IB生物学 DNA复制 转录 翻译 蛋白质合成

IB生物学 DNA复制 转录 翻译 蛋白质合成

DNA carries the genetic blueprint of life, but that information must be copied, read, and translated into functional molecules for cells to survive. The three core processes of molecular biology: DNA replication, transcription, and translation: form the foundation of how genetic information flows and is expressed in all living organisms. This article covers the IB Biology Higher Level syllabus for these interconnected topics.
DNA承载着生命的遗传蓝图，但这些信息必须被复制、读取并翻译成功能性分子，细胞才能生存。分子生物学的三个核心过程：DNA复制、转录和翻译：构成了所有生物体中遗传信息流动和表达的基础。本文涵盖IB生物学高级课程中这些相互关联的主题。

1. DNA结构与复制概述 Introduction to DNA Structure and Replication

DNA (deoxyribonucleic acid) is a double-stranded helical polymer of nucleotide monomers. Each nucleotide has a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), or guanine (G). The strands run antiparallel (5′ to 3′ and 3′ to 5′). A pairs with T via two hydrogen bonds; C pairs with G via three. DNA replication is semiconservative: each daughter molecule contains one parental strand and one newly synthesised strand, as demonstrated by the Meselson-Stahl experiment using N-15 and N-14 isotopes.
DNA（脱氧核糖核酸）是一种双螺旋核苷酸聚合物。每个核苷酸有脱氧核糖、磷酸基团和四种含氮碱基之一：腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）或鸟嘌呤（G）。链反向平行（5’到3’和3’到5’）。A与T通过两个氢键配对；C与G通过三个氢键配对。DNA复制是半保留的：每个子代分子含有一条亲本链和一条新合成链，由Meselson-Stahl实验使用N-15和N-14同位素证明。

Replication begins at origins of replication. Prokaryotes like E. coli have a single origin (oriC); eukaryotes have multiple origins per chromosome. Helicase unwinds the double helix by breaking hydrogen bonds, creating a replication fork. Single-strand binding proteins (SSBs) coat separated strands to prevent re-annealing. DNA gyrase (a topoisomerase) relieves torsional stress ahead of the fork by introducing negative supercoils.
复制从复制起点开始。原核生物如大肠杆菌有单一起点（oriC）；真核生物每条染色体有多个起点。解旋酶通过断裂氢键解开双螺旋，形成复制叉。单链结合蛋白（SSB）覆盖分离的链防止重新结合。DNA旋转酶（一种拓扑异构酶）通过引入负超螺旋缓解复制叉前方的扭转应力。

2. DNA复制的酶学机制 Enzymology of DNA Replication

DNA polymerase III is the main replicative enzyme in prokaryotes. It synthesises new DNA strands in the 5′ to 3′ direction only, adding nucleotides to the free 3′-OH group of the growing chain. This directional constraint creates a fundamental asymmetry at the replication fork. The leading strand is synthesised continuously in the same direction as the replication fork moves. The lagging strand is synthesised discontinuously as short fragments called Okazaki fragments, each requiring its own RNA primer. These fragments are later joined by DNA ligase.
DNA聚合酶III是原核生物中主要的复制酶。它只能沿5’到3’方向合成新的DNA链，将核苷酸添加到生长链的游离3′-OH基团上。这种方向性限制在复制叉处产生了根本性的不对称。前导链沿与复制叉移动相同的方向连续合成。后随链以不连续的方式合成为短片段，称为冈崎片段，每个都需要自己的RNA引物。这些片段随后由DNA连接酶连接。

DNA polymerase cannot initiate synthesis de novo; it requires a primer with a free 3′-OH group. RNA primase synthesises short RNA primers (~10 nucleotides) as starting points. DNA polymerase I then removes the RNA primers and fills the gaps with DNA. The replisome includes helicase, primase, DNA polymerase III, DNA polymerase I, DNA ligase, SSBs, and DNA gyrase working together at the replication fork. Students should know each enzyme’s role and predict consequences of inhibition.
DNA聚合酶不能从头合成；需要带有游离3′-OH基团的引物。RNA引物酶合成短RNA引物（约10个核苷酸）作为起点。DNA聚合酶I随后移除RNA引物并用DNA填补空隙。复制体包括在复制叉处协同工作的解旋酶、引物酶、DNA聚合酶III、DNA聚合酶I、DNA连接酶、SSB和DNA旋转酶。学生应了解每种酶的作用并预测抑制的后果。

