Course: Virology
Lecturer: Dr. Weam SaadLecture: Genome and Capsid of viruses
Viral Genome(viral core):
The viral genome consists of either DNA or RNA, a virus does not contain both DNA and RNA. Virus Core Structure, except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core. Small virions are simple nucleocapsids containing 1 to 2 protein species. The larger viruses contain in a core the nucleic acid genome complex with basic protein(s) and protected by a single- or double layered capsid (consisting of more than one species of protein) or by an envelope.The DNA can be:
Single-stranded (ss) called linear, e.g. (parvoviruses and circoviruses).
Double-stranded (ds) (polyomaviruses, adenoviruses, herpesviruses)
Partially double-stranded (hepadnaviruses).
(Single stranded ss virus)
(Double stranded ss virus)
(Partially double-stranded)
The DNA genome have ends covalently linked to each other (circular = polyomavirus, circoviruses) or not linked (linear = adenovirus, herpesvirus, parvoviruses). The genome of poxviruses is ssDNA have ends attached covalently to each other.
RNA viral genomes are all linear. Most of these are single-stranded and a few are double stranded (reoviruses, bornaviruses).
Most RNA viruses have genomes in a single piece (monopartite) while others segmented in 10 segments (reoviruses), 7 or 8 segments (orthomyxoviruses), three segments (bunyaviruses) and two segments (arenaviruses).
The ssRNAs, can be found in two sequences:
If the viral ssRNA act as a message for translation (the same sense of mRNA), it is referred to positive-sense.
If the viral RNA is antisense (or complementary) to that of mRNA – and thus cannot be translated directly - it is called negative-sense.
In some viruses (Arenavirus and Bunyaviruses) portions of the RNA genome are transcribed for generating mRNA, which are then translated. This arrangement is unique (special) among viruses and called ambisense.
The genes contained within the genome may encode from a few gene (e.g. Polyomavirus, 6 - 7 genes, 5000 nucleotides in length) to greater than 70 – 100 different gene products (Herpesviridae, 60 to 120 genes, 120,000 - 220,000 nucleotide bases pairs in length).
In general, RNA virus genomes are smaller, with a 30,000 nucleotide maximum size seen in the Coronavirus. One hypothesis for this is that the viral RNA polymerases are more error prone compared with viral DNA polymerases. Thus, replication can limit size. But, DNA virus genomes can reach up to 300,000 nucleotides as seen in some species of Herpesviridae.
The Viral Capsid
The function of the capsid is to protect the viral genome during its transfer from cell to cell. Capsids are made up of multiples copies of one single protein or by association of several different proteins. Capsids made up of multiple copies of a single protein due to a single gene can encode the products needed to form a capsid for the whole genome.The capsid of a virus can be in different geometric shapes (symmetry) that are characteristic in viral families. These include:
Icosahedral structure or Cubic Symmetry:
Can be naked e.g. (picornaviruses, polyomaviruses), or enveloped e.g. (herpesviruses). This geometric shape has several triangular faces and corners. The number of faces and corners may differ according to the number and type of structural proteins and units. All cubic or icosahedral symmetry observed with animal viruses are icosahedral, the arrangement for protein subunits is a closed shell.
The icosahedron has 20 faces (each an equilateral triangle). There are 60 identical subunits on the surface of an icosahedron (viral shells are composed of multiples of 60 structural units), 12 vertices, threefold, and twofold axes of rotational symmetry. The viral nucleic acid is condensed within the capsid for packaging.
Helical structure (Helical Symmetry):
Can be naked e.g. (tobacco mosaic virus) or enveloped e.g. (rabies virus), the protein subunits are bound in a periodic way to the viral nucleic acid making a helix. The filamentous viral nucleic acid protein complex (nucleocapsid) is then coiled inside a lipid-containing envelope. An enveloped virus with helical nucleocapsid symmetry, e.g. paramyxovirus. All known examples of animal viruses with helical symmetry contain RNA genomes, but the exception is rhabidoviruses, which have flexible nucleocapsids that form a ball inside envelopes.
Complex Viral structure:
Complex virus structures have a combination of icosahedral and helical shape and may have a complex outer wall or head-tail morphology. The head-tail morphology structure is unique (special) to viruses that only infect bacteria and are known as bacteriophages. The head of the virus has an icosahedral shape with a helical shaped tail; the head has a core contain the nucleic acid.The bacteriophage uses its tail to attach to the bacterium, creates a hole in the cell wall, and then inserts its DNA into the cell using the tail as a channel. The Poxvirus is one of the largest viruses in size and has a complex structure with a unique outer wall and capsid. One of the most famous types of poxviruses is the variola virus which causes smallpox.
Bacteriophage Structure
