📚 Nucleic Acids and Protein Synthesis | 核酸与蛋白质合成
Nucleic acids are the molecules of heredity and information flow in all living cells. Deoxyribonucleic acid (DNA) stores the genetic instructions, while ribonucleic acid (RNA) helps convert those instructions into proteins. Protein synthesis is the process by which the sequence of bases in DNA is transcribed into messenger RNA and then translated into the amino acid sequence of a polypeptide.
核酸是所有活细胞中负责遗传和信息传递的分子。脱氧核糖核酸（DNA）储存遗传指令，而核糖核酸（RNA）帮助把这些指令转化为蛋白质。蛋白质合成是指 DNA 中的碱基序列先被转录为信使 RNA，再被翻译为多肽链中氨基酸序列的过程。
1. Nucleotide Structure and Nucleic Acids | 核苷酸结构与核酸
A nucleotide is the monomer of nucleic acids and consists of three parts: a phosphate group, a pentose sugar, and a nitrogenous base. In DNA the pentose sugar is deoxyribose, while in RNA the sugar is ribose.
核苷酸是核酸的单体，由三部分组成：磷酸基团、五碳糖和含氮碱基。DNA 中的五碳糖是脱氧核糖，而 RNA 中的五碳糖是核糖。
Nucleotide = phosphate group + pentose sugar + nitrogenous base
The nitrogenous bases are divided into purines and pyrimidines. Adenine and guanine are purines with a double-ring structure, while cytosine, thymine, and uracil are pyrimidines with a single-ring structure.
含氮碱基分为嘌呤和嘧啶两类。腺嘌呤和鸟嘌呤是双环结构的嘌呤，而胞嘧啶、胸腺嘧啶和尿嘧啶是单环结构的嘧啶。
- DNA bases: adenine (A), thymine (T), cytosine (C), guanine (G)
- RNA bases: adenine (A), uracil (U), cytosine (C), guanine (G)
Adjacent nucleotides are joined by condensation reactions between the phosphate group of one nucleotide and the hydroxyl group on carbon 3 of the sugar of another nucleotide. This forms a phosphodiester bond and creates a sugar-phosphate backbone.
相邻核苷酸通过一个核苷酸的磷酸基团与另一个核苷酸糖分子 3 号碳上的羟基发生缩合反应而连接。这形成磷酸二酯键，并构建出糖-磷酸骨架。
2. DNA Double Helix and Base Pairing | DNA 双螺旋与碱基配对
DNA consists of two polynucleotide strands wound around each other to form a double helix. The two strands run in opposite directions, meaning they are antiparallel: one strand runs from 5′ to 3′ while the other runs from 3′ to 5′.
DNA 由两条多核苷酸链相互缠绕形成双螺旋。两条链方向相反，即它们是反向平行的：一条链从 5′ 到 3′ 方向，另一条从 3′ 到 5′ 方向。
The sugar-phosphate backbones lie on the outside of the helix, and the nitrogenous bases project inward. Hydrogen bonds form between complementary base pairs, holding the two strands together.
糖-磷酸骨架位于螺旋外侧，含氮碱基朝向内部。互补碱基对之间形成氢键，将两条链维系在一起。
Adenine is complementary to thymine and forms two hydrogen bonds, while cytosine is complementary to guanine and forms three hydrogen bonds. This complementary base pairing ensures that the sequence of one strand determines the sequence of the other.
腺嘌呤与胸腺嘧啶互补，形成两个氢键；胞嘧啶与鸟嘌呤互补，形成三个氢键。这种互补碱基配对保证了其中一条链的序列决定另一条链的序列。
| Adenine (A) | Thymine (T) in DNA / Uracil (U) in RNA | 2 hydrogen bonds |
| Cytosine (C) | Guanine (G) | 3 hydrogen bonds |
3. RNA Structure and Types | RNA 结构与类型
RNA is usually a single-stranded polynucleotide, but it can fold into complex three-dimensional shapes. Its sugar is ribose, and it contains uracil instead of thymine.
