DNA Metabolism
DNA replication: processes by which copies of DNA molecules are faithfully made. DNA repair: processes by which the integrity of DNA are maintained. DNA recombination: processes by which the DNA sequences are rearranged.
Map of the E. coli chromosome.
DNA Replication Is Semiconservative.
Replication Forks may Move Either Unidirectionally or Bidirectionally
Replication Begins at an Origin and Proceeds Bidirectionally in Many Bacteria Such as E. coli.
DNA synthesis is catalyzed by DNA polymerases in the presence of (i) primer, (ii) template, (iii) all 4 dNTP, and (iv) a divalent cathion such as Mg++.
DNA Synthesis Can’t be Continuously on Both Strands (because the DNA duplex is antiparallel and all DNA polymerases synthesize DNA in a 5’ to 3’ direction)
What is the source of primer used for lagging strand synthesis?
DNA Replication is Very Accurate
Base selection by DNA polymerase is fairly accurate (about 1 error per 104) Proofreading by the 3’ to 5’ exonuclease associated with DNA polymerase improves the accuracy about 102 to 103-fold. Mismatch repair system repairs any mismatched base pairs remaining after replication and further improves the accuracy.
An Example of Proofreading by the 3’ to 5’ Exonuclease of DNA Polymerase I of E. coli
Large (Klenow) fragment of DNA polymerase I retains polymerization and proofreading (3’ to 5’ exo)
DNA polymerase I has 5’ to 3’ exonuclease and can conduct Nick Translation
Holoenzyme consists of two cores, two b subunits and one g complex held together by a dimer of t. So it is an asymmetric dimer.
DNA polymerase III
The two b subunits of PolIII form a circular clamp that surrounds DNA
DNA Replication requires many enzymes and protein factors
Helicases: separation of DNA duplex. Topoisomerase: relieves topological stress Single-strand DNA binding proteins: stabilizes separated DNA strands. Primase: synthesizes RNA primer. DNA Pol I: removes RNA in Okazaki fragments and fills the gaps between Okazaki fragments. Ligase: seals nicks.
Replication of the E. coli chromosome
Initiation. Elongation. Termination.
Initiation begins at a fixed origin, called oriC, which consists of 245 bp bearing DNA sequences that are highly conserved among bacterial replication origins.
Model for initiation of replication at oriC.
Proteins involved in Elongation of DNA
Elongation: Synthesis of Okazaki fragments
Model for the synthesis of DNA on the leading and lagging strands by the asymmetric dimer of PolIII
Pol I can remove RNA primer and synthesize DNA to fill the gap
Termination: When the two opposing forks meet in a circular chromosome. Replication of the DNA separating the opposing forks generated catenanes, or interlinked circles.
Termination sequences and Tus (termination utilization substance) can arrest a replication fork
Replication in eukaryotic cells is more complex
Contains many replicons. How is DNA replication initiated in each replicon is not well understood. Yeast cells appears to employ ARS (autonomously replicating sequences) and ORC (origin recognition complex) to initiate replication. More than one DNA polymerase are used to replicate DNA. End-replication problem of linear DNA.
DNA Repair
DNA damage may arise: (i) spontaneously, (ii) environmental exposure to mutagens, or (iii) cellular metabolism. DNA damage may be classified as: (I) strand breaks, (ii) base loss (AP site), (iii) base damages, (iv) adducts, (v) cross-links, (vi) sugar damages, (vii) DNA-protein cross links. DNA damage, if not repaired, may affect replication and transcription, leading to mutation or cell death.
Ames test for mutagens (carcinogens)
Methylataion and Mismatch Repair
Model for Mismatch Repair
Base-Excision Repair
Nucleotide-Excision Repair in E. coli and Humans
Direct Repair: Photoreactivation by photolyase
Alkylation of DNA by alkylating agents
O6-methyl G, if not repaired, may produce a mutation
Direct Repair: Reversal of O6 methyl G to G by
methyltransferase
D i r e c t
Direct repair of alkylated bases by AlkB.
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Effect of DNA damage on replication: (i) coding lesions won’t interfere with replication but may produce mutation, (ii) non-coding lesions will interfere with replication and may lead to formation of daughter-strand gaps (DSG) or double-strand breaks (DSB).
DSG and DSB may be repaired by recombination process, to be discussed in the following section.
DNA repair and cancer
Defects in the genes encoding the proteins involved in nucleotide-excision repair, mismatch repair, and recombination repair have all been linked to human cancer. Examples are: (i) xeroderma pigmentosum (or XP) patients with defects in nucleotide-excision repair, (ii) HNPCC (hereditary nonpoplyposis colon cancer) patients with defects in hMLH1 and hMSH2, and (3) breast cancer patients with inherited defects in BRCA1 and Brca2, which are known to interact with Rad 51 (the eukaryotic homolog of RecA) and therefore may have defective recombination repair.
DNA Recombination
Homologous recombination or generalized recombination. Site-specific recombinataion. Transposition.
Pairing of homologous chromosomes and crossing-over in meiosis.
Recombination during meiosisis initiated by double-strand breaks.
Homologous recombination is catalyzed by enzymes
The most well characterized recombination enzymes are derived from studies with E. coli cells. Presynapsis: helicase and/or nuclease to generate single-strand DNA with 3’-OH end (RecBCD). Synapsis: joint molecule formation to generate Holliday juncture (RecA). Postsynapsis: branch migration and resolution of Holliday juncture (RuvABC).
Helicase and nuclease activities of the RecBCD
RecA forms nucleoprotein filament on single-strand DNA
RecA promotes joint molecule formation and strand exchange
Model for DNA strand exchange mediated by RecA
Models for recombinational DNA repair
Models for recombinational DNA repair of stalled replication fork
Site-specific Recombination: Bacteriophage lambda
integration in E. coli
Effects pf site-specific recombination on DNA structure
A site-specific recombination reaction (eg. catalyzed by Int of bacteriophage lambda)
XerCD site-specific recombinataion system can resolve dimer into monomer
Immunoglobulin Genes Are Assembled by V(D)J Recombination
Mechanism of V(D)J Recombination
Transposition
Transposition is mediated by transposable elements, or transposons. Transposons of bacteria: IS (insertion sequences) contains only sequences required for transposition and proteins (transposases) that promote the process. Complex transposons contain genes in addition to those needed for transposition. Transposition is characterized by duplication of direct repeats (5-9 bps) at target site. Transposition, in some instances, may be mediated through a RNA intermediate.
Duplication of the DNA sequence at a target site when a transposon is inserted
Models for Direct and Replicative Transposition