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DNA: The Stuff of Life Griffith and Transformation • Griffith and Transformation • In 1928, British scientist Fredrick Griffith was trying to learn how certain types of bacteria caused pneumonia. • He isolated two different strains of pneumonia bacteria from mice and grew them in his lab. Griffith and Transformation Performed the first major experiment that led to the discovery that genetic information could be passed between organisms. Griffith and Transformation – Transformation • Griffith determined that bacteria could pass genetic information from one to another. • Griffith called this process transformation because one strain of bacteria (the harmless strain) had changed permanently into another (the diseasecausing strain). • Transformation = the exchange of information between genetic material. Oswald Avery 1931 - Oswald Avery repeated Griffith’s work to determine which molecule was most important for transformation. Identified the molecule that transformed the R strain of bacteria into the S strain Concluded that when the S cells were killed, DNA was released R bacteria incorporated this DNA into their cells and changed into S cells. The Hershey-Chase Experiment • The Hershey-Chase Experiment – Alfred Hershey and Martha Chase studied viruses—nonliving particles smaller than a cell that can infect living organisms. The Hershey-Chase Experiment • Bacteriophages • A virus that infects bacteria is known as a bacteriophage. • Bacteriophages are composed of a DNA or RNA core and a protein coat. The Hershey-Chase Experiment • If Hershey and Chase could determine which part of the virus entered an infected cell, they would learn whether genes were made of protein or DNA. • They grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S). The Hershey-Chase Experiment Hershey and Chase Used radioactive labeling to trace the DNA and protein Concluded that the viral DNA was injected into the cell and provided the genetic information needed to produce new viruses Chargaff Chargaff’s rule: C = G and T = A •The percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA. •The percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA. Two Groups of Bases in DNA • Pyrimidines are single ring bases. – Thymine & Cytosine N N C O C C N C N • Purines are double ring bases. – Adenine & Guanine N C C CN N C NC X-ray Diffraction Rosalind Franklin’s X-ray diffraction data helped solve the structure of DNA Indicated that DNA was a double helix Watson and Crick Built a model of the double helix that conformed to the others’ research 1. Model was a double helix 2. Backbone made of sugars and phosphates 3. Base pairs attached to deoxyribose with A & T in equal amounts and C & G in equal amounts. DNA Structure DNA often is compared to a twisted ladder. Rails of the ladder are represented by the alternating deoxyribose and phosphate. The pairs of bases (cytosine–guanine or thymine–adenine) form the steps. The Components and Structure of DNA • Watson and Crick discovered that hydrogen bonds can form only between certain base pairs—adenine and thymine, and guanine and cytosine. • This principle is called base pairing. Copyright Pearson Prentice Hall • DNA Double Helix DNA Structure Nucleotides Consist of a five-carbon sugar, a phosphate group, and a nitrogenous base DNA Replication 1 DNA 2 identical DNA’s DNA Replication – Duplicating DNA • Before a cell divides, it duplicates its DNA in a process called replication. • Replication ensures that each resulting daughter cell will have a complete set of DNA. • Occurs in the S phase of Interphase. DNA Replication • The DNA molecule produces 2 IDENTICAL new strands. • Each strand of the original DNA serves as a template for the new strand • Following the rules of base pairing: A-T, G-C, the new side is the compliment of the parent side. DNA Replication • Semiconservative Model: 1. Watson and Crick showed: the two strands of the parental molecule separate, and each functions as a template for synthesis of a new complementary strand. . Parental DNA DNA Template New DNA DNA Replication New Strand Original strand Nitrogen Bases Replication Fork The sites where separation and replication occur are called replication forks. Replication Fork DNA Polymerase DNA Replication • DNA must first unwind and “unzip”, exposing the nitrogenous bases on each strand of the parent molecule. • In large eukaryotic molecules, the parent DNA molecule may have many replication sites at once. • Next, the enzyme DNA polymerase helps to add new complimentary nitrogenous bases to the exposed sides of the parent molecule. • DNA Replication • The final result is two identical DNA strands, each with one original parent strand, and one new complimentary strand. DNA Replication • Give the complementary sequence for the following strand of DNA: – DNA A T C C G A A G C T T – DNA T A G G C T T C G A A