Essential Cell Biology Chapter 6: DNA Replication - PDF

CHAPTER 6 Lecture Slides DNA Replication and Repair Copyright © 2023 by W. W. Norton & Company, Inc.  CHAPTER CONTENTS DNA REPLICATION DNA REPAIR  Differences in DNA can produce the variations that underlie the differences between individuals of the same For survival andspec...

CHAPTER 6 Lecture Slides DNA Replication and Repair Copyright © 2023 by W. W. Norton & Company, Inc.  CHAPTER CONTENTS DNA REPLICATION DNA REPAIR  Differences in DNA can produce the variations that underlie the differences between individuals of the same For survival andspecies—even proliferation, a vast within amount the same family of the cell's genetic information needs to be accurately copied through a process called DNA replication. There is a proof reading system that ensure precise duplication. However, error takes place during copying the genetic material Look at this picture. What does it tell? How many different species can you see here? How about member within a species? Are they look identical? Ans. If they look different, it is because there is a variation in their DNA.  DNA REPLICATION  Base-pairing Enables DNA Replication  DNA Synthesis Begins at Replication Origins  Two Replication Forks Form at Each Replication Origin  DNA Polymerase Synthesizes DNA Using a Parent Strand as a Template  The Replication Fork Is Asymmetrical  DNA Polymerase Is Self-Correcting  Short Lengths of RNA Act as Primers for DNA Synthesis  DNA REPLICATION  Proteins at a Replication Fork Cooperate to Form a Replication Machine  Telomerase Replicates the Ends of Eukaryotic Chromosomes  Telomere Length Varies by Cell Type and with Age  DNA REPLICATION  Base-pairing Enables DNA Replication  Base-pairing Enables DNA Replication Each of the two complementary strands acts as a template for the synthesis of a new strand during DNA replication. Thus, a single DNA double helix can be copied accurately to produce two identical double helices. The process seems to be simple, but it is awe-inspiring as billions of nucleotides are copied literally flawlessly at an incredible speed While 1000 nucleotides of bacterial DNA are copied in a second, it takes 1 sec to copy DNA replication: each strand serves as a template for a new complementary strand DNA replication is semiconservative, with each daughter helix consisting of one old strand and one new strand.  DNA REPLICATION  DNA Synthesis Begins at Replication Origins  DNA Synthesis Begins at Replication Requirement Origins of replication Base paired  Three different models for DNA replication Each parent Each The first strand acts as replicated round of a template for DNA replication synthesizing a generation produces new daughter is a mosaic one strand of parent original and newly and one synthesized entirely DNA new double helix How can we decide which model DNA replication does follow son and Stahl Experiments to confirm the model of DNA re Bacteria are grown for several generations in a medium with either 15N (heavy isotope) or 14N (light isotope) to label their DNA. The cells are broken open, and the DNA is loaded into an ultracentrifuge tube containing a cesium chloride salt solution. The tubes are centrifuged at high speed for 2 days, forming a gradient of low density at the top and high density at the bottom. As the gradient forms, the DNA migrates to the region where its density matches that of the surrounding salt. The heavy and light DNA son and Stahl Experiments to confirm the model of DNA re Bacteria grown in light medium (containing 14N) yield DNA that forms a band near the top of the centrifuge tube, whereas bacteria grown in 15N- containing heavy medium son and Stahl Experiments to confirm the model of DNA re Bacteria grown in 15N-containing heavy medium produce DNA that reaches a position farther down the tube son and Stahl Experiments to confirm the model of DNA re When bacteria are transferred from heavy to light medium, they form a band midway between heavy and light DNA, ruling out the conservative replication model. However, it doesn't distinguish between semiconservative and dispersive models, both predicting intermediate DNA densities. growing in heavy medium growing in light medium on and Stahl Experiments to confirm the model of DNA re  How could Meselson and Stahl confirm that DNA replication follow the semiconservative model not the dispersive model? After transferring bacteria from heavy to light medium, they collected the DNA from the bacterial population Heated Heat treatment break the bonds between the complementary strands and cause them to separate. Density gradient centrifugation Resulted in two separate layers, with one located in the upper layer and the other at the bottom. This suggested that DNA replication follows the semi-  DNA REPLICATION  Two Replication Forks Form at Each Replication Origin  Two Replication Forks Form at Each Replication Origin DNA synthesis occurs at Y-shaped junctions called replication forks, which move away from each other as replication proceeds  The two replication forks formed at each replication origin move away in opposite directions These drawings represent the same portion of a eukaryotic DNA molecule as it might appear at different times during replication What players are involved in DNA synthesis?  DNA REPLICATION  DNA Polymerase Synthesizes DNA Using a Parent Strand as a Template DNA Polymerase Synthesizes DNA Using a Parent StrandNucleoside as a Template =base + sugar A nucleotide is a nucleoside with one or more phosphate What is nucleotide? groups attached to the sugar Long chains of nucleotides (DNA or What is nucleic acid? RNA) that carry genetic information. DNA Strand Synthesis in the 5′-to-3′ Direction 1. New DNA strand is synthesized in the 5′-to-3′ direction. 2. Nucleoside triphosphates are selected by base pairing with the template strand: A- T, T-A, C-G, G-C. 3. Nucleotides are added to  DNA Polymerase and DNA Strand Synthesis DNA polymerase adds deoxyribonucleotides to the 3′ end of a growing strand. Nucleoside triphosphates pair with the template strand and are covalently attached to the 3′ end. DNA synthesis occurs in the 5′-to-3′ direction. Energy for the process comes from breaking high-energy phosphate bonds in nucleoside triphosphates.  DNA Polymerase and DNA Strand Synthesis 1. DNA polymerase catalyzes the reaction and guides incoming nucleoside triphosphates to the template strand. 2. Proper base pairing allows the 5′ triphosphate to react with the 3′- hydroxyl on the growing strand. 3. Polymerase moves in the direction indicated by the gray arrow. 4. This ensures the correct addition of nucleotides during DNA synthesis  DNA REPLICATION  The Replication Fork Is Asymmetrical  The Replication Fork Is Asymmetrical DNA Strands have opposite polarities at the Replication Fork At replication fork, one newly synthesized strand is being made on a template strand that runs 5’ → 3’ direction and the other new one is being synthesized on a template strand, running 3’→5’ direction which makes the replication fork asymmetrical olymerase 3 (DNA pol 3) always builds new strand 5’ → 3’ dire DNA pol 3 reads template 3’ → 5’ direction but writes 5’ → 3’ direction Open Lagging strand Okazaki fragment 5’ Discontinuous 3’ 3’ 5’3’ 5’ 5’ 5’ Continuous 3’ 3’ Leading strand