RNA 通常是单链多核苷酸，但可以折叠成复杂的三维形状。其糖为核糖，并且含有尿嘧啶而不是胸腺嘧啶。
There are three main types of RNA involved in protein synthesis: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Each type has a distinct role in translating genetic information into protein.
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）。每种类型在将遗传信息翻译为蛋白质的过程中都有不同的作用。
- mRNA carries the genetic code from DNA in the nucleus to ribosomes in the cytoplasm.
- tRNA brings specific amino acids to the ribosome and matches them to the mRNA codon.
- rRNA, together with proteins, forms the structure of ribosomes and catalyses peptide bond formation.
mRNA 将遗传密码从细胞核中的 DNA 运送到细胞质中的核糖体。tRNA 将特定氨基酸带到核糖体，并与 mRNA 密码子匹配。rRNA 与蛋白质一起构成核糖体的结构，并催化肽键形成。
4. Semi-Conservative DNA Replication | DNA 半保留复制
DNA replication occurs during the S phase of the cell cycle before cell division. The process is semi-conservative because each new DNA molecule contains one original parental strand and one newly synthesised daughter strand.
DNA 复制发生在细胞分裂前细胞周期的 S 期。该过程是半保留的，因为每个新的 DNA 分子包含一条原有的母链和一条新合成的子链。
The enzyme DNA helicase unwinds the double helix and breaks the hydrogen bonds between complementary bases, creating a replication fork. Single-strand binding proteins keep the two strands apart.
DNA 解旋酶解开双螺旋并断裂互补碱基之间的氢键，形成复制叉。单链结合蛋白使两条链保持分离。
DNA polymerase catalyses the formation of new phosphodiester bonds by adding complementary nucleotides to the exposed template strand. DNA polymerase can only add nucleotides to the 3′ end of the growing strand, so the new strand is synthesised in a 5′ to 3′ direction.
DNA 聚合酶通过在暴露的模板链上添加互补核苷酸来催化形成新的磷酸二酯键。DNA 聚合酶只能在生长链的 3′ 端添加核苷酸，因此新链按 5′ 到 3′ 方向合成。
The leading strand is synthesised continuously, while the lagging strand is synthesised discontinuously as a series of Okazaki fragments. These fragments are later joined by DNA ligase.
前导链是连续合成的，而后随链是不连续合成的，形成一系列冈崎片段。这些片段随后由 DNA 连接酶连接起来。
5. Meselson and Stahl Experiment | Meselson 与 Stahl 实验
The semi-conservative model of DNA replication was confirmed by the Meselson and Stahl experiment in 1958. They grew Escherichia coli for many generations in a medium containing the heavy isotope ¹⁵N, so all bacterial DNA contained heavy nitrogen.
DNA 复制的半保留模型由 1958 年 Meselson 和 Stahl 的实验证实。他们让大肠杆菌在含有重同位素 ¹⁵N 的培养基中生长多代，使所有细菌 DNA 都含有重氮。
The bacteria were then transferred to a medium containing the lighter isotope ¹⁴N and allowed to divide once. The DNA was extracted and separated by density-gradient centrifugation.
然后将细菌转移到含有较轻同位素 ¹⁴N 的培养基中，并让其分裂一次。提取 DNA 后通过密度梯度离心进行分离。
After one round of replication, all DNA molecules had an intermediate density between ¹⁵N-DNA and ¹⁴N-DNA. This result ruled out the conservative model, which would have produced one heavy and one light DNA molecule.
经过一轮复制后，所有 DNA 分子的密度介于 ¹⁵N-DNA 和 ¹⁴N-DNA 之间。这一结果排除了保守模型，因为保守复制会产生一条重 DNA 和一条轻 DNA。
After two rounds of replication in ¹⁴N medium, half of the DNA molecules had intermediate density and half had light density. This pattern was exactly as predicted by the semi-conservative model.
