Episome (外质体)

在中文里，“episome”可以翻译为“外质体”。外质体是一种遗传元素，它可以作为独立的环状DNA分子存在，也可以整合到宿主的染色体中。外质体与宿主的染色体DNA分开复制，并在细胞分裂过程中在细胞之间传递。无论是原核生物还是真核生物，都可以找到外质体。

在细菌中，外质体通常是质粒，质粒是一种小的环状DNA分子，携带对宿主有益的基因，如抗生素抗性或产生毒素的能力。细菌质粒可以通过称为“共轭”的过程在细菌细胞之间传递，从而促进有益基因在细菌种群中的传播。

在真核细胞中，例如人类细胞，外质体可以是病毒基因组，如Epstein-Barr病毒（EBV）基因组。感染后，EBV使其线性DNA环化并作为外质体建立潜伏在宿主细胞的细胞核中。在潜伏期间，病毒外质体在宿主细胞中保持存在，但不会立即造成损害，病毒可以在以后重新激活，可能导致疾病。作为外质体，病毒基因组可以避免被宿主免疫系统检测和清除，同时保持在重新激活后复制并产生新的病毒颗粒的能力。

An episome is a genetic element that can exist either independently as a circular DNA molecule or integrate into a host's chromosome. Episomes replicate separately from the host's chromosomal DNA and can be transferred between cells during cell division. They are found in both prokaryotic and eukaryotic organisms.

In bacteria, episomes are often plasmids, which are small, circular DNA molecules that carry genes beneficial to the host, such as antibiotic resistance or the ability to produce toxins. Bacterial plasmids can be transferred between bacterial cells through a process called conjugation, which promotes the spread of beneficial genes within a bacterial population.

In eukaryotic cells, such as human cells, episomes can be viral genomes, like the Epstein-Barr virus (EBV) genome. Upon infection, EBV circularizes its linear DNA and establishes latency as an episome in the host cell's nucleus. During latency, the viral episome persists in the host cell without causing immediate damage, and the virus can reactivate at a later time, potentially causing disease. By existing as an episome, the viral genome can avoid detection and clearance by the host's immune system while maintaining the ability to replicate and produce new virus particles upon reactivation.

Human gammaherpesvirus 4 (HHV-4) is another name for Epstein-Barr virus (EBV). EBV is a member of the Herpesviridae family and belongs to the Gammaherpesvirinae subfamily. It is a double-stranded DNA virus that infects humans and is associated with various diseases, including infectious mononucleosis, certain types of lymphomas, and nasopharyngeal carcinoma.

EBV establishes a latent infection in the host's B cells, where it can persist as an episome. The virus can reactivate at a later time, potentially leading to disease.

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