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Unformatted text preview: Chapter 6 Notes The ability of a cell to maintain order in a chaotic environment depends on the accurate duplication of the vast quantity of genetic information carried in its DNA Duplication process DNA replication must occur before a cell can produuce 2 genetically identical daughter cells DNA is subject to damage from chemicals and radiation from the environment Mutations in the DNA often affect the information it encodes Base Pairing Enables DNA Replication Hereditary information is passed faithfully from one generation to the next Each strand can act as a template for the synthesis of a new complementary strand In about 8 hours, a dividing animal cell will copy the equivalent of 1000 books like this one and, on average, get no more than a single letter or two wrong Because each parental strand serves as the template for one new strand, each of the daughter DNA double helices ends up with one of the original (old) strands plus one strand that is completely new; this style of replication is said to be semiconservative DNA Synthesis Begins at Replication Origins Only temperatures approaching those of boiling water provide enough thermal energy to separate the strands To be used as a template, the double helix must first be opened up and the two strands separated to expose unpaired bases The process of DNA replication is begun by initiator proteins that bind to the DNA and pry the 2 strands apart, breaking the hydrogen bonds between the bases Individually each hydrogen bond is weak The positions at which the DNA is first opened are called replicatio origins, and they are normally marked by a particular sequence of nucleotides A-T stretches, which have fewer hydrogen bonds, are typically found at replication origins Bacterial genome, typically contained in circular DNA molecule of several million nucleotides pairs, has a single origin of replication Human genome, approximately 10,000 origins Once an initiator protein binds to DNA at the replication origin and locally opens up the double helix, it attracts a group of proteins that carry out DNA replication In each round of replication, each of the two strands of DNA is used as a template for the formation of a complementary DNA strand New DNA Synthesis Occurs at Replication Forks Replication forks move away in both directions from multiple replication origins in a eukaryotic chromosome Y- shaped junction in the DNA replication forks At the forks, the replication machine is moving along the DNA, opening up the two strands and using them to make the daughter strand 2 replication forks are formed starting from each replication origin DNA replication in bacterial and eukaryotic chromosomes is termed bidirectional Forks move every rapidly 1000 nucleotide pairs per second in bacteria, 100 nucleotide pairs per second in humans DNA polymerase synthesizes new DNA using one of the old strands as a template...
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