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Skip to 0 minutes and 14 seconds Herpes viruses have been known for long. It was the Greeks who coined the term “herpes”, meaning to crawl, in reference to the aspect of Herpes lesions. In human medicine, herpes are known mostly because they cause cold sores. Herpes are very large viruses, up to 200nm in diameter. Virus particles are pleomorphic, predominantly spherical,

Skip to 0 minutes and 45 seconds where it is possible to differentiate from the inside to the outside: an icosahedral capsid that contains the genome of the virus, an amorphous layer called the tegument, made of protein an envelope where a series of projections may be seen, similar to spikes, evenly distributed, which correspond to glycoproteins. The genome is a molecule of linear double-stranded DNA of up to 240-kilo base pairs, which contains terminal and internal repeated sequences of variable length. It comprises around 220 genes that encode proteins involved in the entry in and exit from the infected cell, the replication of the viral genome and the formation of the virion.

Skip to 1 minute and 32 seconds Similarly to most DNA viruses, the replication cycle of herpes virus takes place in the nucleus of the infected cell. The cycle begins with the interaction of the envelope glycoproteins with receptors and co-receptors on the plasma membrane. After the fusion of the envelope with the plasma membrane, the capsid and the tegument proteins are set free in the cytosol and are transported to the nucleus, using the cellular system of microtubules. There, the genetic material is injected through the nuclear pores, adopting a circular form in the interior of the nucleus. The expression of the genetic information contained in the genome takes place in a regulated cascade consisting of three phases called immediate, early and late expression.

Skip to 2 minutes and 24 seconds Transcription occurs always in the interior of the nucleus and the messenger RNAs go out to the cytoplasm to be translated by the cell ribosomes. In the late phase, the proteins that form the capsid and the viral glycoproteins that form the envelope are expressed. The proteins that will form the viral capsids penetrate in the nucleus through the nuclear pores and are partially assembled, while the glycoproteins of the envelope are inserted in the nuclear membrane. Replication of the genome of the virus occurs at the same time. The new genomes are packaged within the capsids. The viruses acquire a first envelope from the internal lamina of the nuclear membrane, which it loses when it fuses with the external nuclear membrane.

Skip to 3 minutes and 14 seconds The capsid is freed in the cytosol and is directed toward the Golgi apparatus, acquiring the tegument proteins. In the Golgi vesicles, it is coated with a new envelope (sometimes even without losing the first one) and the new virions leave by exocytosis, first the Golgi vesicles, and later the cell. After the primary infection, newly formed viruses enter cells in which they establish latency and can persist as non-integrated DNA (episome) in the nucleus. During latency, only a limited number of viral genes is expressed, and viral particles are not produced. Herpes virus affecting both animals and human beings are grouped according to their biological characteristics in three subfamilies known as Alpha-, Beta - and Gammaherpesvirinae.

Skip to 4 minutes and 9 seconds Both alfaherpesvirus and betaherpesvirus originate epithelial damage, while gammaherpesvirus produce lymphoproliferative diseases. They have varying specificities of hosts.

Skip to 4 minutes and 25 seconds Latency is also established in different types of cells: alfaherpesvirus in nervous tissue, such as sensorial ganglia or neurons, betaherpesvirus in secretory glands and in cells of the lympho-reticular system, and gammaherpesvirus in lymphoid tissue. In this step we have learned the basics of the herpesviruses, their replication cycle and how they are grouped in different subfamilies, depending on, among other things, of the locations in which they establish latency. All this will help us to understand better the pathogenesis which we will see in a later activity.

What is a herpesvirus?

This video shows you how to recognise the key features that define the herpesviruses. It also outlines the different steps they follow for replicating and producing many more viral particles.

Have your say

Compare the size of herpesvirus particles with other viruses that you know. Are herpesvirus genomes the largest of the known viruses? Human herpesviruses are well known; have you contracted the chickenpox? And herpes zoster? Share your experience with other learners.

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Animal Viruses: Their Transmission and the Diseases They Produce

Complutense University of Madrid

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