Nucleic Acids and Replication

Nucleic Acids

The knowledge of genetics was widened by the discovery that DNA is not the only possible harbor of viral genes: some viral genes may consist of RNA instead. Viruses contain either DNA or RNA, but neither of both in the same virion. This, of course, is in similr to all cellular forms of life, which without exception contain both types of nucleic acids in each cell. Furthermore, the genome of higher organisms similar as animals and plants consists of double-stranded DNA (dsDNA). But the DNA of a virion can consist of DNA or RNA that is either double-stranded or single-stranded. All four types of genomes have been found in the bacterial, animal, and plant viruses-double-stranded DNA (dsDNA), single- stranded DNA (ssDNA), double-stranded RNA (dsRNA), and single-stranded RNA (ssRNA). The amount of nucleic acid present may be in different groups of viruses. Small viruses usually parvoviruses and picornaviruses each contain about three or four genes, while large viruses such as the herpesviruses and poxviruses haves several hundred genes per virion.

Moreover, the structure of double-stranded or single-stranded DNA in the virion may be either Linear or Circular. For instance, simian vacuolating virus(SV 40), found in monkey kidney cells, has circular dsDNA, while herpesvirus has linear dsDNA. In contrast, the RNA in animal viruses exists only as Linear double-stranded or single-stranded molecules. However, some plant viruses appear to have a genome of circular dsDNA.

Different from viral DNA, the RNA within a virion may also exist as a segmented genome (in several separate molecules). For instance, the genome of most influenza viruses consists of eight separate segments of ssRNA; reoviruses contain 10 different segments of ssRNA; retroviruses contain two identical single-stranded genomes. Just a complicated organization of genetic material requires unique mechanisms for ensuring its proper distribution during replication. But it also provides the viruses with unique opportunities to vary their genomes.

These viruses containing single-stranded RNA that acts directly as messenger RNA (mRNA) have been found as plus-strand, or (+) strand, viruses; such ssRNA molecules are known as the plus, or (+), strands. Those viruses that must usually replicate their RNA to form a complementary strand (which acts as the mRNA) are designated as minus-strands, or (-) strands, viruses, and their RNA molecules (which serve as templates) are known as the minus, or (-), strands. Duplication of the minus RNA strand is always catalyzed by an RNA polymerase carried within the virion.

Viral DNA can also live as plus or minus ssDNA. But ssDNA must be transformed to dsDNA in order to be transcribed by RNA polymerase to form mRNA. During transcribed of dsDNA, generally only one strand is real and it is considered the plus strands; its complementary strand is considered the minus strand.

Replication

The small Virus particles out of a host cell have no independent metabolic activity and are incapable of reproduction by processes characteristic of other microorganisms (for example, binary fusion in bacteria). Reproduction of virus takes place by Replication, in which the viral protein and nucleic acid components are reproduced within susceptible host cells. The viruses effectively redirect the metabolic methods of many host cells to produce new virion than new host-cell material.

Most of the virus-specific enzymes, which are those encoded by viral genes, are usually not part of the virion itself. This includes those necessary for replication. The smallest naked virions (without an envelope) do not contain any preformed enzymes. Larger virion may harbour one or a few enzymes, which usually function in aiding the virus to penetrate the host cell or to replicate its own nucleic acid. Those enzymes that are lacking may be synthesized only when the virion is within a host cell, which spends part of its energy making viral enzymes rather ten cellular components.

The entire sequence of cell infection by viruses may be generalized as follows. The virion attaches to a susceptible host cell as a specific site. Ether the whole virus or its viral nucleic acid penetrate the cell interior. If a total virus entered the cell, uncoating of the virus must take place to release the viral nucleic acid, which is then free to convert the cell into a factory for the production of viral progeny. The specific intercellular site of progeny assembly and maturation is characteristics for each group of viruses. Until assembled and mature, the virions are release from the host cell.

Despite these basics similarities, there are several distinctive differences between bacteriophage replication and plant and animal virus replication. Plant and animal viruses differ from the phages in their mechanism of entry into the host cell. Usually inside the host cell, plant and animal viruses also differ from phages in the synthesis of the new viral components, partly because of the difference between the bacterial prokaryotic cell and the plant or animal eukaryotic cell. Finally, the process of the maturation and release, and the effect of the host cell differ between plant and animal viruses and those of phages.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.