在 ¹⁴N 培养基中复制两轮后，一半 DNA 分子为中间密度，一半为轻密度。这一模式与半保留模型的预测完全一致。
6. Transcription: From DNA to mRNA | 转录：从 DNA 到 mRNA
Transcription is the first stage of protein synthesis and takes place in the nucleus in eukaryotic cells. It produces a messenger RNA molecule that is complementary to the template strand of a gene.
转录是蛋白质合成的第一阶段，在真核细胞的细胞核中进行。它产生一个与基因模板链互补的信使 RNA 分子。
RNA polymerase binds to the promoter region of the gene and unwinds the DNA double helix. It then moves along the template strand, adding complementary RNA nucleotides to form a single-stranded mRNA molecule.
RNA 聚合酶与基因的启动子区域结合，解开 DNA 双螺旋。然后它沿着模板链移动，添加互补的 RNA 核苷酸，形成单链 mRNA 分子。
In RNA, adenine pairs with uracil and cytosine pairs with guanine. The mRNA is assembled in a 5′ to 3′ direction, using the 3′ to 5′ DNA strand as the template.
在 RNA 中，腺嘌呤与尿嘧啶配对，胞嘧啶与鸟嘌呤配对。mRNA 以 3′ 到 5′ 方向的 DNA 链为模板，按 5′ 到 3′ 方向组装。
When RNA polymerase reaches a terminator sequence, transcription stops and the pre-mRNA is released. In eukaryotes, introns are removed and exons are spliced together before the mature mRNA leaves the nucleus through nuclear pores.
当 RNA 聚合酶到达终止序列时，转录停止，前体 mRNA 被释放。在真核生物中，内含子被切除，外显子被剪接在一起，成熟的 mRNA 随后通过核孔离开细胞核。
7. The Genetic Code: Triplets and Codons | 遗传密码：三联体与密码子
The genetic code is the set of rules by which the sequence of bases in mRNA specifies the sequence of amino acids in a polypeptide. The code is read in groups of three bases called codons.
遗传密码是 mRNA 中的碱基序列决定多肽链中氨基酸序列的一套规则。密码以三个碱基为一组读取，这些三碱基组称为密码子。
There are 64 possible codons: 61 code for amino acids and 3 are stop codons that do not code for any amino acid. The start codon AUG also codes for methionine.
共有 64 种可能的密码子：61 种编码氨基酸，3 种是不编码任何氨基酸的终止密码子。起始密码子 AUG 也编码甲硫氨酸。
Number of possible codons = 4 × 4 × 4 = 64
The genetic code is degenerate and universal. Degenerate means that most amino acids are coded for by more than one codon; universal means that the same codons specify the same amino acids in almost all organisms.
遗传密码具有简并性和通用性。简并性意味着大多数氨基酸由多个密码子编码；通用性意味着几乎所有生物体中相同的密码子都指定相同的氨基酸。
8. Translation: From mRNA to Polypeptide | 翻译：从 mRNA 到多肽
Translation is the second stage of protein synthesis and occurs at ribosomes in the cytoplasm. The ribosome reads the mRNA sequence in groups of three bases and links amino acids together in the correct order.
翻译是蛋白质合成的第二阶段，发生在细胞质中的核糖体上。核糖体以三个碱基为一组读取 mRNA 序列，并按正确顺序将氨基酸连接在一起。
Each tRNA molecule has a specific anticodon of three unpaired bases at one end and an amino acid binding site at the opposite end. The anticodon binds to the complementary codon on mRNA by hydrogen bonding.
每个 tRNA 分子的一端有一个由三个不配对碱基组成的特定反密码子，另一端有一个氨基酸结合位点。反密码子通过氢键与 mRNA 上互补的密码子结合。
The ribosome has three binding sites for tRNA: the A site, the P site, and the E site. Translation begins when the small ribosomal subunit binds to the mRNA at the start codon AUG.
