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12.2.1. Viral morphology

Viral morphology

The morphologically complete virus particle or virion is made up of a nucleic acid  enveloped by a protein capsid or coat. Some viruses also have a membranous envelope  that surrounds them.

The viruses  are composed of:

Nucleic acid

All viruses contain a nucleic acid that can be DNA (in DNA viruses) or RNA (in RNA viruses or retroviruses), but never DNA and RNA in the same virus. This nucleic acid can be circular or linear, and single-stranded or double-stranded. Nucleic acid can be made up of a single molecule, as in the case of DNA viruses, or segmented, as in some RNA viruses .

  • DNA viruses: Most DNA viruses have double-stranded DNA, and will take advantage of the cellular mechanism to replicate their DNA and synthesize the mRNA that produces the proteins necessary for the virus . Examples of viral DNA are adenoviruses, herpesviruses, and bacteriophage T4.
  • RNA viruses: There are several types, depending on whether the RNA sequence is directly translated into proteins (equivalent to mRNA, as in poliovirus) or not (complementary to mRNA, as rhabdoviruses). In this case, the RNA polymerase enzyme will form the mRNA, or, as in retroviruses, such as HIV , the reverse transcriptase enzyme , which forms DNA from viral RNA and integrates it into the host cell's DNA.

All viruses that have double-stranded RNA, and some single-stranded RNA, have several independent RNA molecules. They are said to have a fragmented genome. DNA viruses have only one DNA molecule.

The simplest viruses can only code for about eight proteins, but the most complex viruses can code for up to two hundred proteins. These proteins can be:

  • Structural proteins: are responsible for forming the capsid.
  • Enzyme proteins: they are involved in the synthesis of new viral nucleic acids.
  • Binding proteins: facilitate adherence to the host cell membrane.

Capsid or capsid

The capsid is a protein structure that surrounds and protects nucleic acid. It is formed by the union of capsomeres (globular proteins). Although its main function is to protect the nucleic acid, in viruses that are not enveloped by a membrane, it is also responsible for recognizing the cells it is going to parasitize.

The capsid is usually formed by the repetition of capsomeres with the same type of proteins, so the virus can create a complex capsid , with many capsomeres , with little genetic information. The more proteins you need to synthesize, the more genetic information you need, and the space for nucleic acid in a virus is very limited.

The term nucleocapsid refers to the genetic material wrapped in its capsid.

The capsomeres, to the joining, can form three types of capsids: icosahedral , helical and complex.

  • The icosahedral capsid is a polyhedral structure with 12 vertices, 20 triangular faces and 30 edges, in which the capsomeres are arranged to form an icosahedron. This arrangement allows to have a closed structure with few capsomeres.

For example, they are virus icosahedral the virus influenza  and virus human warts.

By Dr. Richard Feldmann (photographer) (National Cancer Institute here) [Public domain], via Wikimedia Commons

  • The helical capsid is made up of capsomers arranged in a helical manner, forming a tubular structure inside which is the nucleic acid. They have this type of capsid, the tobacco mosaic virus (VMT), the rabies virus or the measles virus.

By Thomas Splettstoesser (www.scistyle.com) (Own work) [CC BY-SA 4.0], via Wikimedia Commons

  • The complex capsid is characteristic of bacteriophages , viruses specialized in parasitizing bacteria.

      The virion has at least two parts:

  • An icosahedral head or nucleocapsid that contains the nucleic acid .
  • A tail (viruses that present a tail are called urophagi), of helical and contractile symmetry, ending in a basal plate equipped with spines and anchoring fibers (generally of a protein nature), adapted for the injection of nucleic acid into the inside the bacteria. At the base of the tail there may be enzymes and ATP, whose function is to destroy the bacterial wall.

By NoeTico1516 (Own work) [CC BY-SA 4.0], via Wikimedia Commons

Enveloped virus

The viral envelope is a membranous structure that surrounds the viral capsid of some viruses. It usually comes from the host's plasma membrane. In addition, the envelope contains glycoproteins in its outer part that are encoded by the viral genome. These glycoproteins are involved in the recognition, in the adherence to the host cell and in inducing the penetration of the virion  into the cell by phagocytosis.

The enveloped viruses do not necessarily have to destroy the host cell for her to leave because they can get out by budding and cause persistent infections, such as HIV, the virus flu, or the smallpox.

The viruses that have envelope called virus naked, such as virus polio and bacteriophage T4.

Fundamental ideas about viral morphology

The viruses  are microscopic particles  acellular  genetic material that are able to reproduce inside living cells.

The viruses  are composed of: