Transmission of Genetic Information DNA DNA BiosynthesisReplication DNA

第三篇基因信息的传递 Transmission of Genetic Information

第十章 DNA的生物合成 (复制) DNA Biosynthesis，Replication

DNA Replicating Itself

第一节复制的基本规律 Basic Rules of DNA Replication

l 复制的方式 ——半保留复制(semi-conservative replication) l 复制的高保真性(high fidelity) l 双向复制(bidirectional replication) l 半不连续复制(semi-discontinuous replication)

A G G T A C T G C C A C T G G T C C A T G A C G G T G A C C 母链DNA A AT G T C G G C GC T C TA A A AT C T G TA G A C GC C G A T C G T A G C 复制过程中形成的复制叉 AT GC GC TA AT CG TA GC CG CG AT CG TA GC GC + AT GC GC TA AT CG TA GC CG CG AT CG TA GC GC 子代DNA 目录

• Half of the parental DNA molecule is conserved in each new double helix, paired with a newly synthesized complementary strand. This is called semiconservative replication.

Each replication fork has a leading and a lagging strand leading strand (synthesized continuously) replication fork 5’ 3’ 3’ 5’ lagging strand (synthesized discontinuously) • The leading and lagging strand arrows show the direction of DNA chain elongation in a 5’ to 3’ direction • The small DNA pieces on the lagging strand are called Okazaki fragments (100 -1000 bases in length)

RNA primer direction of leading strand synthesis 3’ 5’ replication fork 5’ 3’ 3’ 5’ direction of lagging strand synthesis

Movement of the replication fork 5’ RNA primer Okazaki fragment RNA primer

第二节 DNA复制的酶学 The Enzymology of DNA Replication

• 聚合反应的特点 DNA 新链生成需引物和模板；新链的延长只可沿 5 → 3 方向进行。 DNA polymerases require both templates and primer. DNA polymerases polymerize nucleotides in the 5’ to 3’ direction.

（一）原核生物的DNA聚合酶 Prokaryotic DNA polymerases DNA-pol Ⅰ DNA-pol Ⅱ DNA-pol Ⅲ

DNA Polymerase of E. coli Characteristic Polymerase III pol. A 103, 000 pol. B 90, 000 pol. C 130, 000 400 10 3' exonuclease yes yes* 5' exonuclease yes no no DNA repair, RNA primer excision SOS DNA repair (? ) replicative chain growth gene MW molecules /cell biological function Polymerase I

（二）Eukaryotic DNA polymerases DNA-pol Priming replication DNA-pol Mitochondeial DNA replication DNA-pol Major replication polymerases DNA-pol

• DNA polymerases helps to ensure accuracy in two ways, initial selection of the proper nucleotide to add and enzymatic proofreading. • The initial selection is based on the fit of incoming nucleotides into the enzyme’s active site. An incoming nucleotide that makes the proper hydrogen bonds with the nucleotide on the template strand can be added to the growing chain.

• In the second way, DNA polymerases increase accuracy is by proofreading step. This is carried out by a 3’ to 5’ exonucleolytic activity that removes mispaired nucleotides from the 3’ end of the chain.

• 解螺旋酶(helicase) ——利用ATP供能，作用于氢键，使DNA双链解开成为两条单链 • To separate the parental DNA at physiological temperatures, cells use enzymes call helicase. Helicase binds to single-stranded DNA and moves along it in a fixed direction, with each step requiring hydrolysis of ATP.

• 引物酶(primase) – ——复制起始时催化生成RNA引物的酶 – DNA polymerases require primers that provide a free 3’-OH to which the deoxyribonucleotide can be added covalently. The primers are short of RNA. Synthesized by a special enzyme called a primase.

（二）DNA拓扑异构酶(DNA topoisomerase) 局部解链后 10 8

五、DNA连接酶 DNA ligase • DNA连接酶作用方式连接DNA链 3 -OH末端和相邻DNA链 5 -P 末端，使二者生成磷酸二酯键，从而把两段相邻的DNA链连接成一条完整的链。 This enzyme catalyzes the formation of phosphodiester bonds between the 3’hydroxy group at the end of one DNA chain and 5’phosphate group at the end of the other.

第三节 DNA生物合成过程 The Process of DNA Replication

1. DNA解链 1 13 17 29 32 44 GATTNTTTATTT ··· GATCTNTTNTATT ··· GATCTCTTATTAG ··· 3 5 5 3 串联重复序列反向重复序列 ···TGTGGATTA-‖-TTATACACA-‖-TTTGGATAA-‖-TTATCCACA 58 66 174 201 E. coli复制起始点 ori. C 209 237 245

3' 5' 5' 3' DNA-pol OH 3' d. CTP d. TTP d. GTP d. ATp d. ATP d. GTP d. CTP d. TTP 目录

• 端粒酶(telomerase) 组成 • 端粒酶RNA (human telomerase RNA, h. TR) • 端粒酶协同蛋白(human telomerase associated protein 1, h. TP 1) • 端粒酶逆转录酶(human telomerase reverse transcriptase, h. TRT)

第四节逆转录和其他复制方式 Reverse Transcription and Other DNA Replication Ways

• D环复制(D-loop replication) 是线粒体DNA (mitochondrial DNA，mt. DNA) 的复制形式。 DNA-pol γ d. NTP

第五节 DNA损伤（突变）与修复 DNA Damage (Mutation) and Repair

三、突变的分子改变类型 • 错配 (mismatch) • 缺失 (deletion) • 插入 (insertion) 框移 (frame-shift) • 重排 (rearrangement)

（一）错配 DNA分子上的碱基错配称点突变(point mutation)。 Mutations that are changes of a single base pair are called point mutation. 1. 转换发生在同型碱基之间，即嘌呤代替另一嘌呤，或嘧啶代替另一嘧啶。 Transitions are point mutations in which one purine is substituted for another or one pyrimidine is substituted for another.

2. 颠换发生在异型碱基之间，即嘌呤变嘧啶或嘧啶变嘌呤。 Transversions are point mytations in which a purine is substituted for a pyrimide, or vice versa.

• Insertion or deletions of one more base pairs can lead to frameshifts ( if the number of base paire is not a multiple of 3 ), disrupting the coding of a protein.