Viral Capsids

Dinah McAlister, Jeremy Battista
  • Author
    Dinah McAlister

    Dinah Mcalister teaches 7-12 Science. Courses taught include Chemistry, Biology, System's Go!, Physical Science, Life Science and Earth Science. She has 25 years of teaching experience. Dinah received her Master's degree in Secondary Education with an emphasis in Curriculum and InstructionFrom Texas Tech University in Lubbock, Texas.

  • Instructor
    Jeremy Battista

    Jeremy has a master of science degree in education.

Learn about viral capsids, including the definition of capsid. Discover the function of capsids, study the three capsid shapes, understand capsid protein structures, and learn whether all viruses have capsids. Updated: 02/01/2022

Table of Contents


What is a Viral Capsid?

A capsid is a protein covering viral nucleic acid. A virus is a nonliving particle that contains nucleic acid (DNA or RNA) in a protein coat (viral capsid) that will cause disease. Viruses are often characterized by the organism they infect. These organisms include animals, plants, or bacteria.

Virus structures

Some viruses have a capsid that is surrounded by an envelope. The capsid is made up of proteins, while the envelope is a membrane made up of lipids. However, not all viruses have this membrane envelope.

What is a Nucleocapsid?

A nucleocapsid is an inclusive term for the protein coat and the nucleic acid of a virus, including DNA or RNA. The nucleocapsid would be similar to the nucleus of an animal or plant cell containing these organisms' genetic materials. The nucleocapsid can be surrounded by a lipid membrane outside the protein capsid. Nucleocapsids are fully assembled and infectious viruses.

Do all Viruses have Capsids?

All viruses have a capsid made of protein that surrounds and protects the viral nucleic acid, DNA, or RNA. In addition, many viruses contain a lipid membrane envelope that surrounds the capsid.

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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
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What is the structure of a Capsid?

The composition of the viral capsid can classify viruses. The capsid surrounds the viral nucleic acid, genetic material, and is composed of protein subunits called protomers. The protomers are composed of several different polypeptide chains. The protomers make up capsomeres, and capsomeres form the capsid. The proteins that make up the capsid are known as capsid proteins or viral coat proteins. The capsid that encloses the nucleic acid is known as a nucleocapsid which makes it possible for the virus to penetrate a host's cell membrane, insert viral nucleic acid into the cell and take over cell functions. The nucleocapsid is considered an infectious virion, a fully assembled, infectious virus.

Viruses can also be classified by the shape of the viral capsid, which includes the number of capsid proteins and the placement or arrangement of the capsid proteins. Viral capsids can be found in shapes, such as helical, polygonal, and sphere. Helical-shaped capsids are named because of the spiral shape of the capsid, but the shape of the nucleic acid and the protein subunits called capsomeres are in rod or filament shapes. Polygonal-shaped capsids, also known as icosahedral, are 20-sided polygons. Sphere-shaped capsids are round-shaped.

What is a Capsid Protein?

A capsid protein is composed of several different types of proteins. The nucleocapsid (N) protein is made of protomers and capsomeres. The protomers are structural units of oligomeric proteins which means they are composed of more than one polypeptide chain (chain of amino acids that make up proteins). Capsomeres are individual proteins composed of protomers. Protomers make up capsomeres which form the capsid, a protein coat. Spike (S) proteins allow viruses to penetrate host cells and cause infection. Spike (S) proteins are made up of glycoproteins. Glycoproteins are composed of proteins and carbohydrates that play an important role in immunity. A membrane (M) protein is either attached to or a part of the membrane of a virus. An envelope (E) protein has several functions, including assembly during the viral life cycle, envelope formation, and pathogenesis (ability to cause disease).

Capsid Proteins.

What are the Shapes of Capsids?

Capsid Shape is one way to classify viruses, along with the composition of the capsid. Capsids come in various shapes, but the following three shapes are the most common.

  • Helical capsid shape: Spiral-shaped capsid.

Helical Capsid shape

  • Icosahedral capsid shape: Polygonal (20-sided) shaped capsid.

Icosahedral shape

  • Prolate capsid shape: Modified shape of Icosahedral (cylinder shape with Icosahedral shaped caps on end)

Prolate capsid shape in a bacteriophage

Viruses can be classified according to the composition and shape of the capsid and the proteins that make up the capsid.

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Frequently Asked Questions

What is the function of a capsid?

The function of the capsid also known as the viral coat is to surround and protect the nucleic acid (DNA or RNA).

What does capsid mean in a virus?

The capsid is also known as the nucleocapsid and surrounds the nucleic acid of a virus. The nucleocapsid includes the nucleic acid and viral protein coat.

What is the purpose of the capsid in a virus?

The main purpose of the capsid is to surround and protect the DNA or RNA (nucleic acid) of the virus. Another purpose is to help the virus overtake the host cell's functions.

What is a viral capsid made of?

The capsid also known as the viral coat is made up of proteins and its function is to surround and protect the nucleic acid (DNA or RNA).

What are the 2 major viral capsid types?

There are two major types of capsids. The first type is helical which can be described as spiral shaped. The second type is icosahedral shape which is a polygonal (20-sided) shaped capsid. There is a third common type which is sphere. Sphere is a round shaped capsid.

What viruses have a capsid?

All viruses have a capsid that surrounds and protects the nucleic acid of a virus. Only certain viruses contain a lipid membrane envelope.

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