26.2 Viruses replicate only in host cells

Each virus has a host range 寄主範圍, a limited number of host cells that it can infect

病毒藉由 lock and key(專一性鎖鑰)，將表面上的蛋白質和在宿主細胞表面上的特定受器分子結合

General Features of Viral Replicative Cycles
The virus makes use of host
 * enzymes
 * ribosomes
 * tRNAs
 * amino acids
 * ATP


 * and other molecules

Replicative Cycles of Phages
Phages have two reproductive mechanisms:

the lytic cycle 溶菌週期
A phage that reproduces only by the lytic cycle is called a virulent phage 毒性噬菌體

Bacteria have defenses against phages, including restriction enzymes 限制酶 that recognize and cut up certain phage DNA

lysogenic cycle 潛溶菌週期
This integrated viral DNA is known as a prophage 原噬菌體

Every time the host divides, it copies the phage DNA and passes the copies to daughter cells

An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode

Phages that use both the lytic and lysogenic cycles are called temperate phages 溫和性噬菌體 viruses are obligate intracellular parasites 病毒是專性細胞內寄生

Reproductive Cycles of Animal Viruses
i. DNA or RNA?

ii.Single-stranded or double-stranded?

以下斜體有封套*

(a).雙股 DNA

 * Adenovirus (腺病毒)
 * Papillomavirus (乳頭瘤病毒)
 * Polyomavirus (多瘤病毒)
 * Herpesvirus (皰疹病毒)*
 * Poxvirus (痘病毒)*

(b).單股 DNA
(c).雙股 RNA
 * Parvovirus (多泡病毒)
 * Reovirus (呼腸孤病毒)

(d)作為 mRNA 的單股 RNA

 * Picornavirus (微小核糖核酸病毒)
 * Coronavirus (冠狀病毒)*
 * Flavivirus (黃病毒)*
 * Togavirus (批蓋病毒)*

(e).作為 mRNA 模板的單股 RNA

 * Fliovirus (絲狀病毒)*
 * Orthomyxovirus (正黏液病毒)*
 * Paramyxovirus (副黏液病毒)*
 * Rhabdovirus (彈狀病毒)*

(f). 作為 DNA 合成版的單股 DNA

 * Retrovirus (反轉錄病毒)*

Viral Envelopes
Other viral membranes form from the host’s nuclear envelopeand are then replaced by an envelope made from Golgi apparatus membrane

RNA as Viral Genetic Material
The broadest variety of RNA genomes is found in viruses that infect animals

Retroviruses 反轉錄病毒 use reverse transcriptase to copy their RNA genome into DNA

HIV (human immunodeficiency virus 人類免疫缺陷病毒) is the retrovirus that causes AIDS (acquired immunodeficiency syndrome 後天天疫缺陷症候群)

The viral DNA that is integrated into the host genome is called a provirus 原病毒

Unlike a prophage 原噬菌體, a provirus remainsa permanent 永遠 resident of the host cell

The RNA molecules function both as
 * mRNA for synthesis of viral proteins
 * genomes for new virus particles released from the cell

Candidates for the source of viral genomes

 * plasmids 質體
 * circular DNA in bacteria and yeasts 細菌和酵母的環形DNA
 * transposons 轉座子
 * small mobile DNA segments

Plasmids, transposons, and viruses are all mobile genetic elements 移動性遺傳物質