📚 RNA Structure and Function | RNA的结构与功能
Ribonucleic acid (RNA) is a nucleic acid found in all living cells. It carries genetic information, helps build proteins, and controls how genes are expressed. In IGCSE Biology, you need to know the structure of RNA and how it works with DNA during protein synthesis.
核糖核酸（RNA）是存在于所有活细胞中的一种核酸。它携带遗传信息，帮助制造蛋白质，并控制基因的表达方式。在IGCSE生物学中，你需要了解RNA的结构以及它在蛋白质合成过程中如何与DNA配合工作。
1. The RNA Nucleotide | RNA核苷酸的结构
RNA is a polymer made of many smaller units called nucleotides. Each RNA nucleotide consists of three parts: a ribose sugar (a five-carbon sugar), a phosphate group, and one of four nitrogenous bases: adenine (A), guanine (G), cytosine (C), and uracil (U).
RNA是由许多叫做核苷酸的小单元组成的聚合物。每个RNA核苷酸由三个部分构成：一个核糖（五碳糖）、一个磷酸基团，以及四种含氮碱基之一：腺嘌呤（A）、鸟嘌呤（G）、胞嘧啶（C）和尿嘧啶（U）。
RNA nucleotide = ribose sugar + phosphate group + nitrogenous base (A, G, C or U)
2. Key Differences Between RNA and DNA | RNA与DNA的主要区别
RNA differs from DNA in three important ways. First, RNA contains the sugar ribose, while DNA contains deoxyribose. Second, RNA uses uracil instead of thymine, so base pairing is A-U and C-G. Third, RNA is usually single-stranded, while DNA is double-stranded and forms a double helix.
RNA与DNA在三个方面有重要区别。第一，RNA含有核糖，而DNA含有脱氧核糖。第二，RNA使用尿嘧啶代替胸腺嘧啶，因此碱基配对为A-U和C-G。第三，RNA通常是单链，而DNA是双链并形成双螺旋结构。
| Feature | DNA | RNA |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, G, C | A, U, G, C |
| Strand | Double-stranded | Single-stranded |
| Length | Very long | Relatively short |
| Location | Mainly nucleus | Nucleus and cytoplasm |
3. Messenger RNA (mRNA) | 信使RNA（mRNA）
mRNA carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm. It is made during transcription and contains codons, which are groups of three bases. Each codon codes for a specific amino acid. For example, the codon AUG codes for methionine and also acts as the start signal.
mRNA将遗传密码从细胞核中的DNA携带到细胞质中的核糖体。它是在转录过程中合成的，包含密码子，密码子由三个碱基组成一组。每个密码子编码一种特定的氨基酸。例如，密码子AUG编码甲硫氨酸，同时也作为起始信号。
Codons are read in groups of three bases, e.g. AUG, GGC, UAC
4. Transfer RNA (tRNA) | 转运RNA（tRNA）
tRNA is a small RNA molecule that carries amino acids to the ribosome during translation. It has a cloverleaf shape with two key regions: an amino acid attachment site at one end, and an anticodon at the other end. The anticodon pairs with a complementary codon on mRNA, ensuring the correct amino acid is added to the growing protein chain.
tRNA是一种小RNA分子，在翻译过程中将氨基酸运送到核糖体。它呈三叶草形状，有两个关键区域：一端是氨基酸连接位点，另一端是反密码子。反密码子与mRNA上的互补密码子配对，确保正确的氨基酸被添加到正在生长的蛋白质链上。
tRNA anticodon UAC binds to mRNA codon AUG
5. Ribosomal RNA (rRNA) | 核糖体RNA（rRNA）
rRNA is a component of ribosomes, the organelles where proteins are assembled. Ribosomes are made of both rRNA and proteins. rRNA helps hold the ribosome together and ensures that tRNA molecules align correctly with mRNA during translation. It also has an enzymatic role in forming peptide bonds between amino acids.
rRNA是核糖体的组成成分，核糖体是组装蛋白质的细胞器。核糖体由rRNA和蛋白质共同构成。rRNA帮助维持核糖体的结构，并确保翻译过程中tRNA分子与mRNA正确对齐。它还在氨基酸之间形成肽键的过程中起酶催化作用。
6. Transcription: DNA to RNA | 转录：从DNA到RNA
Transcription occurs in the nucleus. The enzyme RNA polymerase binds to a region of DNA and separates the two strands. It then uses one strand, called the template strand, to build a complementary mRNA molecule. Where the DNA base is adenine, the RNA base added is uracil; where DNA has thymine, RNA adds adenine; cytosine adds guanine, and guanine adds cytosine.
