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本课件为《分子生物学》第13章DNA复制（DNA replication），依据 Gene VII 教材编排，沿DNA polymerases等小节推进，讲解本章核心概念与分子机制，配逐页图示与文字版。
Chapter 13 DNA replication
Replisome is the multiprote in structure that assembles at the bacterial replicating fork to undertake synthesis of DNA. Contains DNA polymerase and other enzymes.
Replication of duplex DNA takes place by synthesis of two new strands that are complementary to the parental strands. The parental duplex is replaced by two identical daughter duplexes, each of which has one parental strand and one newly synthesized strand. It is called semiconservative because the conserved units are the single strands of the parental duplex.
Figure 13.1 Semiconservative replication synthesizes two new strands of DNA.
Figure 13.2 Repair synthesis replaces a damaged strand of DNA.
Figure 13.3 DNA synthesis occurs by adding nucleotides to the 3-OH end of the growing chain, so that the new chain is synthesized in the 5-3direction. The precursor for DNA synthesis is a nucleoside triphosphate, which loses the terminal two phosphate groups in the reaction.
Figure 13.4 Only one DNA polymerase is the replicase. The others participate in repair of damaged DNA.
Nick translation describes the ability of E. coli DNA polymerase I to use a nick as a starting point from which one strand of a duplex DNA can be degraded and replaced by resynthesis of new material; is used to introduce radioactively labeled nucleotides into DNA in vitro.
Proofreading refers to any mechanism for correcting errors in protein or nucleic acid synthesis that involves scrutiny of individual units after they have been added to the chain.
Figure 13.6 Bacterial DNA polymerases scrutinize the base pair at the end of the growing chain and excise the nucleotide added in the case of a misfit.
Figure 13.5 Nick translation replaces part of a pre-existing strand of duplex DNA with newly synthesized material.
13.3 DNA polymerases have various nuclease activities
13.3 DNA polymerases have various nuclease activities
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Figure 13.12 There are several methods for providing the free 3-OH end that DNA polymerases require to initiate DNA synthesis.
Figure 13.7 Crystal structure of phage T7 DNA polymerase has a right hand structure. DNA lies across the palm and is held by the fingers and thumb. Photograph kindly provided by Charles Richardson and Tom Ellenberger.
Figure 13.8 The catalytic domain of a DNA polymerase has a DNA-binding cleft created by three subdomains. The active site is in the palm. Proofreading is provided by a separate active site in an exonuclease domain.
Lagging strandof DNA must grow overall in the 3′-5′ direction and is synthesized discontinuously in the form of short fragments (5′-3′) that are later connected covalently.
Figure 13.9 The leading strand is synthesized continuously while the lagging strand is synthesized discontinuously.
SSB is the single-strand protein of E. coli, a protein that binds to single-stranded DNA.
Figure 13.22 Synthesis of Okazaki fragments requires priming, extension, removal of RNA, gap filling, and nick lig at ion.
Figure 13.23 DNA ligase seals nicks between adjacent nucleotides by employing an enzyme-AMP intermediate.
Figure 13.10 fX174 DNA can be separated into single strands by the combined effects of 3 functions: nicking with A protein, unwinding by Rep, and single-strand stabiliz at ion by SSB.
Figure 13.13 Initiation requires several enzymatic activities, including helicases, single-strand binding proteins, and synthesis of the primer.
Figure 13.13 Initiation requires several enzymatic activities, including helicases, single-strand binding proteins, and synthesis of the primer.
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Figure 13.12 Leading and lagging strand polymerases move apart.
Figure 13.18 DNA polymerase III holoenzyme assembles in stages, generating an enzyme complex that synthesizes the DNA of both new strands.
Figure 13.19 The b subunit of DNA polymerase III holoenzyme consists of a head to tail dimer (the two subunits are shown in red and orange) that forms a ring completely surrounding a DNA duplex (shown in the center). Photograph kindly provided by John Kuriyan.
Figure 13.20 Each catalytic core of Pol III synthesizes a daughter strand. DnaB is responsible for forward movement at the replication fork.