In eukaryotes, enzymes have different names but analogous functions. DNA polymerases α, δ, and ε replace the prokaryotic polymerases. Telomerase solves the end-replication problem: DNA polymerase cannot replicate linear chromosome ends, so telomeres shorten with each round. Telomerase, a ribonucleoprotein with its own RNA template, extends the 3′ overhang, allowing replication without information loss. It is active in germ and stem cells but not most somatic cells, linking telomere shortening to ageing.
在真核生物中，酶有不同名称但功能类似。DNA聚合酶α、δ和ε取代了原核聚合酶。端粒酶解决末端复制问题：DNA聚合酶无法复制线性染色体末端，因此端粒随每轮缩短。端粒酶是一种带有自身RNA模板的核糖核蛋白，延伸3’突出端，使复制无信息丢失。它在生殖和干细胞中活跃，但在大多数体细胞中不活跃，将端粒缩短与衰老联系起来。

3. 转录：从DNA到RNA Transcription: From DNA to RNA

Transcription is the process by which DNA is copied into complementary RNA. Only one DNA strand serves as template: the antisense (template) strand. The RNA transcript matches the sense (coding) strand sequence, with thymine replaced by uracil. RNA polymerase catalyses transcription and, unlike DNA polymerase, can initiate without a primer. In prokaryotes, a single RNA polymerase transcribes all RNA; in eukaryotes, RNA polymerase II transcribes mRNA, while polymerases I and III handle rRNA and tRNA.
转录是将DNA复制到互补RNA的过程。只有一条DNA链作为模板：反义（模板）链。RNA转录本与有义（编码）链序列匹配，胸腺嘧啶被尿嘧啶取代。RNA聚合酶催化转录，与DNA聚合酶不同，可在无引物下启动。原核生物中单一RNA聚合酶转录所有RNA；真核生物中RNA聚合酶II转录mRNA，聚合酶I和III处理rRNA和tRNA。

The transcription process has three stages: initiation, elongation, and termination. In prokaryotic initiation, sigma factor guides RNA polymerase to the promoter with conserved sequences at -10 (TATAAT, Pribnow box) and -35. Once bound, DNA unwinds into an open complex and transcription begins. In eukaryotes, initiation requires general transcription factors (TFIID, TFIIB) assembling at the TATA box before RNA polymerase II can bind. Enhancers and silencers regulate transcription from a distance via DNA looping.
转录过程有三个阶段：起始、延伸和终止。在原核起始中，sigma因子引导RNA聚合酶到启动子，启动子在-10（TATAAT，Pribnow盒）和-35有保守序列。一旦结合，DNA解开成开放复合体，转录开始。在真核生物中，起始需要通用转录因子（TFIID、TFIIB）在TATA盒组装，然后RNA聚合酶II才能结合。增强子和沉默子通过DNA环化从远处调控转录。

During elongation, RNA polymerase moves along the template 3′ to 5′, synthesising RNA 5′ to 3′. Nucleoside triphosphates (NTPs) are added by base-pairing: A-U, T-A, C-G, G-C. Termination in prokaryotes can be Rho-dependent or Rho-independent (GC-rich hairpin + poly-U). Eukaryotic termination involves cleavage at the polyadenylation signal (AAUAAA) and poly-A tail addition.
在延伸过程中，RNA聚合酶沿模板3’到5’移动，沿5’到3’合成RNA。核苷三磷酸（NTP）按碱基配对添加：A-U、T-A、C-G、G-C。原核终止可以是Rho依赖型或Rho非依赖型（GC富集发夹+poly-U）。真核终止涉及在polyadenylation信号（AAUAAA）处切割并添加poly-A尾。

4. 真核生物中的RNA加工 RNA Processing in Eukaryotes

In eukaryotes, the primary transcript (pre-mRNA) undergoes extensive processing in the nucleus before translation. Three major modifications occur. First, a 5′ cap (7-methylguanosine) is added, protecting mRNA from exonuclease degradation and aiding ribosome binding. Second, a 3′ poly-A tail (~200 adenines) enhances stability and nuclear export. Third, splicing removes introns (non-coding) and joins exons (coding) to produce a continuous open reading frame.
在真核生物中，初级转录本（pre-mRNA）在翻译前于细胞核中经历广泛加工。三个主要修饰发生：首先添加5’帽（7-甲基鸟苷），保护mRNA免受外切核酸酶降解并帮助核糖体结合。其次添加3’poly-A尾（约200腺嘌呤），增强稳定性和核输出。第三，剪接去除内含子（非编码）并连接外显子（编码），产生连续开放阅读框。

