基因表达与调控

DNA carries genetic information as a double helix 双螺旋 of two strands. Its nucleotides 核苷酸 pair by rule – A with T, G with C (complementary base pairing 互补配对) – so one strand specifies the other. The strands run antiparallel 反平行. RNA is single-stranded, uses uracil (U) instead of thymine, and has ribose sugar.

How the DNA is packaged differs between the domains, and the CED asks for both.

• Prokaryotic organisms typically have a single circular chromosome 环状染色体 in the cytoplasm, often with extra small circles called plasmids.
• Eukaryotic organisms typically have multiple linear chromosomes 线性染色体, and each is condensed by wrapping around proteins called histones 组蛋白. That packing is what fits two metres of DNA into a nucleus a few micrometres across — and because a tightly wound region is hard for the transcription machinery to reach, the packing is itself a level of gene regulation.

Before a cell divides, DNA is copied semiconservatively 半保留复制: the helix unwinds and each old strand templates a new one, so each daughter helix has one old and one new strand. DNA polymerase 聚合酶 adds nucleotides following base-pairing rules, building the new strand and proofreading as it goes.

The rule that explains the fork. The two strands of DNA are antiparallel: one runs $5'\rightarrow3'$, the other $3'\rightarrow5'$. DNA polymerase can add nucleotides only to the 3′ end of a growing strand, so every new strand grows $5'\rightarrow3'$. Each enzyme at the fork has one job:

• helicase 解旋酶 unwinds the double helix and separates the strands at the fork.
• topoisomerase 拓扑异构酶 works ahead of the fork, relieving the twisting that unwinding causes.
• primase 引物酶 lays down a short RNA primer RNA引物, because DNA polymerase cannot start a new strand on its own; it can only extend one.
• DNA polymerase adds nucleotides to the 3′ end, and later replaces the RNA primers with DNA.
• ligase 连接酶 joins the separate pieces of DNA into one strand.

The leading strand 前导链 grows towards the fork, in the same direction as the fork moves, so it is made in one piece. The lagging strand 后随链 must grow away from the fork, so it is made in short Okazaki fragments 冈崎片段. Each needs its own primer, and ligase joins them.

Transcription 转录 copies a gene's DNA into messenger RNA 信使核糖核酸 (mRNA). RNA polymerase reads the template strand and builds a complementary RNA. In eukaryotes the mRNA is then processed: a cap and tail are added, and introns 内含子 are removed by splicing, leaving the exons 外显子 in the mature transcript. Some retained exon regions are untranslated, so exon does not mean only protein-coding sequence.

The 5′ cap added during processing is a modified guanine nucleotide — a GTP cap — and its job is recognition: it helps the ribosome recognise and bind the mRNA, as well as protecting the transcript from degradation as it leaves the nucleus.

Translation 翻译 builds a protein from the mRNA at the ribosome 核糖体. The mRNA is read in three-base codons 密码子, each specifying an amino acid or a stop signal (the genetic code). Transfer RNA 转运核糖核酸 brings the matching amino acid, and the ribosome links them into a polypeptide until a stop codon ends it. This is the "central dogma": DNA → RNA → protein.

Inside the ribosome. The ribosome has three sites for tRNA: A (where a new tRNA arrives), P (holding the tRNA with the growing chain) and E (exit).

1. Initiation: the small subunit binds the mRNA and finds the start codon 起始密码子 AUG. A tRNA carrying methionine pairs with it, and the large subunit joins.
2. Elongation: a tRNA whose anticodon 反密码子 is complementary to the next codon enters the A site. A peptide bond 肽键 joins the chain in the P site to the new amino acid. The ribosome then moves one codon towards the 3′ end, and the empty tRNA leaves from the E site.
3. Termination: a stop codon 终止密码子 (UAA, UAG or UGA) reaches the A site. No tRNA matches it, so a release factor frees the finished polypeptide.

