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Lecture 12

Biology Lecture 12 notes

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Western University
Biology 1202B
Brenda Murphy

Bio 1 Biology Exam Review Lecture 11: Chapter 12—DNA Structure and Organization DNA structure  Watson and Crick won Nobel Prize for discovery  Franklin played role because she helped discover it through X-Ray Diffraction— died because of things she worked with and her notes ended up with Watson. X- shaped pattern of spots was correctly interpreted by Franklin to indicate that DNA has a helical structure similar to spiral staircase Nucleotides  DNA contains nucleotides joined to form a polynucleotide chain. In a polynucleotide chain, the deoxyribose sugars are linked by phosphate groups in an alternating sugar-phosphate-sugar-phosphate pattern, forming a sugar- phosphate backbone  Each phosphate group is a ―bridge‖ between the 3’ carbon of one sugar and the 5’ carbon on the next sugar; the entire linkage, including the bridging phosphate group is called a phosphodiester bond  At one end, a phosphate group is bound to the 5’ carbon of a deoxyribose sugar (5’ end), whereas at the other end, a hydroxyl group is bonded to the 3’ carbon of a deoxyribose sugar (3’ end)  Each of the 2 sugar-phosphate backbones of the double helix qualifies as a molecule—double helix is technically composed of 2 poly-nucleotide molecules held together by hydrogen bonds  Full turn takes 3.4nm therefore there are 10 base pairs in a full turn Semiconservative replication 1. Complementary base pairing in the DNA double helix: A pairs with T; G with C 2. The 2 chains unwind and separate 3. Each ―old‖ strand is a template for the addition of bases according to the base- pairing rules 4. The result is 2 DNA helices that are exact copies of the parental DNA molecule with one ―old‖ strand and one ―new‖ strand DNA replication  DNA polymerase catalyzes assembly of template strand  Can add a nucleotide only to the 3’ end of an existing nucleotide chain  As DNA is assembled, a 3’ –OH group is always exposed at its ―newest‖ end.  ―Oldest‖ end of the new chain has an exposed 5’ phosphate  DNA polymerase assemble nucleotide chains 5’ 3’ direction  Read in the 3’  5’ direction for new synthesis  Key molecular events Bio 2 1. The 2 strands of the DNA molecule unwind for replication to occur 2. DNA polymerase can add nucleotides only to an existing chain 3. The overall direction of new synthesis is in the 5’ 3’ direction, which is a direction antiparallel to that of the template strand 4. Nucleotides enter into a newly synthesized chain according to the A-T and G- C complementary base-pairing rules Lecture 12: Chapter 12—DNA Replication Semi-conservative: double helix opens, each strand is a template, after replication, each strand has a new and an old Conservative: double helix opens, parental stands pair back up, there is a new double strand and an old double strand Dispersive: splits into double stranded segments, on which new double stranded segments form. Some new parts, some old parts Meselson and Stahl showed that DNA replication is semiconservative…  Grew bacteria for several generations in 15N (heavy) medium—incorporated into nitrogenous bases of DNA  Transferred bacteria to 14N (normal/light) medium and allowed it to grow and divide for several generations  Mixed with CsCL and centrifuged—things with highest density go to bottom Results… Supports the semi conservative model Bio 3 DNA replication  Unwinding of DNA occurs at specific sequence in bacterial chromosome called origin of replication (ori)  Proteins bind to ori and promote binding of DNA helicase, which unwinds the DNA strands and produces a replication fork, which consists of the 2 unwound template strands transitioning to double helical DNA  SSBs keep the strands from binding back
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