📚 Nucleic Acids and Protein Synthesis | 核酸与蛋白质合成
Nucleic acids and protein synthesis form the molecular foundation of all living organisms. DNA stores hereditary information, while RNA decodes it to produce proteins – the workhorses of the cell. This article explores the structure of nucleic acids, the mechanisms of DNA replication, transcription and translation, as well as how genetic mutations can alter protein function. Understanding these processes is essential for grasping how cells maintain life and how errors can lead to disease.
核酸与蛋白质合成是所有生物体的分子基础。DNA 储存遗传信息，RNA 将其解码以产生蛋白质——细胞的工作分子。本文探讨核酸的结构、DNA 复制、转录和翻译的机制，以及基因突变如何改变蛋白质功能。理解这些过程对掌握细胞如何维持生命以及错误如何导致疾病至关重要。
1. Nucleic Acids: The Blueprint of Life | 核酸：生命的蓝图
Nucleic acids are large polymers composed of nucleotide monomers. Each nucleotide consists of a pentose sugar, a phosphate group and a nitrogenous base. There are two types of nucleic acid: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
核酸是由核苷酸单体组成的大分子聚合物。每个核苷酸由一个戊糖、一个磷酸基团和一个含氮碱基构成。核酸分为脱氧核糖核酸 (DNA) 和核糖核酸 (RNA) 两种类型。
DNA stores genetic information in a stable, double-stranded form and is located mainly in the nucleus of eukaryotic cells. RNA is typically single-stranded and acts as a messenger and machinery in protein synthesis. The sequence of nitrogenous bases along a nucleic acid chain encodes the instructions for building proteins.
DNA 以稳定的双链形式储存遗传信息，主要位于真核细胞的细胞核中。RNA 通常为单链，在蛋白质合成中充当信使和工厂。核酸链上含氮碱基的序列编码了构建蛋白质的指令。
In DNA, the sugar is deoxyribose; in RNA, it is ribose. DNA contains the bases adenine (A), thymine (T), cytosine (C) and guanine (G), whereas RNA replaces thymine with uracil (U). These subtle differences are crucial for the distinct roles of the two molecules.
DNA 中的糖是脱氧核糖，RNA 中则为核糖。DNA 含有腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 和鸟嘌呤 (G)，而 RNA 用尿嘧啶 (U) 代替了胸腺嘧啶。这些细微的差异对两种分子截然不同的功能至关重要。
2. Structure of DNA: The Double Helix | DNA的结构：双螺旋
DNA consists of two polynucleotide strands wound into a right-handed double helix. The sugar–phosphate backbones run antiparallel – one strand runs 5' to 3', the other 3' to 5'. The nitrogenous bases point inward and pair specifically: A with T via two hydrogen bonds, and C with G via three hydrogen bonds.
DNA 由两条多核苷酸链缠绕成右手双螺旋。糖-磷酸骨架反向平行排列——一条链方向为 5' 到 3'，另一条为 3' 到 5'。含氮碱基朝内并特异性配对：A 与 T 通过两个氢键配对，C 与 G 通过三个氢键配对。
This complementary base pairing is the key to accurate replication and transcription. The double helix is stabilised by hydrogen bonds between bases and by base-stacking interactions. The uniform diameter of about 2 nm is maintained because a purine (A or G) always pairs with a pyrimidine (T or C).
这种互补碱基配对是精确复制和转录的关键。双螺旋由碱基之间的氢键和碱基堆积作用来稳定。由于嘌呤 (A 或 G) 总是与嘧啶 (T 或 C) 配对，螺旋的直径保持均匀，约为 2 nm。
The double-helix model, proposed by Watson and Crick in 1953, explained how genetic information could be copied and passed from generation to generation. The sequence of bases along a single strand serves as a template for synthesising a new complementary strand.
Watson 和 Crick 于 1953 年提出的双螺旋模型解释了遗传信息如何被复制并代代相传。一条链上的碱基序列可作为合成新的互补链的模板。
3. DNA Replication: Semi-Conservative and Continuous | DNA复制：半保留与连续合成
DNA replication occurs during the S phase of the cell cycle. It is semi-conservative – each new DNA molecule consists of one parental strand and one newly synthesised daughter strand. This was elegantly demonstrated by the Meselson and Stahl experiment using isotopes of nitrogen.