Worked example (DNA to protein). The template strand of a gene reads $3'$-TAC CGA AAG ATT-$5'$.

1. The mRNA is complementary to the template strand, with U instead of T: $5'$-AUG GCU UUC UAA-$3'$.
2. Read the codons from the start codon, using the genetic code table: AUG = Met, GCU = Ala, UUC = Phe, UAA = stop.
3. The polypeptide is Met–Ala–Phe. The stop codon adds no amino acid.

The tRNA anticodons are $3'$-UAC-$5'$, $3'$-CGA-$5'$ and $3'$-AAG-$5'$. They have the same sequence as the template DNA, but with U for T.

Common mistakes.

• Saying DNA polymerase "reads $5'\rightarrow3'$". It builds the new strand $5'\rightarrow3'$, by reading the template $3'\rightarrow5'$.
• Saying the lagging strand is made "more slowly" or "backwards". It is made in the same $5'\rightarrow3'$ direction, but in pieces, because it runs away from the fork.
• Using T in mRNA, or reading the genetic code table with anticodons. The table is written in mRNA codons.
• Giving the stop codon an amino acid.
• Saying a ribosome makes the amino acids. It joins amino acids brought to it by tRNA.

Cells control which genes are expressed and when. In prokaryotes, operons 操纵子 switch groups of genes on or off. In eukaryotes, regulation happens at many levels – which genes are transcribed (transcription factors, promoters, enhancers), RNA processing, and after translation. This lets a cell respond to its environment without changing its DNA.

Every cell in a body has the same DNA, yet cells differ because they express different genes – differential gene expression 差异表达. This is how one fertilized egg produces many specialized cell types (muscle, nerve, skin); signals during development turn specific genes on and off.

A mutation 突变 is a change in the DNA sequence. Point mutations change one base (silent, missense, or nonsense); insertions/deletions can cause a frameshift 移码 that garbles everything downstream. Mutations in gametes are heritable; they may be harmful, neutral, or beneficial – and beneficial ones supply the variation for natural selection.

Whole-chromosome errors change the phenotype too. Mutation is not only a change of base: errors in mitosis or meiosis can result in changes in phenotype. If chromosomes fail to separate — nondisjunction 不分离 — a gamete receives an extra copy or none, and the resulting zygote has an abnormal chromosome number (aneuploidy 非整倍性). Trisomy 21 is the familiar example. Because a whole chromosome carries hundreds of genes, the phenotypic effect is far broader than a point mutation's.

Biotechnology 生物技术 tools manipulate genetic material: PCR 聚合酶链反应 copies DNA, gel electrophoresis 凝胶电泳 separates DNA fragments by size, restriction enzymes and cloning move genes between organisms, and CRISPR edits sequences. These techniques enable genetic testing, engineered organisms, and medical treatments.

Worked example. A template DNA strand 3'-TAC GGA TTC-5' is transcribed into mRNA 5'-AUG CCU AAG-3'. A ribosome reads three codons: AUG = Met (start), CCU = Pro, AAG = Lys — coding for Met–Pro–Lys. Changing the third base of a codon often gives the same amino acid, because the genetic code is degenerate, which softens the effect of many mutations.

Reading DNA identifies individuals. Because the number of repeats at certain non-coding loci varies from person to person, analysis of DNA can be used for forensic identification: amplify those regions by PCR, separate the fragments by gel electrophoresis, and compare the pattern of band positions with a reference sample. A match is a statement about probability rather than certainty, which is why several loci are tested rather than one.

• Know the central dogma DNA → RNA → protein: transcription makes mRNA, translation reads it in three-base codons at the ribosome.
• Remember RNA is single-stranded and uses U instead of T; replication is semiconservative.
• Every cell has the same DNA — cells differ because they express different genes (differential gene expression).
• A mutation is a change in the DNA sequence and may be harmful, neutral, or beneficial (the raw material for selection).
• Link biotech tools to their jobs: PCR copies DNA; gel electrophoresis separates fragments by size.