DNA, RNA and Protein Synthesis | DNA、RNA 与蛋白质合成

📚 DNA, RNA and Protein Synthesis | DNA、RNA 与蛋白质合成

DNA stores genetic information, RNA transfers that information, and proteins carry out most cellular functions. In Cambridge A-Level Biology, these processes are studied through the central dogma of molecular biology: DNA is transcribed into RNA, which is translated into protein. Understanding the structure of nucleic acids and the mechanism of protein synthesis is essential for explaining how genes control the phenotype of an organism.

DNA 储存遗传信息，RNA 传递这些信息，而蛋白质执行细胞的大部分功能。在剑桥 A-Level 生物课程中，这些过程通过分子生物学的中心法则来学习：DNA 转录为 RNA，RNA 翻译为蛋白质。理解核酸的结构以及蛋白质合成的机制，对于解释基因如何控制生物体的表型至关重要。

DNA → RNA → Protein

1. Nucleotide Structure and DNA Composition | 核苷酸结构与 DNA 组成

DNA is a polymer made of repeating units called nucleotides. Each nucleotide contains three components: a phosphate group, a deoxyribose sugar, and one of four nitrogenous bases. The deoxyribose sugar has five carbon atoms, numbered 1′ to 5′, and the base is attached to the 1′ carbon while the phosphate group is attached to the 5′ carbon.

DNA 是由称为核苷酸的重复单元组成的多聚体。每个核苷酸含有三个组分：一个磷酸基团、一个脱氧核糖和四种含氮碱基之一。脱氧核糖有五个碳原子，编号为 1′ 到 5’，碱基连接在 1′ 碳上，而磷酸基团连接在 5′ 碳上。

The four bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). Adenine and guanine are purines with a double-ring structure, while cytosine and thymine are pyrimidines with a single-ring structure. This distinction is important because complementary base pairing always involves one purine and one pyrimidine.

DNA 中的四种碱基是腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）和鸟嘌呤（G）。腺嘌呤和鸟嘌呤是双环结构的嘌呤，而胞嘧啶和胸腺嘧啶是单环结构的嘧啶。这一区别很重要，因为互补碱基配对总是涉及一个嘌呤和一个嘧啶。

Nucleotides are joined by phosphodiester bonds between the phosphate group of one nucleotide and the 3′ carbon of the deoxyribose of the next nucleotide. This creates a sugar-phosphate backbone with a free 5′ end and a free 3′ end, giving each DNA strand directionality.

核苷酸通过一个核苷酸的磷酸基团与下一个核苷酸脱氧核糖的 3′ 碳之间的磷酸二酯键连接。这样形成了具有游离 5′ 端和游离 3′ 端的糖-磷酸骨架，使每条 DNA 链具有方向性。

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 5′ to 3′ and the other runs 3′ to 5′. The sugar-phosphate backbones are on the outside of the helix, protecting the bases inside.

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’，另一条链从 3′ 到 5’。糖-磷酸骨架位于螺旋外侧，保护内部的碱基。

The nitrogenous bases are located on the inside of the helix, where complementary base pairing occurs. Adenine always pairs with thymine via two hydrogen bonds, and cytosine always pairs with guanine via three hydrogen bonds. The three hydrogen bonds between C and G make GC-rich regions more stable than AT-rich regions.

含氮碱基位于螺旋内部，在那里发生互补碱基配对。腺嘌呤总是通过两个氢键与胸腺嘧啶配对，胞嘧啶总是通过三个氢键与鸟嘌呤配对。C 和 G 之间的三个氢键使 GC 富集区域比 AT 富集区域更稳定。

This specificity maintains a constant width of the DNA molecule and enables accurate replication and transcription. Because one strand is complementary to the other, the sequence of bases along one strand determines the sequence along the opposite strand.

这种特异性保持了 DNA 分子的恒定宽度，并能够准确复制和转录。由于一条链与另一条链互补，因此一条链上的碱基序列决定了互补链上的序列。

3. Types and Functions of RNA | RNA 的类型与功能

RNA is a single-stranded nucleic acid that contains ribose sugar and the bases adenine, uracil, cytosine, and guanine. Unlike DNA, RNA contains uracil instead of thymine. It is synthesised from a DNA template and is essential for decoding genetic information into proteins.

RNA 是一种单链核酸，含有核糖以及腺嘌呤、尿嘧啶、胞嘧啶和鸟嘌呤碱基。与 DNA 不同，RNA 含有尿嘧啶而不是胸腺嘧啶。它以 DNA 为模板合成，对于将遗传信息解码为蛋白质至关重要。

There are three main types of RNA involved in protein synthesis. Messenger RNA (mRNA) carries the genetic code from DNA in the nucleus to the ribosome in the cytoplasm. Transfer RNA (tRNA) brings specific amino acids to the ribosome and matches them to the correct codons. Ribosomal RNA (rRNA) combines with proteins to form ribosomes and catalyses peptide bond formation.

参与蛋白质合成的 RNA 主要有三种。信使 RNA（mRNA）将遗传密码从细胞核中的 DNA 传递到细胞质中的核糖体。转运 RNA（tRNA）将特定的氨基酸带到核糖体，并使它们与正确的密码子匹配。核糖体 RNA（rRNA）与蛋白质结合形成核糖体，并催化肽键形成。

mRNA is produced in the nucleus during transcription, while tRNA and rRNA are also transcribed from DNA but

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