DNA聚合酶III 同时合成前

1、A9 DNA replicationvDNA replicationvThe replication forkvBacterial DNA replicationvDNA replication in eukaryotes1. DNA replicationvDNA replication is the process by which a cell copies its DNA. DNA polymerases catalyze this reaction, by acting on single-stranded DNA synthesizing a new strand compleme

2、ntary to the original strand.vDNA synthesis always in the 5 3 direction. vDNA replication is semi-conservative: copied DNA contains one strand derived from the parent molecule and one newly synthesized strandDNA的半保留复制模型的半保留复制模型vThe mechanism of DNA replication: very similar in most organisms In prok

3、aryotes, DNA polymerase and In eukaryotes, five DNA polymerase (delta) Proofreading activity achieved through the reverse (3 5) exonuclease activity of DNA polymerase ensures the replication accuracy2. The replication forkvReplication origin: replication initiation site, usually contain sequences ri

4、ch in weak A-T base pairsvReplication fork: the region where the helix unwinds and new DNA is synthesized. 复制起始时打开复制起始时打开DNA双螺旋的过程双螺旋的过程How to open double helix at initiation stage?vA number of several distinct events occur at this site:Separation of the double helix: helicase separate the double he

5、lix and single-strand binding (SSB) protein attaches to the DNA and prevents the double helix from reformingSynthesis of leading and lagging strand: leading strand is synthesized continuously, lagging strand discontinuously, a series of Okazaki fragments半不连续复制半不连续复制Semi-discontinuous replicationPrim

6、ing: primase synthesizes a short RNA primer sequence on the DNA template creating a short double-stranded region required by DNA polymerase to initiate DNA synthesis. In E coli, DNA polymerase synthesizes DNA beginning at the RNA primer. On the lagging strand, DNA polymerase removes the RNA primer r

7、eplacing it with DNA. In eukaryotes, DNA polymerase has internal primase activity.In eukaryotes, DNA is replicated by DNA polymerase and with synthesizing the lagging strand and synthesizing the leading strand. DNA polymerase is involved in DNA repair and DNA polymerase replicates mitochondrial DNA.

8、 Ligation: for lagging strand synthesis. Carried out by a DNA ligase enzyme.DNA合成的起始合成的起始：先合成一段互补的先合成一段互补的RNA引物引物Initiation of DNA synthesis:a complement RNA primer synthesisDNA聚合酶聚合酶III 同时合成前导链和后随链同时合成前导链和后随链Concurrent synthesis on leading and lagging strand by DNA Pol III发生在复制叉处的发生在复制叉处的DNA合成过程合成过

9、程Summary of DNA synthesis at a single replication fork3. Bacterial DNA replicationvCircular DNA replication: a single replication origin, two replication forks progress in opposite directions. Form. Forks meet, fuse and replication terminates.v Supercoiling and topoisomerases: for circular DNA, no f

10、ree ends, unwinding DNA helix produces supercoiling. To overcome this, topoisomerases are required:Topoisomerase : produces a transient break in the polynucleotide backbone of one of the DNA strands a short distance ahead of the replication fork. Rejoin the broken ends.Topoisomerase : cause transien

11、t breaks in both strands of one of the DNA molecules allowing the other DNA molecule to pass through, thus separating the two daughter molecules. Rejoin the broken strands.E. coli DNA复制复制:型型DNA replication of E. coli chromosome: type4. DNA replication in eukaryotesvCell cycle: G1S G2 M G1 or G0. Bef

12、ore a cell divide it must replicate its DNA.vInitiated at multiple origins and replication forks proceed in either direction. Form replication bubbles which eventually meet and merge.vA typical mammalian cell: 50-100000 replicons, each replicates 40-200kb of DNA真核生物染色体上的复制真核生物染色体上的复制：多个复制起始位点多个复制起始位

13、点(multiple origins)vTranscriptionally active genes replicate first, nonactive regions replicate later.vDNA must unwind from the nucleosome for replication to occurvReplication of linear eukaryotic chromosome: The extreme 5 end of the lagging strand cannot be replicated because no room is available f

14、or an RNA primer to initiate replication.vTo overcome this problem, (1) the telomere contains tandem repeats of a simple non-coding sequence. e.g. in human, 5 TTAGGG 3, (2) the 3 end of the leading strand extends beyond the 5 end of the lagging strand; (3) The enzyme telomerase contains an RNA molec

15、ule which partly overlaps with and binds to the repeat sequence on the leading strand; (4) The telomerase extends the leading strands using RNA as a template, dissociate and binds to the new telomere end so that the leading strand is extended again; (5) The newly extended leading strand then acts as a template for replicating of the 5 end of the lagging strand. By this, the overall length of the chromosome remains approximately the same.真核生物染色体末端的复制真核生物染色体末端的复制：端粒