转录发生在细胞核中。RNA聚合酶与DNA的某个区域结合并解开两条链。然后它以其中一条链（称为模板链）为模板，构建互补的mRNA分子。当DNA碱基是腺嘌呤时，加入的RNA碱基是尿嘧啶；当DNA碱基是胸腺嘧啶时，RNA加入腺嘌呤；胞嘧啶对应鸟嘌呤，鸟嘌呤对应胞嘧啶。
DNA A-T → RNA A-U and DNA C-G → RNA C-G
Once completed, the mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm.
一旦合成完成，mRNA通过核孔离开细胞核，并在细胞质中附着到核糖体上。
7. Translation: RNA to Protein | 翻译：从RNA到蛋白质
Translation is the process by which the sequence of codons on mRNA is used to build a protein. A ribosome attaches to the mRNA and reads the codons from the 5′ end to the 3′ end. Each codon attracts a tRNA molecule with a complementary anticodon. The tRNA carries its specific amino acid, and as successive tRNA molecules bind, the amino acids are joined by peptide bonds to form a polypeptide chain.
翻译是利用mRNA上的密码子序列来构建蛋白质的过程。核糖体附着在mRNA上，从5’端向3’端读取密码子。每个密码子吸引带有互补反密码子的tRNA分子。该tRNA携带其特定的氨基酸，随着后续tRNA分子依次结合，氨基酸通过肽键连接形成多肽链。
mRNA codon → tRNA anticodon → amino acid
Translation continues until a stop codon, such as UAA, UAG or UGA, is reached. The finished polypeptide then folds into a functional protein.
翻译一直持续到遇到终止密码子（如UAA、UAG或UGA）为止。最终形成的多肽随后折叠成有功能的蛋白质。
8. Comparing the Three Main Types of RNA | 三种主要RNA类型比较
Each type of RNA has a different role in protein synthesis, so their structures match their functions.
每种RNA在蛋白质合成中都有不同的作用，因此它们的结构与功能相适应。
| Type | Role | Where it works |
| mRNA | Carries genetic code from DNA to ribosome | Nucleus → cytoplasm |
| tRNA | Carries amino acids to ribosome | Cytoplasm |
| rRNA | Forms ribosome and catalyses peptide bonds | Ribosome in cytoplasm |
9. Other RNA Molecules | 其他RNA分子
Besides the three main types, cells contain other RNA molecules. MicroRNA (miRNA) and small interfering RNA (siRNA) regulate gene expression by binding to mRNA molecules and blocking translation or causing mRNA breakdown. These RNA molecules are important in controlling which proteins are produced and in defending against viruses.
除了三种主要类型外，细胞还含有其他RNA分子。微小RNA（miRNA）和小干扰RNA（siRNA）通过结合mRNA分子、阻止翻译或促使mRNA降解来调节基因表达。这些RNA分子在控制产生哪些蛋白质以及防御病毒方面非常重要。
10. Common Exam Points | 常见考点提醒
Students often lose marks on the following points, so remember them clearly:
学生经常在以下知识点上失分，请务必记清楚：
- RNA contains uracil, not thymine. The pairing is therefore A-U and C-G.
- RNA is single-stranded, not double-stranded like DNA.
- mRNA carries the code, tRNA carries the amino acid, and rRNA forms the ribosome.
- A codon is on mRNA, while an anticodon is on tRNA.
- Transcription happens in the nucleus, but translation happens in the cytoplasm at ribosomes.
- RNA含尿嘧啶，不含胸腺嘧啶，因此配对是A-U和C-G。
- RNA是单链，不像DNA那样是双链。
- mRNA携带密码，tRNA携带氨基酸，rRNA构成核糖体。
- 密码子在mRNA上，反密码子在tRNA上。
- 转录发生在细胞核中，而翻译发生在细胞质中的核糖体上。
11. Summary | 总结
RNA is a single-stranded nucleic acid made from ribose, phosphate and bases A, U, G and C. It exists in three main forms: mRNA carries the genetic message, tRNA delivers amino acids, and rRNA builds ribosomes. Together with DNA, RNA controls protein synthesis through the processes of transcription and translation.
RNA是由核糖、磷酸以及碱基A、U、G、C组成的单链核酸。它有三种主要形式：mRNA携带遗传信息，tRNA运送氨基酸，rRNA构成核糖体。RNA与DNA共同通过转录和翻译过程控制蛋白质的合成。
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