Capsid: Definition, Function & Structure

Capsid: Definition, Function & Structure
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  • 0:00 Understanding a Virus
  • 0:50 So, What is a Capsid?
  • 0:54 So, What is a Capsid?
  • 1:34 Capsid Function
  • 2:30 Lesson Summary
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Lesson Transcript
Instructor: Jeremy Battista
Viruses cause diseases and they like to stick around long past the point we want them to. Viruses like to stay around so they can multiply. One way they accomplish this is through capsids. In this lesson learn about capsids and their function.

Understanding A Virus

A virus is not considered a living thing. It is essentially just a strand of genetic material that is coded to replicate itself. Viruses are infectious particles that are nucleic acids encased in a protein coat and sometimes further enclosed in a membrane. The genomes, or genetic materials, of viruses may be DNA (deoxyribonucleic acid), but sometimes the genomes are single stranded DNA, double stranded RNA (ribonucleic acid), or even single stranded RNA.

Basically, a virus is just encapsulated genetic material that gets injected into the host cell. It then begins to copy, or replicate itself, and produce more and more viruses. This is how something or someone would become 'infected.' As you can see, the squiggly lines in the head-like structure is the genome, and the head is really the capsid.

The genome and capsid of a virus.
Basic Virus

So, What Is A Capsid?

A capsid is a protein shell that encloses the viral genome (RNA, DNA, etc.). Capsids come in about three different shapes, although there can easily be more complex ones. The most common shapes are icosahedral, prolate, and helical. Understanding some of these are very complex, but picture a capsid as a slightly odd shaped soccer ball. Instead of being nice and round, though, picture it with more angles, almost like a pentagon-type shape.

This is a helical capsid, one of the many different kinds.
Helical Capsid

The capsid is built from different protein subunits that are known as capsomeres. These capsomeres may number in the thousands, but the actual different proteins that make them up are small.

This is the capsid of the Adenovirus. This capsid is called an icosahedral shape. This is one of the more complex types of capsid.
Adenovirus

Capsid Function

The capsid's main function is to protect the virus. That virus wants to attach to a host and get inside. It needs the capsid to make sure that nothing destroys the genome floating around inside of it. Remember, something so small is susceptible to a multitude of things that could destroy it.

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