Splicing is catalysed by the spliceosome, a ribonucleoprotein complex of snRNAs (U1, U2, U4, U5, U6) and proteins. It recognises conserved sequences: 5′ splice site (GU), branch point (A), and 3′ splice site (AG). Two transesterification reactions occur: first, the branch point adenosine attacks the 5′ splice site, forming a lariat; second, the upstream exon attacks the 3′ splice site, joining exons and releasing the intron. Alternative splicing produces multiple protein isoforms from a single gene, a key source of proteomic diversity.
剪接由剪接体催化，这是由snRNA（U1、U2、U4、U5、U6）和蛋白质组成的核糖核蛋白复合体。它识别保守序列：5’剪接位点（GU）、分支点（A）和3’剪接位点（AG）。两个转酯反应发生：首先分支点腺苷攻击5’剪接位点，形成套索；其次上游外显子攻击3’剪接位点，连接外显子并释放内含子。可变剪接从单基因产生多种蛋白质异构体，是蛋白质组多样性的关键来源。

5. 翻译：从mRNA到蛋白质 Translation: From mRNA to Protein

Translation is the process by which the genetic code in mRNA is decoded to synthesise a polypeptide chain. It occurs on ribosomes and involves mRNA (template), tRNA (amino acid carriers), and ribosomes (catalytic machinery). The genetic code is degenerate: multiple codons can specify the same amino acid. Of 64 possible codons, 61 code for amino acids and 3 are stop codons (UAA, UAG, UGA). AUG is the start codon, coding for methionine. The code is nearly universal, supporting common ancestry.
翻译是将mRNA中的遗传密码解码以合成多肽链的过程。它发生在核糖体上，涉及mRNA（模板）、tRNA（氨基酸载体）和核糖体（催化机制）。遗传密码是简并的：多个密码子可指定同一氨基酸。64个密码子中61个编码氨基酸，3个是终止密码子（UAA、UAG、UGA）。AUG是起始密码子，编码甲硫氨酸。密码接近通用，支持共同祖先。

Transfer RNA (tRNA) molecules have a cloverleaf secondary structure and L-shaped tertiary structure. Each tRNA carries a specific amino acid at its 3′ end and has an anticodon loop with three nucleotides complementary to the mRNA codon. Aminoacyl-tRNA synthetases attach amino acids to cognate tRNAs with high specificity: a critical quality-control step since the ribosome cannot verify which amino acid is attached. Each of the 20 amino acids has its own specific synthetase. The charging reaction requires ATP: formation of an aminoacyl-AMP intermediate, then transfer to tRNA.
转移RNA（tRNA）分子具有三叶草二级结构和L形三级结构。每个tRNA在其3’端携带特定氨基酸，并具有与mRNA密码子互补的三核苷酸反密码子环。氨酰tRNA合成酶以高特异性将氨基酸连接到同源tRNA：关键的质量控制步骤，因为核糖体无法验证连接的氨基酸。20种氨基酸各有特异性合成酶。装载反应需要ATP：形成氨酰-AMP中间体，然后转移到tRNA。

Ribosomes consist of two subunits (large and small) made of rRNA and ribosomal proteins. Prokaryotic ribosomes (70S: 50S + 30S) and eukaryotic ribosomes (80S: 60S + 40S) differ in size. Three tRNA binding sites exist: A (aminoacyl) for incoming aminoacyl-tRNA, P (peptidyl) holding the growing peptide chain, and E (exit) for uncharged tRNA departure. The large subunit catalyses peptide bond formation via peptidyl transferase activity, performed by 23S rRNA in prokaryotes: a classic ribozyme.
核糖体由两个亚基（大和小）组成，由rRNA和核糖体蛋白质构成。原核核糖体（70S：50S+30S）和真核核糖体（80S：60S+40S）大小不同。三个tRNA结合位点：A（氨酰）接纳进入的氨酰tRNA，P（肽基）容纳增长中的肽链，E（出口）供未装载tRNA离开。大亚基通过肽基转移酶活性催化肽键形成，原核中由23S rRNA执行：经典核酶。

