📚 The Structure of DNA and RNA | DNA 和 RNA 的结构
Nucleic acids are the macromolecules that store, transmit and express genetic information in all living organisms. The two main forms, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are polymers built from nucleotide monomers. Although they share a common building-block logic, their structural differences allow DNA to act as a stable long-term store of genes, while RNA serves as a more versatile intermediary in gene expression.
核酸是所有生物体中储存、传递和表达遗传信息的大分子。两种主要形式——脱氧核糖核酸（DNA）和核糖核酸（RNA）——都是由核苷酸单体构成的多聚体。尽管它们遵循相同的组装逻辑，但结构差异使 DNA 能够充当基因的稳定长期储存库，而 RNA 则在基因表达中发挥更灵活的中介作用。
1. Nucleotide Monomers | 核苷酸单体
Each nucleotide consists of three components: a pentose sugar, a phosphate group and a nitrogenous base. In DNA the sugar is deoxyribose; in RNA it is ribose. The phosphate group is attached to the 5' carbon of the sugar, and the nitrogenous base is attached to the 1' carbon. This common structure allows nucleotides to be linked into long chains.
每个核苷酸由三个部分组成：五碳糖、磷酸基团和含氮碱基。DNA 中的糖是脱氧核糖，RNA 中是核糖。磷酸基团连接在糖的 5' 碳上，含氮碱基连接在 1' 碳上。这种共同结构使核苷酸能够连接成长链。
When nucleotides join, the phosphate group of one nucleotide reacts with the hydroxyl group on the 3' carbon of another sugar. A water molecule is removed, so this is a condensation reaction. The resulting covalent link is the phosphodiester bond, and the repeating chain of sugar-phosphate units forms the backbone of the nucleic acid.
当核苷酸连接时，一个核苷酸的磷酸基团与另一个糖的 3' 碳上的羟基发生反应。反应脱去一分子水，因此属于缩合反应。形成的共价键就是磷酸二酯键，重复出现的糖-磷酸单元链构成了核酸的骨架。
2. Pentose Sugars: Deoxyribose and Ribose | 五碳糖：脱氧核糖与核糖
The pentose sugars give each nucleic acid its name. Ribose has a hydroxyl group (-OH) on the 2' carbon, whereas deoxyribose has only a hydrogen atom at this position. The prefix deoxy means without oxygen, which refers to this missing oxygen atom on carbon 2'.
五碳糖决定了两类核酸的名称。核糖在 2' 碳上有一个羟基（-OH），而脱氧核糖在该位置只有一个氢原子。前缀 deoxy 意为脱氧，指的就是 2' 碳上缺少了这个氧原子。
This small structural difference has major chemical consequences. The absence of the 2' -OH group makes DNA more resistant to hydrolysis and therefore more stable, which suits its role as a long-term store of genetic information. RNA, with its reactive 2' -OH group, is more easily broken down, allowing cells to control RNA levels rapidly.
这个微小的结构差异具有重要的化学意义。缺少 2'-OH 基团使 DNA 更耐水解，因而更稳定，这适合它长期储存遗传信息的功能。RNA 带有活泼的 2'-OH 基团，更容易被分解，使细胞能够快速调控 RNA 的水平。
3. Nitrogenous Bases: Purines and Pyrimidines | 含氮碱基：嘌呤与嘧啶
DNA and RNA both use four main nitrogenous bases, divided into two structural families. Purines are larger double-ring molecules: adenine (A) and guanine (G). Pyrimidines are smaller single-ring molecules: cytosine (C), thymine (T) and uracil (U). The base is linked to the 1' carbon of the sugar by a glycosidic bond.
DNA 和 RNA 都使用四种主要含氮碱基，分为两个结构家族。嘌呤是较大的双环分子：腺嘌呤（A）和鸟嘌呤（G）。嘧啶是较小的单环分子：胞嘧啶（C）、胸腺嘧啶（T）和尿嘧啶（U）。碱基通过糖苷键连接在糖的 1' 碳上。
DNA contains the bases A, T, C and G, whereas RNA contains A, U, C and G. Thymine is replaced by uracil in RNA. Uracil differs from thymine by lacking a methyl group on the pyrimidine ring. This is one of the key chemical distinctions between the two nucleic acids.
DNA 含有的碱基是 A、T、C、G，而 RNA 含有的碱基是 A、U、C、G。在 RNA 中，胸腺嘧啶被尿嘧啶取代。尿嘧啶与胸腺嘧啶的区别在于嘧啶环上缺少一个甲基。这是两种核酸之间关键的化学差异之一。
4. Phosphodiester Bonds and Backbone Direction | 磷酸二酯键与骨架方向
Adjacent nucleotides are joined by phosphodiester bonds. A single phosphodiester bond links the 5' phosphate of one nucleotide to the 3' hydroxyl of the neighbouring sugar. Repeating this linkage creates a sugar-phosphate backbone with a clear direction: one end has a free 5' phosphate and the other has a free 3' hydroxyl.
相邻核苷酸通过磷酸二酯键连接。一个磷酸二酯键将一个核苷酸的 5' 磷酸基团与相邻糖的 3' 羟基连接起来。这种连接不断重复，形成具有明确方向的糖-磷酸骨架：一端有游离的 5' 磷酸，另一端有游离的 3' 羟基。
5' phosphate + 3' OH → phosphodiester bond + H₂O
The sequence of bases along a nucleic acid is conventionally written from the 5' end to the 3' end. This directionality is essential in both DNA replication and transcription, because polymerases can only add new nucleotides to the 3' end of a growing chain.
