Chp 6 notes - Chapter 6: DNA Replication and Repair I....

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Chapter 6: DNA Replication and Repair I. General process of DNA Replication A. Because each strand of DNA contains a sequence of nucleotides that is complementary to the nucleotide sequence of the partner strand- each strand can act as a t emplat e (or a mold) for the synthesis of a new complementary strand. One double helix is replicated to form two identical double helicies- each with one of the original DNA strands and one new DNA strand ( Figure 6- 3 ). The original strands thus remain intact through many cell generation. Thus the process is said to be semiconservative (each product of DNA replication consists of one old strand and one newly synthesized strand). B. DNA synthesis begins at replication origins . 1. bacterial cells have a single origin (one circular chromosome) 2. human genome has ~10,000 origins spread out over the different chromosomes 3. the origins are marked by a particular sequence of nucleotides 4. a particular set of proteins binds to the DNA at the origin C. Once the initiator proteins bind to the origin- they open up a small stretch of the helix , by breaking the hydrogen bonds between bases, to expose single strands (Figure 6-5) - this action attracts a set of proteins that start replicating the DNA. 1. You can see DNA in the process of being replicated using the Electron Microscope (Figure 6- 9) - one thing that is visible are replication forks - Y-shaped junctions in the DNA. As DNA replication proceeds, the replication machine moves along the DNA in both directions away from the origin- so replication is bidirectional . Forks move at 100 nucleotides per second in humans and 1000 nucleotides per second in bacteria. 2. DNA polymerase is the enzyme that synthesizes DNA- it adds nucleotides to the 3’ end of a growing DNA strand using the old strand as a template . Phosphodiester bonds are formed between the 3’OH of the last nucleotide in the strand and the 5’ phosphate group of the incoming nucleotide. Nucleotides enter the rxn as energy rich nucleoside triphosphates and the energy in the bonds between the phosphate groups (phosphoanhydride bonds) is used to perform the condensation reaction that adds the incoming nucleotide. (Figure 6-10) 3. DNA polymerase stays attached to the growing chain (rather than dissociating after the addition of a single nucleotide). 4. So DNA is synthesized in the 5’ to 3’ direction ; nucleotides are added to the 3’ end of a DNA strand 5. The replication fork is asymmetrical -the two newly synthesized strands of DNA are made in different ways. 6. Remember that the two strands of DNA are antiparallel- so one end of a DNA double helix has the 5’ end of one strand and the 3’ end of another strand. Nucleotides can only be added to the 3’ end of a DNA strand. So, one new strand is being made on a template that runs 5’ to 3’ while another is made on a strand that runs 3’ to 5’. Both strands appear to grow in the same direction, which would defy the rule that states that nucleotides are only added to the 3’ end. DNA is only synthesized in the 5’ to 3’ direction
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Chp 6 notes - Chapter 6: DNA Replication and Repair I....

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