30 Chargaff’s Rule # of A’s = # of T’s # of C’s = # of G’s If it’s determined that a sample of DNA consists of 23% Adenine, how much of the sample is Guanine?27%
31 DNA strands are antiparallel Numbering of strands is based on position of deoxyribose sugarsAntiparallel - strands run in opposite directions.Chemists assign numbers to differently positioned carbons in organic molecules. The 5 carbons of deoxyribose are numbered 1’-5’. Bases are attached to the 1’C, while the phosphate groups are attached to the 5’C.Notice that the 5’C is uppermost on the left strand, and the 3’C is uppermost on the right strand.
32 DNA is wrapped tightly around proteins & folded DNA must unwind for replication to occur
33 DNA Replicationoccurs during the S phase of Interphaseis semi-conservative (Meselson & Stahl)Used 14N and 15N – different densities
34 Overview of DNA Replication: Unreplicated DNA. Strands “unzip” at several points creating replication forks. Each strand serves as template for complementary nucleotides to H-bond. New nucleotides of each daughter strand are linked.
35 Steps in DNA Replication: Helicase breaks hydrogen bonds.Binding proteins stabilize strands; prevent them from rejoining.Primase makes an RNA primer.RNA primer is a short segment of RNA that functions to attract an enzyme called DNA polymerase.
36 Free nucleotides move in & H-bond; DNA polymerase links nucleotides to each other starting at primer & working in the 5’ to 3’ directionDNA polymerase “proofreads” new strand (replaces incorrect bases); leaves errors 1/1,000,000,000 base pairingsDNA polymerase works directionally. It can add new nucleotides only to the 3’ end of a growing strand. Thus, DNA polymerase moves along parental strands in opposite directions.
37 DNA replication is continuous on one strand (leading strand) DNA replication is discontinuous on other strand (lagging strand), producing Okazaki fragments
38 Repair enzymes remove RNA primers; Ligase connects Okazaki fragments. Keep in mind that DNA replicates at hundreds of points along a chromosome, and then the newly made pieces merge (hooked together by ligase) to make one complete DNA molecule from each parent strand.
39 Origin of replicationReplication forksReplication bubbles