核酸中的碱基序列通常按 5' 端到 3' 端的方向书写。这种方向性在 DNA 复制和转录中至关重要，因为聚合酶只能在生长链的 3' 端添加新的核苷酸。
5. DNA Double Helix: Antiparallel Strands | DNA 双螺旋：反向平行链
In DNA, two polynucleotide strands wind around each other to form a double helix. The two strands run in opposite directions, a property described as antiparallel: one strand runs 5' to 3', while its partner runs 3' to 5'. The helix is right-handed and has a constant diameter of about 2 nm.
在 DNA 中，两条多核苷酸链相互缠绕形成双螺旋。两条链方向相反，这一特性称为反向平行：一条链从 5' 到 3' 延伸，另一条链从 3' 到 5' 延伸。双螺旋为右手螺旋，直径恒定为约 2 纳米。
The sugar-phosphate backbones lie on the outside of the helix, while the nitrogenous bases project inwards. This arrangement protects the bases from chemical damage and allows the two strands to be held together by hydrogen bonds between the inward-facing bases. The twisting of the backbone also creates major and minor grooves on the surface of the molecule.
糖-磷酸骨架位于螺旋外侧，含氮碱基伸向内侧。这种排列保护碱基免受化学损伤，并使两条链能够通过朝内的碱基之间的氢键连接在一起。骨架的扭转还在分子表面形成大沟和小沟。
6. Complementary Base Pairing and Hydrogen Bonds | 互补碱基配对与氢键
The two DNA strands are held together by hydrogen bonds between complementary base pairs. Adenine pairs with thymine by two hydrogen bonds, and guanine pairs with cytosine by three hydrogen bonds. This complementarity means that the sequence of one strand determines the sequence of the other.
两条 DNA 链由互补碱基对之间的氢键连接。腺嘌呤与胸腺嘧啶通过两个氢键配对，鸟嘌呤与胞嘧啶通过三个氢键配对。这种互补性意味着一条链的序列决定了另一条链的序列。
A = T (2 hydrogen bonds) C ≡ G (3 hydrogen bonds)
The pairing rule also explains the constant diameter of the double helix: a purine always pairs with a pyrimidine, so each base pair has roughly the same width. Erwin Chargaff's data supported this model by showing that in any DNA sample the amount of adenine equals thymine, and the amount of guanine equals cytosine.
碱基配对规则还解释了双螺旋直径恒定的原因：嘌呤总是与嘧啶配对，因此每一对碱基的宽度大致相同。Erwin Chargaff 的数据支持了这一模型，他证明在任何 DNA 样品中腺嘌呤的数量等于胸腺嘧啶，鸟嘌呤的数量等于胞嘧啶。
7. RNA Structure: Usually Single-Stranded | RNA 结构：通常为单链
Unlike DNA, RNA is usually single-stranded. A single RNA molecule consists of one polynucleotide chain, although it can be much shorter than a DNA molecule. The sugar-phosphate backbone still has 5' to 3' directionality, but there is no complementary strand running alongside the entire length.
与 DNA 不同，RNA 通常是单链的。一个 RNA 分子由一条多核苷酸链组成，但通常比 DNA 分子短得多。糖-磷酸骨架仍然具有 5' 到 3' 的方向性，但整条链旁边没有与之互补的另一条链。
However, the RNA chain can fold back on itself, allowing complementary regions to pair and form short double-stranded sections such as hairpin loops. These internal pairings give each RNA molecule a distinctive three-dimensional shape that is closely linked to its function. Some viruses also use double-stranded RNA as their genetic material, but this is less common in cells.
然而，RNA 链可以自身回折，使互补区域配对并形成短的双链区段，例如发夹环。这些内部配对使每个 RNA 分子具有独特的三维形状，与其功能密切相关。一些病毒也使用双链 RNA 作为遗传物质，但这种情况在细胞中较少见。
8. Types of RNA: mRNA, tRNA and rRNA | RNA 的类型：mRNA、tRNA 和 rRNA
Cells contain three major types of RNA, each with a structure suited to its role. Messenger RNA (mRNA) is a linear copy of a gene that carries the genetic code from DNA in the nucleus to ribosomes in the cytoplasm. Its sequence of codons determines the amino acid sequence of a polypeptide.
细胞含有三种主要类型的 RNA，各自的结构与功能相适应。信使 RNA（mRNA）是基因的线性拷贝，将遗传密码从细胞核中的 DNA 携带到细胞质中的核糖体。它的密码子序列决定多肽的氨基酸序列。
Transfer RNA (tRNA) is a small folded molecule, often drawn as a cloverleaf shape. It has an anticodon of three bases at one end that recognises a complementary codon on mRNA, and an amino acid attachment site at the other end. Ribosomal RNA (rRNA) combines with proteins to form ribosomes, which are the molecular machines of translation. The folded structure of rRNA provides catalytic and structural support for protein synthesis.
转运 RNA（tRNA）是一种小型折叠分子，常画成三叶草形。它的一端有一个由三个碱基组成的反密码子，用于识别 mRNA 上的互补密码子；另一端是氨基酸结合位点。核糖体 RNA（rRNA）与
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