DNA Replication: Lebanese International University EENG304 Notes PDF

1 Lebanese International University EENG304 CHAPTER 12: DNA: The Carrier of Genetic Information  Structure of DNA DNA is a polymer of nucleotides Each nucleotide subunit contains a nitrogenous...

1 Lebanese International University EENG304 CHAPTER 12: DNA: The Carrier of Genetic Information  Structure of DNA DNA is a polymer of nucleotides Each nucleotide subunit contains a nitrogenous a pentose sugar a phosphate base (deoxyribose) group purines pyrimidines (adenine or (thymine or guanine) cytosine)  Structure of DNA Backbone: alternating sugar and phosphate groups joined by covalent phosphodiester linkages 3′ carbon of the next deoxyribose Phosphate group attaches to: 5′ carbon of one deoxyribose  DNA nucleotides DNA Molecule 2 polynucleotide chains arranged in a coiled double helix (Watson and Crick Model) Because exactly 10 base pairs are present in each full turn of the helix, each turn is 3.4 nanometers high each pair of bases is exactly 0.34 nanometer from DNA molecule is 2.0 nm wide. the adjacent pairs above and below  Structure of DNA Molecule The sugar-phosphate Two polynucleotide chains backbones of the two chains associated as double helix form the outside of the helix Two chains are bidirectional and The bases belonging to the two chains antiparallel (running in opposite associate as pairs in the center directions) 5′ end 3′ end phosphate hydroxyl attached to 5′ attached to 3′ deoxyribose deoxyribose carbon carbon  Base-pairing rules for DNA Hydrogen bonding between base pairs holds together the two chains of helix Adenine (A) forms Guanine (G) forms 2 hydrogen bonds 3 hydrogen bonds with thymine (T) with cytosine (C)  Chargaff’s rules : A=T G=C Complementary base pairing between A and T; G and C therefore A = T; G = C The sequence of bases in the two chain are complementary to each other If base sequence of 1 strand is known, base sequence of other strand can be predicted  DNA replication The process by which DNA makes a copy of itself Occurs during interphase, prior to cell division  Models of DNA Replication DNA replication is semiconservative: Hypotheses: 1- semiconservative: one original strand from the parent molecule and one newly synthesized complementary strand 2- conservative: both parent strands would remain together and the two newly synthesized strands form a second double helix 3- dispersive: parental and newly synthesized strands become randomly mixed during the replication process Models of DNA Replication  Meselson-Stahl Experiment E. coli grown in medium containing heavy nitrogen (15N) incorporated 15N into DNA Transferred from 15N to 14N medium after one or two generations, DNA density supported semiconservative replication Meselson-Stahl Experiment  14N (light) DNA 14N– 15N 14N– 15N hybrid DNA hybrid DNA 15N (heavy) DNA Before transfer One cell generation Two cell generations to 14N after transfer to 14N after transfer to 14N The location of DNA molecules within the centrifuge tube can be determined by UV optics. DNA solutions absorb strongly at 260 nm. Fig. 12-8b, p. 269  Semiconservative Replication Each daughter double helix consists of 1 original strand from parent molecule 1 new complementary strand  DNA replication It begins at specific sites on the DNA (origins of replication) 2 strands of double helix unwind, each is template for complementary strand Origin of replication on DNA polymerase DNA molecule 3 3 ’ ’ 5 5 ’ ’  DNA replication: Steps & other Enzymes DNA helicases: open (unwind) the double helix Single-strand binding (SSB) proteins: stabilize single DNA strands; prevents the double helix from re-forming until the strands are replicated. It also prevent the hydrolysis of the single-strand regions by other enzymes Topoisomerases: prevent tangling and knotting and supercoiling Twist introduced into the helix by Single-strand binding unwinding proteins RNA primer DNA polymerase 3’ DNA helicase 3’ 3’ 5’ 3’ 5’ Direction of replication RNA primer  DNA replication DNA strand grows DNA polymerase adds nucleotide subunits at the 3’end of an existing growing polynucleotide strand Replication is initiated DNA primase synthesizes RNA primer DNA replication  DNA replication DNA synthesis always proceeds in 5′ → 3′ direction Strands replicate at replication fork (Junction between unwound part and open part, Y-shaped structure)  DNA replication Two DNA polymerase molecules catalyze replication Leading strand: synthesized continuously, toward replication fork Lagging strand: synthesized discontinuously, away from replication fork short Okazaki fragments DNA primase synthesizes RNA primers DNA ligase links Okazaki fragments  DNA Synthesis DNA replication: Bidirectional Replication (proceeds in both directions)  DNA replication in Prokaryotes and Eukaryotes Eukaryotic chromosome: Multiple, linear chromosome DNA replication occur during interphase may have multiple origins of replication may replicate at many points at same time (replication bubbles) Prokaryotic chromosome: Single, circular chromosome DNA replication occur during binary fission One origin of replication Two replication forks