Translation proceeds through three phases: initiation, elongation, and termination. In prokaryotic initiation, the small ribosomal subunit binds to the Shine-Dalgarno sequence (AGGAGG) on mRNA, positioning the start codon in the P site. The initiator tRNA-fMet binds, and the large subunit joins. In eukaryotes, the small subunit with initiator tRNA-Met scans from the 5′ cap to find the first AUG in a Kozak sequence context. Elongation cycles through codon recognition, peptide bond formation, and translocation, consuming two GTP per cycle. Termination occurs when a stop codon enters the A site; release factors trigger hydrolysis of the peptidyl-tRNA bond, releasing the completed polypeptide.
翻译经过三个阶段：起始、延伸和终止。在原核起始中，小核糖体亚基与mRNA上的Shine-Dalgarno序列（AGGAGG）结合，将起始密码子定位在P位点。起始tRNA-fMet结合，大亚基加入。在真核生物中，携带起始tRNA-Met的小亚基从5’帽扫描找到Kozak序列中的第一个AUG。延伸通过密码子识别、肽键形成和转位的循环进行，每循环消耗两个GTP。终止时终止密码子进入A位点；释放因子触发肽基tRNA键水解，释放完成的多肽。

6. 中心法则与超越 The Central Dogma and Beyond

Francis Crick articulated the Central Dogma of molecular biology in 1958: information flows from DNA to RNA to protein, and cannot flow back from protein to nucleic acids. This framework has been refined by subsequent discoveries. Reverse transcriptase (discovered by Temin and Baltimore, 1970) synthesises DNA from an RNA template, enabling retroviruses like HIV to integrate into host DNA. The discovery of prions, misfolded proteins that propagate conformational information, challenged strict unidirectionality but does not violate the Central Dogma as originally formulated.
Francis Crick于1958年阐明了中心法则：信息从DNA流向RNA再流向蛋白质，不能从蛋白质流回核酸。该框架已被后续发现完善。逆转录酶（Temin和Baltimore，1970年）从RNA模板合成DNA，使逆转录病毒如HIV能整合到宿主DNA中。朊病毒（传播构象信息的错误折叠蛋白质）的发现挑战了严格单向性，但不违反最初的中心法则。

Non-coding RNAs have expanded our understanding beyond the protein-centric view. MicroRNAs (miRNAs, ~22 nucleotides) bind complementary sequences in target mRNAs, causing translational repression or degradation. Small interfering RNAs (siRNAs) operate similarly, forming the basis of RNA interference (RNAi), a powerful gene-silencing tool. Long non-coding RNAs (lncRNAs) act as scaffolds, decoys, or guides in transcription and chromatin regulation. The ENCODE project showed that while only ~1.5% of the human genome codes for proteins, over 80% is transcribed, highlighting the vast regulatory potential of non-coding RNA.
非编码RNA扩展了我们的理解，超越了以蛋白质为中心的观点。微RNA（miRNA，约22核苷酸）结合目标mRNA的互补序列，导致翻译抑制或降解。小干扰RNA（siRNA）类似地运作，构成RNA干扰（RNAi）的基础。长链非编码RNA（lncRNA）在转录和染色质调控中充当支架、诱饵或向导。ENCODE项目显示虽然仅约1.5%人类基因组编码蛋白质，但超过80%被转录，突显了非编码RNA的巨大调控潜力。

7. 关键实验与证据 Key Experiments and Evidence

The IB syllabus expects understanding of experimental evidence for replication, transcription, and translation. The Meselson-Stahl experiment (1958) confirmed semiconservative replication: E. coli grown in N-15 medium (heavy DNA), transferred to N-14, and analysed by CsCl centrifugation. After one generation, all DNA was intermediate density (one heavy, one light strand), ruling out conservative replication. After two generations, half was intermediate and half light, ruling out dispersive replication.
IB教学大纲要求理解复制、转录和翻译的实验证据。Meselson-Stahl实验（1958年）确认半保留复制：大肠杆菌在N-15培养基中生长（重链DNA），转移到N-14，通过CsCl离心分析。一代后所有DNA为中间密度（一条重链一条轻链），排除保留复制。两代后一半中间一半轻链，排除分散复制。

