📚 The Structure and Life Cycle of Viruses | 病毒的结构与生活史
Viruses are the smallest infectious agents, typically ranging from 20 to 300 nm in diameter. They are obligate intracellular parasites, meaning they can only replicate inside living cells. Unlike bacteria or eukaryotic cells, viruses lack cellular machinery and are considered non-living particles outside a host cell.
病毒是最小的感染性因子，直径通常为20至300纳米。它们是专性细胞内寄生物，意味着只能在活细胞内复制。与细菌或真核细胞不同，病毒缺乏细胞机器，在宿主细胞之外被视为非生命颗粒。
1. The Structural Components of Viruses | 病毒的结构组成
All viruses consist of two fundamental components: a nucleic acid core and a protein capsid. The nucleic acid may be DNA or RNA, single-stranded or double-stranded, depending on the virus type. The capsid is composed of repeating protein subunits called capsomeres, which self-assemble to form a protective shell around the genome.
所有病毒由两个基本组分构成：核酸核心和蛋白质衣壳。核酸可以是DNA或RNA，单链或双链，取决于病毒类型。衣壳由称为衣壳粒的重复蛋白质亚基组成，这些亚基自组装形成围绕基因组的保护壳。
Some viruses, particularly those infecting animal cells, possess an additional lipid envelope. This envelope is derived from the host cell membrane and contains viral glycoprotein spikes, which are essential for attachment to host cells. The envelope also helps the virus evade the host immune system.
一些病毒，特别是感染动物细胞的病毒，具有额外的脂质包膜。该包膜来源于宿主细胞膜，并含有病毒糖蛋白刺突，这对附着于宿主细胞至关重要。包膜还有助于病毒逃避宿主免疫系统。
The complete, fully assembled infectious virus particle is known as a virion. A typical bacteriophage such as T-even phages has a complex structure including a head (containing DNA), a tail, and tail fibres for attachment.
完整组装且具有感染性的病毒颗粒称为病毒粒子。典型的噬菌体如T偶数噬菌体具有复杂结构，包括头部（含DNA）、尾部和用于附着的尾丝。
Virion structure = Nucleic acid + Capsid (± Envelope)
病毒粒子结构 = 核酸 + 衣壳（± 包膜）
2. Classification of Viruses | 病毒的分类
Viruses can be classified based on several criteria, including their genetic material, capsid symmetry, presence or absence of an envelope, and the type of host they infect. The Baltimore classification system groups viruses into seven classes based on their genome type and replication strategy.
病毒可以根据若干标准进行分类，包括其遗传物质、衣壳对称性、是否存在包膜以及它们感染的宿主类型。巴尔的摩分类系统根据病毒基因组类型和复制策略将其分为七类。
- DNA viruses (e.g., adenovirus, herpesvirus) — double-stranded or single-stranded DNA
- RNA viruses (e.g., influenza virus, poliovirus) — double-stranded or single-stranded RNA
- Retroviruses (e.g., HIV) — RNA genome with reverse transcriptase
- DNA病毒（如腺病毒、疱疹病毒）— 双链或单链DNA
- RNA病毒（如流感病毒、脊髓灰质炎病毒）— 双链或单链RNA
- 逆转录病毒（如HIV）— 具有逆转录酶的RNA基因组
Structural classification reflects capsid symmetry: icosahedral (e.g., adenovirus), helical (e.g., tobacco mosaic virus), and complex (e.g., bacteriophages).
结构分类反映衣壳对称性：二十面体（如腺病毒）、螺旋形（如烟草花叶病毒）和复合型（如噬菌体）。
3. The Lytic Cycle in Bacteriophages | 噬菌体的裂解周期
The lytic cycle is a replicative pathway that results in the destruction of the host cell. It can be described in five sequential stages: attachment, entry, replication and synthesis, assembly, and release.
裂解周期是导致宿主细胞破坏的复制途径。它可以分为五个连续阶段：附着、进入、复制与合成、组装和释放。
Stage 1 — Attachment: The phage tail fibres recognise and bind to specific receptor proteins on the bacterial cell wall. This specificity determines the host range of the virus.
阶段一 — 附着：噬菌体尾丝识别并结合细菌细胞壁上的特异性受体蛋白。这种特异性决定了病毒的宿主范围。
Stage 2 — Entry: The tail sheath contracts, and the viral DNA is injected through the cell wall and membrane into the cytoplasm. The empty capsid remains outside the bacterium.
