LS1a_L13_notes08rev

LS1a_L13_notes08rev - PROF. ROBERT LUE OCTOBER 28, 2008 1a...

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Unformatted text preview: PROF. ROBERT LUE OCTOBER 28, 2008 1a L I F E S C I E N C E S 1 Nucleic acids and the chemistry of replicating information II 1. The molecular basis of DNA replication- Semiconservative DNA replication- Nucleotide addition to a growing DNA strand- Is DNA polymerization energetically favorable?- The role of pyrophosphate cleavage 2. DNA replication in the cell- The DNA replication fork- DNA polymerase and the accuracy of replication 3. The Polymerase Chain Reaction- DNA amplification by PCR 2 DNA Polymerase is needed to synthesize a new strand of DNA O P O P O O O O O A T G G T- A C- G A- T G- C A T G G - C T- A C- G A- T G- C dCTP + Very slow (no observable reaction) Fast (~50 bases added per second) O P O P O O O O O A T G G T- A C- G A- T G- C A T G G - C T- A C- G A- T G- C dCTP + DNA polymerase Polymerase adds on to the 3-OH Incoming deoxynucleotides must pair with template Requires primer to initiate The role of DNA polymerase: faster and more accurate DNA replication If you combine a DNA template, a nucleotide primer, and four DNA nucleotide triphosphates in a test tube under physiological conditions, the DNA primer will hybridize to the DNA template, but nothing else will happen on any reasonable time scale. But why is this the case since we know that the reaction in thermodynamically favorable? The answer lies in the kinetic barrier or activation energy hill that must be overcome before reactants can go to products. In other words, there is a rate limiting step with a high G. For DNA replication to take place efciently requires the action of a protein called DNA polymerase. DNA polymerase is an enzyme that catalyzes (accelerates) a chemical reaction. There are tens of thousands of enzymes in your body that are necessary to catalyze all of the chemical reactions that must occur for your survival. The vast majority of enzymes are proteins although scientists discovered more recently (in the past 20 years) that RNA can also catalyze chemical reactions. We will learn much more about enzymes and how they accelerate chemical reactions in later lectures on the proteins involved in HIV and cancer. For now, simply appreciate that DNA polymerase is responsible for accelerating the rate of DNA polymerization in humans to a rate of about 50 bases per second. While this rate may seem impressive, it would still take almost two years to copy your entire 6,000,000,000-base genome. In contrast, your growing cells can divide in about 24 hours, and each cell division requires replicating your entire genome. Therefore DNA replication occurs at many locations on each chromosome simultaneously, and the pieces of copied DNA are then connected together (ligated) to yield a complete copy of the genome. In the case of humans, genome replication is complete within a few hours....
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LS1a_L13_notes08rev - PROF. ROBERT LUE OCTOBER 28, 2008 1a...

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