Figure 13.21 Core polymerase and the b clamp dissociate at completion of Okazaki fragment synthesis and reassociate at the beginning.
Figure 13.16 Tus binds to ter asymmetrically and blocks replication in only one direction.
Figure 13.24 Similar functions are required at all replication forks.
Figure 13.24 Similar functions are required at all replication forks.
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Figure 13.25 The minimal origin is defined by the distance between the out side members of the 13-mer and 9-mer repeats.
Figure 13.26 Pre priming involves formation of a complex by sequential association of proteins, leading to the separation of DNA strands.
Figure 13.27 The complex at oriC can be detected by electron microscopy. Both complexes were visualized with antibodies against DnaB protein. Photographs kindly provided by Barbara Funnell.
13.13 Common events in priming replication at the origin Figure 13.28 Transcription initiating at PR is required to activate the origin of lambda DNA.
Figure 13.29 The lambda origin for replication comprises two regions. Early events are catalyzed by O protein, which binds to a series of 4 sites; then DNA is melted in the adjacent A-T-rich region. Although the DNA is drawn as a straight duplex, it is actually bent at the origin.
13.13 Common events in priming replication at the origin
13.14 Does methylation at the origin regulate initiation?Figure 13.30 Replication of methylated DNA gives hemimethylated DNA, which maintains its state at GATC sites until the Dam methylase restores the fully methylated condition.
13.14 Does methylation at the origin regulate initiation?Figure 13.31 Only fully methylated origins can initiate replication; hemimethylated daughter origins cannot be used again until they have been res to red to the fully methylated state.
13.14 Does methylation at the origin regulate initiation?Figure 13.32 A membrane-bound inhibitor binds to hemimethylated DNA at the origin, and may function by preventing the binding of DnaA. It is released when the DNA is remethylated.
13.15 Licensing factor controls eukaryotic rereplic at ion Figure 13.33 A nucleus injected into a Xenopus egg can replicate only once unless the nuclear membrane is permeabilized to allow subsequent replication cycles.
13.15 Licensing factor controls eukaryotic rereplic at ion Figure 13.34 Licensing factor in the nucleus is inactivated after replication. A new supply of licensing factor can enter only when the nuclear membrane breaks down at mitosis.
13.15 Licensing factor controls eukaryotic rereplic at ion Figure 13.35 Proteins at the origin control susceptibility to initiation.
13.16 SummaryDNA synthesis occurs by semid is continuous replication, in which the leading strand of DNA growing 53 is extended continuously, but the lagging strand that grows overall in the opposite 35 direction is made as short Okazaki fragments, each synthesized 53. The leading strand and each Okazaki fragment of the lagging strand initiate with an RNA primer that is extended by DNA polymerase. Bacteria and eukaryotes each possess more than one DNA polymerase activity. DNA polymerase III synthesizes both lagging and leading strands in E. coli. Many proteins are required for DNA polymerase III action and several constitute part of the replisome within which it functions.
Figure 12.16 The rolling circle generates a multimeric single-stranded tail.
Primer is a short sequence (often of RNA) that is paired with one strand of DNA and provides a free 3′-OH end at which a DNA polymerase starts synthesis of a deoxyribonucleotide chain.
Figure 13.11 A DNA polymerase requires a 3-OH end to initiate replication.
Figure 12.34 Replication of ColE1 DNA is initiated by cleaving the primer RNA to generate a 3-OH end. The primer forms a persistent hybrid in the origin region.
Figure 12.16 The rolling circle generates a multimeric single-stranded tail.
Figure 13.5 Nick translation replaces part of a pre-existing strand of duplex DNA with newly synthesized material.
Figure 12.15 Adenovirus terminal protein binds to the 5end of DNA and provides a C-OH end to prime synthesis of a new DNA strand.
Primosome describes the complex of proteins involved in the priming action that initiates replication on fX-type origins. It is also involved in restarting stalled replication forks.
Figure 13.14 Replication is halted by a damaged base or nick in DNA.
