chapter 12 outline 12.1 genetic information must be accurately copied every time a cell divides, 316...
TRANSCRIPT
Chapter 12 Outline
• 12.1 Genetic Information Must Be Accurately Copied Every Time a Cell Divides, 316
• 12.2 All DNA Replication Takes Place in a Semiconservative Manner, 316
• 12.3 The Replication of DNA Requires a Large Number of Enzymes and Proteins, 324
• 12.4 Recombination Takes Place Through the Breakage, Alignment, and Repair of DNA Strands, 335
12.1 Genetic Information Must Be Accurately Copied Every Time a Cell Divides
• Replication has to be extremely accurate:
• 1 error/million bp leads to 6400 mistakes every time a cell divides, which would be catastrophic.
• Replication also takes place at high speed:
• E. coli replicates its DNA at a rate of 1000 nucleotides/second.
12.2 All DNA Replication Takes Place in a Semiconservative Manner
• Conservative replication model
• Dispersive replication model
• Semiconservative replication
Proposed DNA Replication Models
• Two isotopes of nitrogen:
• 14N common form; 15N rare heavy form
• E. coli were grown in a 15N media first, then transferred to 14N media.
• Cultured E. coli were subjected to equilibrium density gradient centrifugation.
Meselson and Stahl’s Experiment
• Replicons: units of replication
• Replication origin
• Theta replication: circular DNA, E. coli; single origin of replication forming a replication fork, usually a bidirectional replication
• Rolling-circle replication: virus, F factor of E. coli; single origin of replication
Modes of Replication
• Eukaryotic cells; thousands of origins; a typical replicon: 200,000 ~ 300,000 bp in length
Linear Eukaryotic Replication
• Requirements of replication:
• A template strand
• Raw material: nucleotides
• Enzymes and other proteins
Linear Eukaryotic Replication
• Direction of replication:
• DNA polymerase add nucleotides only to the 3′ end of a growing strand.
• The replication can only go 5′3′.
Linear Eukaryotic Replication
• Direction of replication:
• Leading strand: undergoes continuous replication
• Lagging strand: undergoes discontinuous replication
• Okazaki fragment: the discontinuously synthesized short DNA fragments forming the lagging strand
Linear Eukaryotic Replication
12.3 The Replication of DNA Requires a Large Number of Enzymes and Proteins
Bacterial DNA Replication
• Initiation: 245 bp in the oriC (single origin replicon); an initiation protein
• Unwinding of DNA is performed by Helicase. Gyrase removes supercoiling ahead of the replication fork. Single stranded DNA is prevented from annealing by single stranded binding proteins.
• Primers: an existing group of RNA nucleotides with a 3′-OH group to which a new nucleotide can be added; usually 10 ~ 12 nucleotides long
Primase: RNA polymerase
Bacterial DNA Replication
• Elongation: carried out by DNA polymerase III
• Removing RNA primer: DNA polymerase I
• DNA ligase: connecting nicks after RNA primers are removed
• Termination: when a replication fork meets or by termination protein
Bacterial DNA Replication
• The fidelity of DNA replication
• Proofreading: DNA polymerase I: 3′5′ exonuclease activity removes the incorrectly paired nucleotide.
• Mismatch repair: correcting errors after replication is complete
Eukaryotic DNA Replication
• Eukaryotic DNA polymerase• DNA polymerase acts like Primase to initiate• DNA polymerase - replicates lagging strand• DNA polymerase - replicates leading strand
Eukaryotic DNA Replication
• Replication at the ends of chromosomes:
• Telomeres and telomerase