DNA 复制发生在细胞周期的 S 期。复制是半保留的——每个新的 DNA 分子由一条亲代链和一条新合成的子代链组成。Meselson 和 Stahl 使用氮同位素的实验巧妙地证明了这一点。
The enzyme helicase unwinds the double helix by breaking hydrogen bonds, creating a replication fork. Single-strand binding proteins stabilise the separated strands. DNA polymerase III can only add nucleotides in the 5' to 3' direction; therefore, the leading strand is synthesised continuously, while the lagging strand is synthesised discontinuously in short Okazaki fragments.
解旋酶通过断裂氢键解开双螺旋，形成复制叉。单链结合蛋白稳定分离的链。DNA 聚合酶 III 只能沿 5' 到 3' 方向添加核苷酸；因此，前导链是连续合成的，而后随链则以短小的冈崎片段不连续合成。
Primase synthesises a short RNA primer to provide a free 3'-OH group for DNA polymerase. On the lagging strand, multiple primers are required. DNA polymerase I later removes the RNA primers and replaces them with DNA, and DNA ligase seals the gaps between fragments. The result is an accurate copy of the entire genome.
引物酶合成一段短的 RNA 引物，为 DNA 聚合酶提供游离的 3'-OH 末端。在后随链上需要多个引物。随后 DNA 聚合酶 I 切除 RNA 引物并代之以 DNA，DNA 连接酶则封补片段之间的缺口。最终得到整个基因组的精确拷贝。
4. RNA: Types and Functions | RNA的类型与功能
RNA is a single-stranded polynucleotide containing ribose sugar and the base uracil instead of thymine. Three main types of RNA are directly involved in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
RNA 是一种单链多核苷酸，含有核糖和尿嘧啶（替代胸腺嘧啶）。直接参与蛋白质合成的 RNA 主要有三种：信使 RNA (mRNA)、转运 RNA (tRNA) 和核糖体 RNA (rRNA)。
mRNA carries the genetic code from DNA in the nucleus to ribosomes in the cytoplasm. It is a linear copy of a gene and contains codons, each made of three consecutive bases that specify an amino acid. tRNA acts as an adapter molecule: one end binds a specific amino acid, the other bears an anticodon that recognises the complementary codon on mRNA.
mRNA 将遗传密码从细胞核中的 DNA 带到细胞质中的核糖体。它是基因的线性拷贝，含有密码子，每个密码子由三个相邻碱基组成，指定一种氨基酸。tRNA 充当接头分子：一端结合特定的氨基酸，另一端带有反密码子，可识别 mRNA 上互补的密码子。
rRNA combines with ribosomal proteins to form the large and small subunits of ribosomes. rRNA also catalyses peptide bond formation – it is a ribozyme. In eukaryotic cells, additional RNA types such as small nuclear RNA (snRNA) are involved in splicing pre-mRNA.
rRNA 与核糖体蛋白结合，形成核糖体的大亚基和小亚基。rRNA 还催化肽键的形成——它是一种核酶。在真核细胞中，还有诸如小核 RNA (snRNA) 等 RNA 参与前体 mRNA 的剪接。
5. Transcription: Synthesising mRNA from DNA | 转录：从DNA合成mRNA
Transcription is the process by which the information in a DNA sequence is copied into a complementary mRNA strand. The enzyme RNA polymerase binds to a promoter region upstream of the gene and unwinds a short section of DNA.
转录是指将 DNA 序列中的信息拷贝到互补的 mRNA 链上的过程。RNA 聚合酶结合在基因上游的启动子区域，并解开一小段 DNA。
Using the template strand (3' to 5' direction) of the DNA, RNA polymerase builds an mRNA molecule by adding RNA nucleotides according to complementary base pairing, with uracil replacing thymine. The RNA polymerase moves along the DNA in the 3' to 5' direction, synthesising mRNA in the 5' to 3' direction until it reaches a terminator sequence, at which point transcription stops and the pre-mRNA is released.
RNA 聚合酶利用 DNA 的模板链（3' 到 5' 方向），按照互补碱基配对原则添加 RNA 核苷酸，其中尿嘧啶替代胸腺嘧啶。RNA 聚合酶沿着 DNA 的 3' 到 5' 方向移动，以 5' 到 3' 方向合成 mRNA，直到遇到终止序列，转录停止并释放前体 mRNA。
In eukaryotes, the initial transcript (pre-mRNA) undergoes processing: a 5' cap (modified guanine) and a 3' poly-A tail are added for stability, and non-coding introns are spliced out, leaving only exons. This mature mRNA is then exported to the cytoplasm for translation.