阶段二 — 进入：尾鞘收缩，病毒DNA通过细胞壁和细胞膜注入细胞质。空衣壳留在细菌外部。
Stage 3 — Replication and Synthesis: The viral DNA takes over the host’s machinery. Host ribosomes synthesise viral proteins, and the viral genome directs the production of new nucleic acid copies. In many cases, the host DNA is degraded.
阶段三 — 复制与合成：病毒DNA接管宿主的机器。宿主核糖体合成病毒蛋白，病毒基因组指导新核酸拷贝的产生。在许多情况下，宿主DNA被降解。
Stage 4 — Assembly: Newly synthesised capsid proteins self-assemble around the replicated viral genomes to form complete virions.
阶段四 — 组装：新合成的衣壳蛋白围绕复制的病毒基因组自组装形成完整的病毒粒子。
Stage 5 — Release: The bacterial cell lyses (bursts) due to the accumulation of virions, releasing them to infect new host cells. Lysozyme, an enzyme, may be used to weaken the cell wall during this process.
阶段五 — 释放：由于病毒粒子的积累，细菌细胞裂解（破裂），释放它们感染新的宿主细胞。在此过程中可能使用溶菌酶削弱细胞壁。
4. The Lysogenic Cycle | 溶原周期
In contrast to the lytic cycle, the lysogenic cycle is a dormant state in which the viral genome integrates into the host chromosome without producing new virions. The integrated viral DNA is termed a prophage.
与裂解周期相反，溶原周期是病毒基因组整合到宿主染色体中而不产生新病毒粒子的休眠状态。整合的病毒DNA称为前噬菌体。
During the lysogenic cycle, the prophage is replicated along with the host chromosome during binary fission. This means all daughter cells inherit the viral genome. Under certain conditions, such as UV radiation or chemical stress, the prophage can excise itself from the host chromosome and enter the lytic cycle.
在溶原周期中，前噬菌体在二分裂期间与宿主染色体一起复制。这意味着所有子细胞都继承病毒基因组。在某些条件下，如紫外线辐射或化学胁迫，前噬菌体可以从宿主染色体中切除自身并进入裂解周期。
Prophage → induction (UV, stress) → Lytic cycle
前噬菌体 → 诱导（紫外线、压力）→ 裂解周期
A well-known example is the lambda phage (λ phage) infecting Escherichia coli, which can switch between both cycles.
一个著名的例子是感染大肠杆菌的λ噬菌体，它可以在两种周期之间切换。
5. Replication of Animal Viruses | 动物病毒的复制
Animal virus replication differs from bacteriophage replication in several important ways. First, entry into animal cells typically occurs via endocytosis or membrane fusion rather than DNA injection. Second, animal viruses may be enveloped, and release often occurs by budding from the host cell membrane.
动物病毒复制在几个重要方面与噬菌体复制不同。首先，进入动物细胞通常通过内吞作用或膜融合发生，而不是DNA注射。其次，动物病毒可能带有包膜，释放通常通过从宿主细胞膜出芽发生。
For non-enveloped animal viruses, the capsid binds to receptors on the host cell surface, triggering endocytosis. The viral genome is then released into the cytoplasm. Enveloped viruses may fuse their envelope directly with the host membrane, releasing the capsid into the cytoplasm.
对于无包膜动物病毒，衣壳与宿主细胞表面的受体结合，触发内吞作用。病毒基因组随后被释放到细胞质中。包膜病毒可以将其包膜与宿主膜直接融合，将衣壳释放到细胞质中。
DNA viruses generally replicate in the nucleus using host DNA polymerase, while RNA viruses replicate in the cytoplasm using viral RNA polymerase. Translation of viral mRNA produces viral proteins, and new virions are assembled and released either by cell lysis or by budding, which does not necessarily kill the host cell immediately.
DNA病毒通常在细胞核中使用宿主DNA聚合酶复制，而RNA病毒在细胞质中使用病毒RNA聚合酶复制。病毒mRNA的翻译产生病毒蛋白，新病毒粒子组装后通过细胞裂解或出芽释放，出芽不一定会立即杀死宿主细胞。
6. HIV: A Retrovirus Case Study | HIV：逆转录病毒案例研究
Human Immunodeficiency Virus (HIV) is an enveloped retrovirus with an RNA genome. Its structure includes a conical capsid, two copies of single-stranded RNA, the enzyme reverse transcriptase, and an envelope with
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