核糖体有三个 tRNA 结合位点：A 位、P 位和 E 位。当小核糖体亚基在起始密码子 AUG 处与 mRNA 结合时，翻译开始。
Elongation involves repeated cycles: a tRNA carrying an amino acid enters the A site, a peptide bond forms between the new amino acid and the growing polypeptide chain, and the ribosome moves along the mRNA by one codon.
延伸包括循环重复：携带氨基酸的 tRNA 进入 A 位，新氨基酸与不断延长的多肽链之间形成肽键，核糖体沿 mRNA 移动一个密码子。
When a stop codon such as UAA, UAG, or UGA reaches the A site, a release factor binds and causes the completed polypeptide to detach from the ribosome. The ribosomal subunits then dissociate.
当终止密码子如 UAA、UAG 或 UGA 到达 A 位时，释放因子结合并导致完成的多肽从核糖体上脱离。随后核糖体亚基解离。
DNA → mRNA → Polypeptide
9. Polypeptide Folding and Protein Structure | 多肽折叠与蛋白质结构
The polypeptide chain produced during translation must fold into a specific three-dimensional shape to become a functional protein. The sequence of amino acids, known as the primary structure, determines the final conformation.
翻译产生的多肽链必须折叠成特定的三维形状才能成为功能蛋白。氨基酸序列（一级结构）决定了最终的构象。
Secondary structure refers to local folding patterns such as α-helices and β-pleated sheets, which are stabilised by hydrogen bonds between backbone atoms. Tertiary structure is the overall 3D shape, stabilised by hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions.
二级结构指局部折叠模式，如 α 螺旋和 β 折叠片，它们由主链原子之间的氢键稳定。三级结构是整体三维形状，由氢键、离子键、二硫键和疏水相互作用稳定。
Quaternary structure occurs when two or more polypeptide chains associate to form a functional protein. Haemoglobin, for example, consists of four polypeptide subunits.
当两条或更多多肽链结合形成功能蛋白时，就出现四级结构。例如，血红蛋白由四个多肽亚基组成。
10. Mutations and Their Effects on Protein Synthesis | 突变及其对蛋白质合成的影响
A gene mutation is a change in the sequence of nucleotide bases in DNA. Mutations can occur spontaneously during DNA replication or be induced by mutagens such as ultraviolet radiation and certain chemicals.
基因突变是 DNA 中核苷酸碱基序列的改变。突变可以在 DNA 复制过程中自发发生，也可以由紫外线辐射和某些化学物质等诱变剂诱发。
Base substitution mutations replace one base with another. Because the genetic code is degenerate, a substitution may be silent and produce no change in the amino acid sequence, or it may change a single amino acid and alter protein function.
碱基替换突变用一个碱基替换另一个碱基。由于遗传密码具有简并性，替换可能是沉默的，不改变氨基酸序列，也可能改变一个氨基酸并影响蛋白质功能。
Insertion or deletion mutations shift the reading frame of all subsequent codons. These frameshift mutations usually result in a completely different amino acid sequence and a shortened, non-functional polypeptide.
插入或缺失突变会改变后续所有密码子的阅读框。这些移码突变通常导致完全不同的氨基酸序列和缩短的、无功能的多肽。
- Sickle cell anaemia is caused by a single base substitution that changes glutamic acid to valine in the β-globin chain.
- Frameshift mutations are more likely to be harmful than substitutions because they affect many amino acids.
镰状细胞贫血由单个碱基替换引起，使 β-珠蛋白链中的谷氨酸变为缬氨酸。移码突变比替换突变更可能有害，因为它们会影响许多氨基酸。
Mutations are the ultimate source of genetic variation, and natural selection can act on this variation. However, many mutations are neutral or harmful, and only a small proportion may be beneficial.
突变是遗传变异的根本来源，自然选择可以作用于这种变异。然而，许多突变是中性的或有害的，只有一小部分可能是有益的。
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