The Nirenberg-Matthaei experiment (1961) cracked the genetic code using synthetic poly-U RNA in a cell-free system, producing polyphenylalanine and establishing UUU as the phenylalanine codon. Subsequent experiments with mixed copolymers and ribosome-binding assays deciphered the full genetic code by 1966. The discovery of split genes (introns and exons) by Sharp and Roberts (1977) used electron microscopy of adenovirus mRNA-DNA hybrids, revealing unpaired DNA loops corresponding to introns, earning them the 1993 Nobel Prize.
Nirenberg-Matthaei实验（1961年）使用合成的poly-U RNA在无细胞系统中破解遗传密码，产生聚苯丙氨酸，确立UUU为苯丙氨酸密码子。随后混合共聚物和核糖体结合实验在1966年破译了完整密码。Sharp和Roberts（1977年）发现断裂基因（内含子和外显子），使用腺病毒mRNA-DNA杂合体电子显微镜，揭示与内含子对应的未配对DNA环，获得1993年诺贝尔奖。

8. 考试技巧与常见误区 Exam Tips and Common Misconceptions

A common exam error is confusing directionality of different processes. DNA polymerase synthesises 5′ to 3′ and reads template 3′ to 5′. RNA polymerase also synthesises 5′ to 3′ and reads template 3′ to 5′. The ribosome translates mRNA 5′ to 3′, synthesising polypeptide N-terminus to C-terminus. Drawing replication forks incorrectly is another mistake: the lagging strand is synthesised away from the fork, not toward it. Okazaki fragments require multiple RNA primers while the leading strand needs only one.
常见考试错误是混淆不同过程的方向性。DNA聚合酶沿5’到3’合成，沿3’到5’读取模板。RNA聚合酶同样沿5’到3’合成，沿3’到5’读取模板。核糖体沿5’到3’翻译mRNA，从N端到C端合成多肽。错误绘制复制叉是另一错误：后随链远离复制叉合成，而非朝向它。冈崎片段需要多个RNA引物，前导链只需一个。

For transcription questions, distinguish template from coding strand. The template strand (antisense) is read by RNA polymerase; the RNA transcript is complementary to it. The coding strand (sense) matches the RNA sequence (T instead of U). Many students incorrectly write the coding strand when asked for the RNA transcript. In translation, be precise: the tRNA anticodon is complementary and antiparallel to the mRNA codon. To determine amino acid sequence from DNA: transcribe DNA to mRNA (T to U), then use the genetic code table. IB questions frequently ask to identify the template strand from a given mRNA: the strand complementary to the mRNA is the template.
对于转录问题，要区分模板链和编码链。模板链（反义）是RNA聚合酶读取的链；RNA转录本与其互补。编码链（有义）与RNA序列匹配（T代替U）。许多学生被要求写RNA转录本时错误地写了编码链。在翻译中要精确：tRNA反密码子与mRNA密码子互补且反向平行。从DNA确定氨基酸序列：DNA转录为mRNA（T到U），然后使用遗传密码表。IB考题常要求根据给定mRNA识别模板链：与mRNA互补的链就是模板。

Remember that prokaryotic transcription and translation are coupled: ribosomes begin translating mRNA during transcription. In eukaryotes, these processes are separated by the nuclear envelope; mRNA processing must complete before translation. This distinction is a common IB comparative question. For splicing, know the conserved sequences: GU at 5′ splice site, AG at 3′ splice site, and the branch point A. Be able to explain the two transesterification reactions.
记住原核生物转录和翻译是偶联的：核糖体在转录期间就开始翻译mRNA。真核生物中这些过程被核膜分隔；mRNA加工必须在翻译前完成。这个区别是IB常见比较题。对于剪接，了解保守序列：5’剪接位点的GU、3’剪接位点的AG和分支点A。能解释两个转酯反应。

When discussing genetic code universality, note minor exceptions: some mitochondrial genomes and ciliates use stop codons for amino acids. Mentioning these demonstrates higher-level understanding. For Meselson-Stahl data questions, practise interpreting band patterns: one heavy band (generation 0), one intermediate (generation 1), intermediate plus light (generation 2 onwards for semiconservative). Predicting band patterns for each replication model is a classic exam skill.
讨论遗传密码通用性时，注意微小例外：一些线粒体基因组和纤毛虫将终止密码子用于氨基酸。提及这些展示更高层次理解。对于Meselson-Stahl数据题，练习解释条带模式：一条重链带（0代）、一条中间带（1代）、中间加轻链带（半保留复制的2代及以后）。预测每种复制模型的条带模式是经典考试技能。