DNA Replication BIO273 PDF

BIO273 Biology The Semiconservative Replication of DNA Nucleic Acids & Heredity  All living cells have the ability to produce exact replicas of themselves through many generations  Requirements Information is passed unchanged by some means Certain enzymes must be produced a...

BIO273 Biology The Semiconservative Replication of DNA Nucleic Acids & Heredity  All living cells have the ability to produce exact replicas of themselves through many generations  Requirements Information is passed unchanged by some means Certain enzymes must be produced at specific times  Transfer of information & the production of enzymes is accomplished by nucleic acids  No matter what organism, the genetic material is Nucleic Acids SCHOOL OF BIOLOGICAL SCIENCES 2 AND APPLIED CHEMISTRY Cell Division  Single cells replicate by binary fission  Single cell divided into two “daughter” cells  Each “daughter” cell must Receive an exact copy of the DNA from “parent” cell Have the DNA number of Chromosomes as “parent” cell  Humans have 46 Chromosomes (23 pairs) Therefore each “daughter” cell must obtain 46 Chromosomes  Chromosomes in the “parent” cell must double to 92 Chromosomes just prior to cell division SCHOOL OF BIOLOGICAL SCIENCES 3 AND APPLIED CHEMISTRY DNA Replication Chromosome  DNA molecule (double helix) with it’s associated proteins (histones)  Prior to cell division Cell must form exact copies of every Chromosome The nucleotide sequence must be exactly the same How does the cell do this?  The double helix is held together by Hydrogen bonds Hydrogen bonds are individually weak, but strong in numbers  Two DNA strands are separated  Base pairing is complementary (A = T & C Ξ G)  Each strand serves as a template for the formation of a new strand SCHOOL OF BIOLOGICAL SCIENCES 4 AND APPLIED CHEMISTRY DNA Replication Requirements  Template Existing Chromosomes  Raw Materials Deoxyribose Nucleotides in the Nucleus  Enzymes Proteins that Catalyze chemical reactions  Energy Nucleotides have 3 Phosphate groups dATP, dGTP, dTTP, dCTP (deoxyribose) Lose 2 Phosphates releases energy SCHOOL OF BIOLOGICAL SCIENCES 5 AND APPLIED CHEMISTRY Semiconservative Replication  Each DNA strand remains unchanged  Acts as the template to create new strand  Complementary bases are added by enzymes SCHOOL OF BIOLOGICAL SCIENCES 6 AND APPLIED CHEMISTRY Semiconservative Replication Result  Two identical double-stranded DNA molecules  Exact same nucleotide sequence for each  Each new DNA molecule has one original strand one new strand SCHOOL OF BIOLOGICAL SCIENCES 7 AND APPLIED CHEMISTRY DNA Replication Overview  Before Replication One parent DNA molecule Double-stranded Both strands act as templates  Replication Unwinding the DNA strands Priming the DNA template Formation of new DNA strand  After Replication Two DNA molecules are the same as: Each other Parent DNA molecule SCHOOL OF BIOLOGICAL SCIENCES 8 AND APPLIED CHEMISTRY DNA Replication - Unwinding Double-strand unwinds & unzips  Hydrogen bonds are broken between bases  Requires the enzyme Helicase Moves directionally along the molecule  Creates a Replication Fork Site where new strand is formed SCHOOL OF BIOLOGICAL SCIENCES 9 AND APPLIED CHEMISTRY DNA Replication - Unwinding Double-strand unwinds & unzips  Once Helicase unwinds  Single Strand Binding Protein (SSBP) holds strands apart  Topoisomerase releases supercoiling tension causes by unwinding (cuts & reseals DNA) SCHOOL OF BIOLOGICAL SCIENCES 10 AND APPLIED CHEMISTRY DNA Replication – Strand Formation  Free deoxyribose nucleotides bind to complementary bases Hydrogen bonding, one at a time  Requires the enzyme DNA Polymerase Catalyzes the reaction to bond a new Nucleotide to previous one Moves directionally as well – opposite directions on the strands SCHOOL OF BIOLOGICAL SCIENCES 11 AND APPLIED CHEMISTRY DNA Replication – Strand Formation  DNA Polymerase can only add to the 3’ end of the strand Creates Phosphodiester bond between Sugar & Phosphate Joins the Phosphate on the nucleotide to the 3’ Carbon on the Sugar Strands are antiparallel – Strands grow in opposite directions SCHOOL OF BIOLOGICAL SCIENCES 12 AND APPLIED CHEMISTRY DNA Replication - Priming DNA Polymerase can only add to 3’ Carbon on Nucleotide  How does a DNA strand get started? Prior to DNA Polymerase binding  Primase (RNA Polymerase) creates a short RNA primer  Complementary base pairing adding to 3’ Carbon SCHOOL OF BIOLOGICAL SCIENCES 13 AND APPLIED CHEMISTRY DNA Replication - Problem Replication occurs on both template strands  Strands are antiparallel  DNA Polymerase starts from 3’ end of RNA Primer  Joins nucleotides to 3’ Carbon – Grows in 5’ to 3’ direction Solution  One DNA template strand replicates continuously Leading strand  Other DNA template strand replicates as a series of short fragments Lagging strand SCHOOL OF BIOLOGICAL SCIENCES 14 AND APPLIED CHEMISTRY DNA Replication – Lagging Strand  Discontinuous formation of the strand  Numerous RNA primers & DNA fragments (Okazaki fragments)  DNA Polymerase removes RNA primer once it reaches it  DNA Ligase joins the Okazaki fragments together SCHOOL OF BIOLOGICAL SCIENCES 15 AND APPLIED CHEMISTRY DNA Replication SCHOOL OF BIOLOGICAL SCIENCES 16 AND APPLIED CHEMISTRY DNA Replication - Videos https://youtu.be/TNKWgcFPHqw https://youtu.be/7Hk9jct2ozY SCHOOL OF BIOLOGICAL SCIENCES 17 AND APPLIED CHEMISTRY Summary  Coordinated effort of many Enzymes make replication of DNA possible.  DNA is replicated in a semiconservative fashion. Each new double helix contains one complete original & one new strand.  DNA Polymerase can only add on to the 3’ end of the strand.  Strands run antiparallel, one strand is replicated continuously, while the other is replicated discontinuously as Okazaki fragments.  RNA primers are used to initiate the actions of DNA Polymerase. SCHOOL OF BIOLOGICAL SCIENCES 18 AND APPLIED CHEMISTRY