General Structures of viruses

• Virus means poison or toxin in Latin.
• A virus is a small infectious agent that can

  replicate only inside the cells of other organisms.
• Viruses are too small to be seen directly with a

  light microscope. They can be seen by electron

  microscope.
• Viruses infect all types of organisms, from

  animals and plants to bacteria and archaea.
• A virus is a small infectious agent that can replicate only inside the cells of other organisms
• Viruses infect all types of organisms, from animals and plants to bacteria and archaea
• In 1884, French microbiologist Charles Chamberland invented a ceramic filter with pores smaller than bacteria
• In 1892, Russian biologist Dmitry Ivanovsky used this filter to study what is now known as the tobacco mosaic virus
• In 1898, Dutch microbiologist Martinus Beijerinck repeated the experiments and became convinced that this was a new form of infectious agent
• In the same year, 1899, Friedrich

  Loeffler and Frosch passed the agent of

  foot-and-mouth disease (aphthovirus)

  through a similar filter and ruled out

  the possibility of a toxin because of the

  high dilution; they concluded that the

  agent could replicate
• In the early 20th century, the English

  bacteriologist Frederick Twort

  discovered the viruses that infect

  bacteria, which are now called

  bacteriophages
• French-Canadian microbiologist Félix

  d'Herelle described viruses that,

  when added to bacteria on agar,

  would produce areas of dead

  bacteria (plaques).
• In 1931, American pathologist Ernest

  William Goodpasture grew influenza

  and several other viruses in fertilized

  chickens' eggs
• With the invention of electron microscopy in 1931 by the German

  engineers Ernst Ruska and Max Knoll came the first images of viruses
• Viruses consist of two or three parts:

  – all viruses have genes made from either DNA or RNA,

  – all have a protein coat that protects these genes

  – some have an envelope of fat that surrounds them when they are outside a cell.
• Viruses vary from simple helical and icosahedral shapes, to more complex structures.
• Viruses are much smaller than bacteria.Most viruses that have been studied have a diameter between 10 and 300 nanometres. 
• Some filoviruses have a total length of up to 1400 nm and their diameters are only about 80 nm.
• Most viruses cannot be seen with a light microscope so

  scanning and transmission electron microscopes are used to

  visualise virions
• A complete virus particle, known as a virion, consists of nucleic acid surrounded by a protective coat of protein called a capsid.

  These are formed from identical protein subunits called capsomers.
• Viruses can have a lipid "envelope" derived from the host cell membrane.
• The capsid is made from proteins (capsomers) encoded

  by the viral genome and its shape serves as the basis for

  morphological distinction.
• Virally coded protein subunits will self-assemble to form

  a capsid, generally requiring the presence of the virus

  genome.
• Complex viruses code for proteins that assist in the

  construction of their capsid.
• Helical:

  • These viruses are composed of a single type of capsomer stacked around a central axis to form a helical structure,This arrangement results in rod-

  shaped or filamentous virions
• The genetic material, generally single-stranded RNA, but ssDNA in some cases, is bound into the protein helix by interactions between the negatively charged nucleic acid and positive charges on the protein. Tobacco mosaic virus is an example of a helical virus
• Tobacco Mosaic Virus: 1- Nucleic acid (RNA)

  2- Capsomer

  3- Capsid
• Most animal and human viruses are icosahedral or near-spherical with icosahedral symmetry.
• A regular icosahedron is the optimum way of forming a closed shell from identical sub-units. The minimum number of identical

  capsomers required is twelve each composed of five identical sub-units
• Capsomers at the corners are surrounded by five other capsomers and are called pentons. Capsomers on the triangular faces are surround by six others and are call hexons
• Some species of virus envelope themselves in a modified form

  of one of the cell membranes, either the outer membrane

  surrounding an infected host cell, or internal membranes such as nuclear membrane or endoplasmic reticulum
• This membrane is studded with proteins coded for by the viral genome and host genome; the lipid membrane itself and any carbohydrates present originate entirely from the host
• Most enveloped viruses are dependent on the envelope for

  their infectivity
• The genome replication of most DNA viruses takes place in the cell's nucleus.If the cell has the appropriate receptor on its surface, these viruses enter the cell by fusion with the cell membrane or by endocytosis.
• Most DNA viruses are entirely dependent on the host cell's DNA and RNA synthesising machinery, and RNA processing machinery. The viral genome must cross the cell's nuclear membrane to access this machinery.
• These viruses are unique because their genetic information is encoded in RNA. Replication usually takes place in the cytoplasm.
• The polarity (whether or not it can be used directly to make proteins) of the RNA largely determines the replicative mechanism, and whether the genetic material is single-stranded or double-stranded.
• RNA viruses use their own RNA replicase enzymes to create copies of their genomes
• These replicate using reverse transcription, which is the

  formation of DNA from an RNA template.
• Reverse transcribing viruses containing RNA genomes use a DNA intermediate to replicate, whereas those containing DNA genomes use an RNA intermediate during genome replication.