在真核生物中，最初的转录产物（前体 mRNA）需要进行加工：添加 5' 帽（修饰的鸟嘌呤）和 3' 多聚腺苷酸尾以增加稳定性，非编码的内含子被剪接掉，仅保留外显子。成熟的 mRNA 随后被运出细胞核，进入细胞质进行翻译。
6. The Genetic Code: From Nucleotide Triplets to Amino Acids | 遗传密码：从核苷酸三联体到氨基酸
The genetic code is a set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a polypeptide. Each three-base sequence, or codon, specifies a single amino acid. The code is degenerate: most amino acids are specified by more than one codon.
遗传密码是一套规则，将 mRNA 的核苷酸序列翻译为多肽的氨基酸序列。每三个碱基序列（密码子）对应一种氨基酸。密码子具有简并性：大多数氨基酸由不止一个密码子编码。
ee codons (UAA, UAG, UGA) are stop signals that terminate translation. The codon AUG codes for methionine and also serves as the start signal. The genetic code is almost universal across all organisms, which is strong evidence for common ancestry.
共有 64 种可能的密码子（4 种碱基，4³ = 64），但只有 20 种氨基酸。三个密码子 (UAA, UAG, UGA) 是终止信号，结束翻译。AUG 既编码甲硫氨酸，也作为起始信号。遗传密码在几乎所有生物中都是通用的，这是共同祖先的有力证据。
Because of the degeneracy, some point mutations in the third base of a codon may still code for the same amino acid – a silent mutation. The code is also non-overlapping and continuous: the ribosome reads bases three at a time with no skipping or punctuation between codons.
由于简并性，密码子第三个碱基发生的一些点突变可能仍编码相同的氨基酸——即沉默突变。遗传密码还是非重叠且连续的：核糖体每次读取三个碱基，密码子之间没有跳跃或标点。
7. Translation: Protein Synthesis on Ribosomes | 翻译：核糖体上的蛋白质合成
Translation is the process by which ribosomes decode mRNA to assemble a polypeptide chain. It occurs in the cytoplasm and can be divided into three stages: initiation, elongation and termination.
翻译是核糖体解码 mRNA 以组装多肽链的过程。它发生在细胞质中，可分为三个阶段：起始、延伸和终止。
During initiation, the small ribosomal subunit binds to the 5' end of mRNA and moves along until it finds the start codon AUG. The initiator tRNA carrying methionine binds to this codon, and the large ribosomal subunit then attaches, forming a functional ribosome with the initiator tRNA in the P site.
在起始阶段，核糖体小亚基与 mRNA 的 5' 端结合并移动，直到找到起始密码子 AUG。携带甲硫氨酸的起始 tRNA 与该密码子结合，随后核糖体大亚基附着，形成有功能的核糖体，起始 tRNA 占据 P 位点。
In elongation, charged tRNAs enter the A site. If the anticodon matches the codon, the ribosome catalyses the formation of a peptide bond between the amino acid in the P site and the new amino acid in the A site. The ribosome then translocates, moving the empty tRNA to the E site for exit, while the growing polypeptide chain is now held in the P site. The process repeats along the mRNA.
在延伸阶段，携带氨基酸的 tRNA 进入 A 位点。若反密码子与密码子匹配，核糖体催化 P 位点氨基酸与 A 位点新氨基酸之间形成肽键。随后核糖体移位，将空载 tRNA 移至 E 位点并排出，此时增长的多肽链占据 P 位点。该过程沿 mRNA 重复进行。
Termination occurs when a stop codon (UAA, UAG or UGA) reaches the A site. A release factor binds, causing the peptide chain, the mRNA and the ribosomal subunits to dissociate. The newly formed polypeptide can then fold into its functional three-dimensional shape.
当终止密码子 (UAA、UAG 或 UGA) 到达 A 位点时，发生终止。释放因子与之结合，使肽链、mRNA 和核糖体亚基分离。新形成的多肽随后可折叠成具有功能的三维形状。
8. The Roles of tRNA and the Ribosome | tRNA和核糖体的作用
Transfer RNA (tRNA) molecules are folded into a cloverleaf shape held together by hydrogen bonds. Each tRNA carries a specific amino acid attached at the 3' end, and has a three-base anticodon loop that pairs with the complementary codon on mRNA.
转运 RNA (tRNA) 分子折叠成由氢键维持的三叶草形。每种 tRNA 在 3' 端携带一个特定的氨基酸，并含有一个三碱基的反密码子环，可与 mRNA 上互补的密码子配对。
Aminoacyl-tRNA synthetases are enzymes that 'charge' tRNA by attaching the correct amino acid. This process is highly specific and uses ATP. The resulting charged tRNA delivers its amino acid to the ribosome for incorporation into the polypeptide.