Figure 14.35 An E. coli retrieval system uses a normal strand of DNA to replace the gap left in a newly synthesized strand opposite a site of unrepaired damage.
Figure 13.15 The primosome is required to restart a stalled replication fork after the DNA has been repaired.
Figure 12.7 Replication termini in E. coli are located beyond the point at which the replication forks actually meet.
Figure 13.17 Leading and lagging strand polymerases move apart.
Figure 13.26 Pre priming involves formation of a complex by sequential association of proteins, leading to the separation of DNA strands.
13.14 Does methylation at the origin regulate initiation?Figure 12.26 Attachment of bacterial DNA to the membrane could provide a mechanism for segregation.
13.15 Licensing factor controls eukaryotic rereplic at ion Figure 1.10 Replication of DNA is semiconservative.
13.15 Licensing factor controls eukaryotic rereplic at ion Figure 12.10 An ARS extends for ~50 bp and includes a consensus sequence (A) and additional elements (B1-B3).
13.16 SummaryDNA synthesis occurs by semid is continuous replication, in which the leading strand of DNA growing 53 is extended continuously, but the lagging strand that grows overall in the opposite 35 direction is made as short Okazaki fragments, each synthesized 53. The leading strand and each Okazaki fragment of the lagging strand initiate with an RNA primer that is extended by DNA polymerase. Bacteria and eukaryotes each possess more than one DNA polymerase activity. DNA polymerase III synthesizes both lagging and leading strands in E. coli. Many proteins are required for DNA polymerase III action and several constitute part of the replisome within which it functions.
The replisome contains an asymmetric dimer of DNA polymerase III; each new DNA strand is synthesized by a different core complex containing a catalytic () subunit. Processivity of the core complex is maintained by the clamp, which forms a ring round DNA. The looping model for the replication fork proposes that, as one half of the dimer advances to synthesize the leading strand, the other half of the dimer pulls DNA through as a single loop that provides the template for the lagging strand. The transition from completion of one Okazaki fragment to the start of the next requires the lagging strand catalytic subunit to dissociate from DNA and then to reattach to a clamp at the priming site for the next Okazaki fragment.
DnaB provides the helicase activity at a replication fork; this depends on ATP cleavage. DnaB may function by itself in oriC replicons to provide primosome activity by interacting periodically with DnaG, which provides the primase that synthesizes RNA.
Phage T4 codes for a size able replication apparatus, consisting of 7 proteins: DNA polymerase, helicase, single-strand binding protein, priming activities, and accessory proteins. Similar functions are required in other replication systems, including a He La cell system that replicates SV40 DNA. Different enzymes, DNA polymerase and DNA polymerase , initiate and elongate the new strands of DNA.
The common mode of origin activation involves an initial limited melting of the double helix, followed by more general unwinding to create single strands. Several proteins act sequentially at the E. coli origin. DnaA binds to a series of 9 bp repeats and 13 bp repeats, forming an aggregate of 2040 monomers with DNA in which the 13 bp repeats are melted. The helicase activity of DnaB, together with DnaC, unwinds DNA further. Similar events occur at the lambda origin, where phage proteins O and P are the counterparts of bacterial proteins DnaA and DnaC, respectively. In SV40 replication, several of these activities are combined in the functions of T antigen.
The X priming event also requires DnaB, DnaC, and DnaT. PriA is the component that defines the primosome assembly site (pas) for X replicons; it displaces SSB from DNA in an action that involves cleavage of ATP. PriB and PriC are additional components of the primosome.
Several sites that are methylated by the Dam methylase are present in the E. coli origin, including those of the 13-mer binding sites for DnaA. The origin remains hemimethylated and is in a sequestered state for ~10 minutes following initiation of a replication cycle. During this period it is associated with the membrane, and reiniti at ion of replication is repressed.
After cell division, nuclei of eukaryotic cells have a licensing factor that is needed to initiate replication. Its destruction after initiation of replication prevents further replication cycles from occur ring in yeast. Licensing factor cannot be imported into the nucleus from the cytoplasm, and can be replaced only when the nuclear membrane breaks down during mitosis.
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