DNA复试(全英文课件教学)

• Large team of enzymes coordinates replication in eukaryotes.
4、The common features of replication origins
1）Unique DNA segments contain multiple short repeated sequences. 2）These unique DNA segments are recognized by multimeric origin-binding proteins, which in turn assemble other replication enzymes at the origin. 3）Origin regions usually contain an AT-rich stretch.
3、Replication bubbles Prokaryotic genome:
When a replicon is circular, the presence of DNA replication forms the θ-structure. The successive stages of replication of the circular DNA are visualized by electron microscopy as shown.
Replicon is any piece of DNA which replicates as a single unit. It contains an origin and a terminus
Origin is the DNA sequence where a replicon
initiates its replication.
Eukaryotic genome
Replication fork
• As the 2 DNA strands open at the origin, Replication Bubbles form • Prokaryotes (bacteria) have a single bubble • Eukaryotic chromosomes have MANY bubbles

5
3
AP Biology
3
5
Bonding in DNA
5
covalent phosphodiester bonds hydrogen bonds
3
3
5
….strong or weak bonds? AP Biology the bonds fit the mechanism for copying DNA? How do
•Molecular Biology Course

CHAPTER 4-Section E: DNA replication
E1: DNA Replication: An Overview
Semi-conservative mechanism, replicons, semidiscontinous replication, RNA priming
Bidirectional replication
2、Number of Replicon
Single replicon: describes a control system in which there is only one copy of a replicon per unit bacterium. The bacterial chromosome and some plasmids have this type of regulation. Multiple replicons: Each eukaryotic chromosome contains a large number of replicons. So the unit of segregation includes many units of replication. This is a major difference in the organization of bacterial and eukaryotic genomes when they replicate.
E1-1 Semi-conservative mechanism
1958 Meselson-stahl CsCl equilibrium density gradient centrifugation
Semiconservative replication:
is accomplished by separation of the strands of a parental duplex, each then acting as a template for synthesis of a complementary daughter strand. Parental strands of DNA are the two complementary strands of duplex DNA before replication.
Daughter strands: newly synthesized by complementary base pairing.
15N
标记 DNA
15
N
buoyant density N15－DNA 1.742g/ml N15－N14－DNA 1.717g/ml N14－DNA 1.710g/ml
Bubbles Bubbles
15
The rate of replication is ~2000 bp/min, which is much slower than the 50,000 bp/min of bacterial replication fork movement. From the speed of replication, it is evident that a mammalian genome could be replicated in ~1 hour if all replicons functioned simultaneously. But S phase actually lasts for >6 hours in a typical somatic cell, which implies that no more than 15% of the replicons are likely to be active at any given moment.
AP Biology
Semi-discontinuous replication
Leading strand of DNA is synthesized continuously in the 5´-3´ direction. Lagging strand of DNA is synthesized discontinuously in the form of short fragments (5´-3´) that are later connected covalently. Okazaki fragments are the short stretches of 10002000 nt(100-200nt in eukaryotes) produced during discontinuous replication; they are later joined into a covalently intact strand.
Synthesis of Okazaki fragments requires priming, extension, removal of RNA, gap filling, and nick ligation.
标 记 实 验
·细胞生长在15N 标记培养基中 ·转入正常N源培养基中 ·分离各代DNA ·分析各代DNA的浮力密度
1958: Matthew Meselson & Frank Stahl’s Experiment Equilibrium density gradient centrifugation (Box 3.1)
单、双向θ 复制模式图
E1-3 Semi-discontinuous Replication
Semidiscontinuous replication is made in which one new strand is synthesized continuously while the other is synthesized discontinuously.
Replicator sequences include initiator binding sites and easily unwound DNA
大肠杆菌的DNA复制需大约40 min(50kb/min), 细胞分裂需约20 min，所以一个细胞分裂周期应为1 小时。 但实际上大肠杆菌在最适生长条件下约20min 一代。这是因为在快速生长条件下，DNA复制和分 裂是同时进行的，可形成所谓的多复制叉染色体 （multiforked chromosome）。
The substrates for DNA synthesis :dNTP
(dNMP) n
+ dNTP (dNMP) n+1 + ppi
5'
3'
A T G C A A T T G C | | | | T G dTTP 3'
5
T A C
ppi
DNA polymerase
E1-2 Replicons

AP Biology
Anti-parallel strands Nucleotides in DNA
backbone are bonded from phosphate to sugar between 3 & 5 carbons