氨酰-tRNA 合成酶是通过连接正确的氨基酸来“装载” tRNA 的酶。该过程高度特异并消耗 ATP。装载后的 tRNA 将氨基酸运至核糖体，用于合成多肽。
Ribosomes consist of a small subunit that binds mRNA and a large subunit where peptide bond formation occurs. The large subunit has three tRNA binding sites: A (aminoacyl), P (peptidyl) and E (exit). The ribosome moves along the mRNA in a 5' to 3' direction, and the peptidyl transferase activity of the rRNA catalyses the formation of peptide bonds – an example of a ribozyme.
核糖体由结合 mRNA 的小亚基和发生肽键形成的大亚基组成。大亚基有三个 tRNA 结合位点：A 位（氨基酰位）、P 位（肽酰位）和 E 位（出口位）。核糖体沿 mRNA 5' 到 3' 方向移动，rRNA 的肽基转移酶活性催化肽键形成——这是核酶的一个实例。
9. Post-Translational Modifications and Protein Folding | 翻译后修饰与蛋白质折叠
Once the polypeptide chain is released, it must fold into a precise three-dimensional conformation to become functional. The primary structure (amino acid sequence) determines how the chain folds into secondary structures (alpha helices and beta sheets) and then into a unique tertiary structure. Many proteins also assemble into quaternary structures with multiple subunits.
多肽链释放后，必须折叠成精确的三维构象才具有功能。一级结构（氨基酸序列）决定了链如何折叠成二级结构（α-螺旋和 β-折叠），进而形成独特的三级结构。许多蛋白质还组装成由多个亚基组成的四级结构。
Post-translational modifications can alter protein activity, localisation or stability. Examples include the addition of phosphate groups (phosphorylation), carbohydrate chains (glycosylation), or the cleavage of signal peptides. Chaperone proteins assist with correct folding and prevent aggregation.
翻译后修饰可改变蛋白质的活性、定位或稳定性。例如添加磷酸基团（磷酸化）、添加糖链（糖基化）或切除信号肽。分子伴侣蛋白辅助正确折叠并防止聚集。
Misfolded proteins can be non-functional and may form aggregates that cause diseases such as Alzheimer's or Parkinson's. Cells have quality-control mechanisms, including the ubiquitin-proteasome system, to degrade damaged or incorrectly folded proteins.
错误折叠的蛋白质可能失去功能，并可形成聚集体导致阿尔茨海默病或帕金森病等疾病。细胞拥有包括泛素-蛋白酶体系统在内的质量控制机制，来降解受损或错误折叠的蛋白质。
10. Mutations and Their Consequences for Proteins | 突变及其对蛋白质的影响
A gene mutation is a permanent change in the DNA sequence. Mutations can occur spontaneously or be induced by mutagens such as radiation or chemicals. They are the ultimate source of genetic variation, but can also cause disease.
基因突变是 DNA 序列的永久性改变。突变可自发发生，也可由辐射或化学物质等诱变剂诱导。它们是遗传变异的根本来源，但也可能导致疾病。
Substitution mutations replace one base with another. If the new codon still codes for the same amino acid, it is a silent mutation. A missense mutation changes the amino acid – as in sickle cell disease, where glutamic acid is replaced by valine in the β-globin chain. A nonsense mutation converts a codon into a stop codon, resulting in a truncated, usually non-functional protein.
替换突变将一个碱基替换为另一个。若新密码子仍编码相同氨基酸，则为沉默突变。错义突变改变氨基酸——如镰状细胞病中，β-珠蛋白链的谷氨酸被缬氨酸取代。无义突变将一个密码子转变为终止密码子，导致产生截短的、通常无功能的蛋白质。
Insertion or deletion mutations add or remove bases. If the number of added or deleted bases is not a multiple of three, a frameshift occurs, altering the reading frame from that point onward. This often produces a completely different amino acid sequence and a premature stop codon, severely compromising protein function.
插入或缺失突变会增加或删除碱基。若增加或删除的碱基数不是 3 的倍数，则发生移码，从该点开始阅读框被改变。这通常产生完全不同的氨基酸序列和提前终止的密码子，严重破坏蛋白质功能。
Understanding how mutations affect protein synthesis helps explain many genetic disorders and also underpins evolutionary biology. Advances in gene editing technologies, such as CRISPR-Cas9, now offer potential strategies for correcting harmful mutations.
理解突变如何影响蛋白质合成有助于解释许多遗传疾病，也为进化生物学奠定了基础。基因编辑技术（如 CRISPR-Cas9）的进步现在提供了纠正有害突变的潜在策略。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图