Are Viruses eukaryotic
Viruses are submicroscopic infectious agents, which can replicate only inside the living cells of an organism. Viruses are composed of a nucleic acid genome surrounded by a protein coat, also known as capsid. The capsids of most eukaryotic viruses consist of a number of different proteins. Viruses can infect both prokaryotic and eukaryotic organisms. Prokaryotic organisms such as bacteria can be infected by viruses, known as bacteriophages. Eukaryotic cells and their viruses carry out processes same as bacteriophages but the processes are not similar in all aspect, some processes are found only in eukaryotes and their viruses i.e., RNA processing and protein modification (proteolytic cleavage, glycosylation and phosphorylation). Many viruses are pathogenic and cause destruction of the host cell leading to disease in humans and other organisms.
Viruses infecting eukaryotic cells
Viruses with different nucleic acid genomes, which may be single or double stranded DNA or RNA infect eukaryotic cells. In some, the viral genome is a single molecule of nucleic acid whereas in others, the viral genome exist on more than one molecule and it is said to be segmented. Single-stranded (ss) genomes are replicated via a double-stranded (ds) intermediate and it may be positive sense or negative sense. The nucleic acid may be single-stranded or double-stranded, depending on the species. If the ssRNA is able to function as mRNA it is referred to as positive strand or plus strand RNA (+RNA); if it is the equivalent to antisense RNA it is known as minus strand or negative strand RNA (-RNA). In some cases, the genome will encode mRNAs which are of either sense.
Structure of eukaryotic viruses
The viral genome is surrounded by a protein shell known as capsid. Capsid encloses the genetic material of the virus and consists of protein subunits known as capsomeres. The nucleic acid genome plus the protective protein coat is called the nucleocapsid which may have icosahedral, helical or complex symmetry. The majority of viruses have capsids with either helical or icosahedral structure. The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere. All faces of an icosahedron are identical. In the icosahedral structure, the individual polypeptide molecules form a geometrical structure that surrounds the nucleic acid (Figure-1).
|Figure 1: Structure of Icosahedral viruses|
Adenovirus has an icosahedral structure. In the helical or filamentous structure, the polypeptide units are arranged as a helix to form a rod like structure surrounding the nucleic acid genome. Tobacco mosaic virus has a helical structure (Figure-2).
Figure 2: The Tobacco mosaic virus showing helical structure
Some viruses are enveloped (Figure-3), where the capsid is coated with a lipid membrane also known as the viral envelope. The envelope is acquired by the capsid from an intracellular membrane derived from the host cell as the virus is released from the cell. In some cases, the virus buds through the plasma membrane but in other cases the envelope may be derived from internal cell membranes such as those of the Golgi body or the nucleus.
Figure 3: Basic structure of an enveloped virus
Some viruses bud through specialized parts of the plasma membrane of the host cell; for example, Ebola virus associates with lipid rafts that are rich in sphingomyelin cholesterol and glypiated proteins. Poxviruses are exceptional in that they wrap themselves in host cell membranes using a mechanism that is different from the usual budding process used by other viruses. Enveloped viruses do not necessarily have to kill their host cell in order to be released, since they can bud out of the cell. If the envelope remains intact, enveloped viruses behave as readily infectious agents.
The viral glycoproteins play an important role in facilitating infection by interacting with receptor proteins on the surface of the host cell. Once the virus has infected the cell, it will start replicating itself, using the mechanisms of the infected host cell. During this process, new capsid subunits are synthesized according to the genetic material of the virus, using the protein biosynthesis mechanism of the cell.
A DNA virus is a virus that contains DNA as its genetic material and it replicates using a DNA-dependent DNA polymerase. The DNA may be single-stranded or double-stranded and may be linear or circular. Most of the DNA viruses replicate in the nucleus and with the help of the host cell machinery replicate their genomes and express viral genes.
Some viruses have circular genomes whereas others have linear genomes. The eukaryotic viruses having circular double-stranded genomes include baculoviruses, papovavirues and Polydnaviruses. The eukaryotic viruses having linear double-stranded genomes include adenoviruses, Herpes viruses. Parvoviruses are very small, single-stranded non-enveloped DNA viruses.
Baculoviruses are very small viruses having double-stranded circular DNA as their genetic material. Baculoviruses are pathogens that attack insects and other arthopods. The baculoviruses can be divided to two genera: nucleopolyhedroviruses (NPV) and granuloviruses (GV). While granuloviruses contain only one nucleocapsidper envelope, NPVs contain either single (SNPV) or multiple (MNPV) nucleocapsids per envelope. These viruses are excellent candidates for species-specific, narrow spectrum insecticidal applications. They have been shown to have no negative impacts on plants, mammals, birds, fish, or even on non-target insects.
There are two distinct forms of baculovirus exist in its lifecycle, i.e., occlusion derived virus (ODV) which is responsible for the primary host infection and the budded virus (BV) which is released from the infected host cells later during secondary infections. Baculovirus infects in three phases i.e., early phase, late phase and very late phase. The budded virus is produced in the late phase, and the occlusion derived virus form is produced in the very late phase acquiring the envelope from host cell nucleus and embedded in the matrix of occlusion body protein. The infection begins when a susceptible host insect feeds on plants which are contaminated with the occluded form of the virus. The occlusion derived virus is present in a protein matrix. The protein matrix dissolves in the alkaline environment of the host midgut, releasing ODV that then fuse to the columnar epithelial cell membrane of the host intestine and are taken into the cell in endosomes. Nucleocapsids come out from the endosomes and are transported to nucleus. Viral transcription and replication occurs in the cell nucleus and new BV particles are budded out from the basolateral side to spread the infection systemically. During budding, BV acquires a loosely fitting host cell membrane with expressed and displayed viral glycoproteins.
Papovaviruses are viruses containing double-stranded DNA, are icosahedral in shape, and do not have a lipoprotein envelope. All papovaviruses replicate inside nucleus. The Papovavirus family is one of the many virus families associated with human disease. The papovaviruses have oncogenic potential. The Papovavirus is divided into two subfamilies or genera, Polyomavirus and Papillomavirus. They are commonly found in humans and other species, mostly mammals.