DNA DestroyedConclusion:DNA was the transforming factor!
11The Hershey-Chase Experiment Protein coatAlfred Hershey & Martha Chase worked with a bacteriophage:A virus that invades bacteria. It consists of a DNA core and a protein coatDNA
12DNA carries the genetic code! The Hershey-Chase results reinforced the Avery, McCarty, and MacLeod conclusion:DNA carries the genetic code!However, there were still important details to uncover…
13How did DNA: 1. Store information? 2. Duplicate itself easily? These questions would be answered by discovering DNA’s structure
15The Race to Discover DNA’s Structure Linus Pauling1940sDiscovered the alpha-helical structure of proteins.
16The Race to Discover DNA’s Structure Why do you think the bases match up this way?1950Chargaff’s Rule: Equal amounts of Adenine and Thymine, and equal amounts of Guanine and CytosinePurine + Purine = Too widePyrimidine + Pyrimidine = Too NarrowErwin ChargaffPurine + Pyrimidine = Perfect Fit from X-ray data
17The Race to Discover DNA’s Structure X-Ray diffraction image of DNA taken by Franklin in 1951Maurice WilkinsRosalind Franklin
18The Race to Discover DNA’s Structure 1953Compiled data from previous scientists to build a double-helical model of DNAJames WatsonFrancis Crick
19DNA Structure Deoxyribonucleic acid Double helix (twisted ladder or strands) of nucleotides (Sugar (deoxyribose), phosphate and a nitrogen base)Each strand has a sugar and phosphate backbone covalently bonded to a nitrogen base
21DNA StructureDouble helix is made of covalently bonded strands that are hydrogen bonded to complementary covalently bonded strandsOne strand bonds to the second strand via hydrogen bonds (weak enough to break in order to separate the 2 strands)Each strand measures 3.4 nm/twist or 10 base pairs
23DNA StructureStrands of DNA are different – they are oriented in opposite directions to each other – they are ANTIPARALLELEach end has a number (5’ or 3’ – you say 5 prime or 3 prime)
24Four Nitrogen BasesAdenine (A), Guanine (G), Cytosine (C), Thymine (T)Purines (double ring structures) – Adenine and GuaninePyrimidines (single ring structures) – Cytosine and ThymineChargaff rules: A –T and T – AG – C and C - G
26Chromosome StructureDNA packs tightly around histones to form chromatin.DNA and histones form bead-like structures called nucleosomes.Nucleosomes pack together to form supercoils.Supercoils condense to form chromosomes.
27Chromosome Structure Nucleosome Chromosome DNA Coils Supercoils Histones
28DNA Replication The double helix did explain how DNA copies itself We will study this process, DNA replication, in more detail
29How does DNA replicate? Hypotheses: Conservative Semi-Conservative Have students stand in one of three corners in the lab: Conservative; Semi-Conservative and Dispersive. Have them design an experiment that would prove their point.ConservativeSemi-ConservativeDispersive
30DNA Replication DNA copies itself in the “S” phase of interphase. 1 parent DNA molecule produces 2 daughter DNA molecules, each daughter being made up of “parent” DNA and a strand of “new” DNA (semiconservative process)
31Steps of DNA Replication 1. DNA unzips – Helicase enzyme breaks hydrogen bonds, unzipping the double helix at the origin of replication (about 100 on a human chromosome).A replication bubble is formed when DNA unzipsDNA polymerization is bi-directional because of the antiparallel orientation of the DNA strand.
32DNA Replication Section 12-2 Original strand New strand Growth Replication forkDNA polymeraseNew strandOriginal strandNitrogenous bases
34Steps of DNA Replication 2. Bases pair up – DNA Polymerase bonds free nucleotides to complementary basesDNA POL reads DNA in 3’ to 5’ direction, thus a new strand elongates only in the 5’ to 3’ directionNucleotides are added at a rate of about 50 per second in mammals and 500 per second in bacteria.
35Steps of DNA Replication Leading strand has continuous elongation starting at RNA primer since it is read in 3’ to 5’ direction (towards replication fork) by DNA polymeraseLagging strand has discontinuous elongationDNA strand is read 3’ to 5’ away from the replication fork in a series of segments called Okazaki fragmentsOnce fragments are finished, they are joined to previous fragment with enzyme Ligase.
37Steps of DNA Replication 3. Proofreading and repairDNA polymerase “proof-reads” newly created DNA strand and identifies incorrect base pairs.Nuclease (exonuclease) enzyme cuts out the identified incorrect nucleotides.DNA polymerase places correct nucleotides into DNA strand.Ligase fuses these corrected nucleotides into the DNA strand