A-Level CCEA Biology: Genetics Key Points | A-Level CCEA 生物：遗传学 考点精讲

📚 A-Level CCEA Biology: Genetics Key Points | A-Level CCEA 生物：遗传学 考点精讲

Genetics is the study of heredity and variation, examining how traits are passed from one generation to the next. This revision guide covers the core concepts tested in CCEA A-Level Biology, from molecular genetics to population genetics.

遗传学是研究遗传与变异的学科，探索性状如何代代相传。这份复习指南涵盖CCEA A-Level生物考试的核心概念，从分子遗传学到群体遗传学。

1. DNA Structure and Replication | DNA结构与复制

DNA (deoxyribonucleic acid) is a polymer of nucleotides, each consisting of deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, or guanine). The two polynucleotide strands run antiparallel (5′ to 3′ and 3′ to 5′) and are held together by hydrogen bonds between complementary base pairs: A pairs with T (two hydrogen bonds), C pairs with G (three hydrogen bonds).

DNA（脱氧核糖核酸）是由核苷酸组成的聚合物，每个核苷酸包含脱氧核糖、磷酸基团和含氮碱基（腺嘌呤、胸腺嘧啶、胞嘧啶或鸟嘌呤）。两条多核苷酸链反向平行（5’到3’和3’到5’），并通过互补碱基对之间的氢键连接：A与T配对（两个氢键），C与G配对（三个氢键）。

DNA replication is semiconservative, as demonstrated by Meselson and Stahl’s experiment. The enzyme DNA helicase unwinds the double helix, and DNA polymerase synthesises new complementary strands in the 5′ to 3′ direction, using the original strands as templates. On the lagging strand, replication occurs discontinuously, forming Okazaki fragments that are later joined by DNA ligase.

DNA复制是半保留的，这已由Meselson和Stahl的实验证实。DNA解旋酶解开双螺旋，DNA聚合酶以原始链为模板，沿5’到3’方向合成新的互补链。在后随链上，复制不连续进行，形成冈崎片段，随后由DNA连接酶连接。

2. The Genetic Code and Protein Synthesis | 遗传密码与蛋白质合成

The genetic code is a set of rules by which the sequence of nucleotides in DNA or mRNA determines the amino acid sequence of a protein. Each three-base codon specifies one amino acid; the code is degenerate (more than one codon can code for the same amino acid) and universal across most organisms. Start codon AUG codes for methionine, while stop codons (UAA, UAG, UGA) terminate translation.

遗传密码是一套规则，DNA或mRNA中的核苷酸序列决定蛋白质的氨基酸序列。每个三碱基密码子指定一种氨基酸；密码子具有简并性（多个密码子可编码同一种氨基酸），且在大多数生物中通用。起始密码子AUG编码甲硫氨酸，终止密码子（UAA、UAG、UGA）终止翻译。

Transcription occurs in the nucleus: RNA polymerase binds to the promoter region, unwinds the DNA, and synthesises a complementary mRNA strand using one DNA strand as template. After processing (capping, poly-A tail addition, splicing in eukaryotes), the mature mRNA moves to the ribosome. Translation involves the binding of tRNA anticodons to mRNA codons, with peptidyl transferase forming peptide bonds between amino acids to build a polypeptide chain.

转录发生在细胞核中：RNA聚合酶与启动子区域结合，解开DNA，并以一条DNA链为模板合成互补的mRNA链。经加工（加帽、加尾、真核生物中的剪接）后，成熟的mRNA移至核糖体。翻译过程中，tRNA反密码子与mRNA密码子结合，肽基转移酶在氨基酸之间形成肽键，构建多肽链。

3. Gene Mutations | 基因突变

A gene mutation is a change in the nucleotide sequence of DNA. Substitution mutations may be silent (no change in amino acid), missense (change to a different amino acid), or nonsense (introduction of a premature stop codon). Insertion or deletion of nucleotides can cause frameshift mutations, altering the entire reading frame downstream and often producing a non-functional protein.

基因突变是DNA核苷酸序列的改变。替换突变可能为沉默突变（氨基酸不变）、错义突变（变为不同氨基酸）或无义突变（提前出现终止密码子）。核苷酸的插入或缺失

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