Viruses and Plasmids: Structure, Function, and Impact
Plasmids
Plasmids are extrachromosomal, double-stranded circular DNA molecules capable of transferring between cells. They lack a protein envelope, exhibit no extracellular phase, and can acquire new genes. Plasmids are found in varying numbers in all bacterial species and some eukaryotes. While not essential for cell survival, they can induce mutations and facilitate bacterial conjugation. Plasmids can confer antibiotic resistance, provide access to new nutrient sources, and transform pathogenic bacteria. The proteins encoded by plasmid genes are synthesized by the same metabolic machinery as chromosomal proteins. However, plasmids are not used to introduce bacterial genes into the bacterial chromosome, nor can they synthesize proteins encoded in their genome. Plasmid loss is permanent unless transferred from another bacterium. Each plasmid replicates independently, and the copy number within a cell is controlled individually.
Viroids
Viroids are the smallest known infectious agents, consisting of small, circular, single-stranded RNA molecules without a protein coat. They are primarily plant parasites, with their RNA localized in the host cell’s nucleus. Viroids do not function as mRNA; their replication relies on host cell enzymes. They are thought to interfere with nuclear genes without being translated into proteins. Viroid diseases cause symptoms similar to viral infections, including malformation, necrosis, chlorosis, or leaf mottling. Transmission occurs through infected insects or agricultural material.
Viroid-Like RNAs and Satellites
Other infectious circular RNAs exist, including viroid-like RNAs and satellites. These differ from viroids in that they replicate only during coinfection with a helper virus, within which their particles are encapsidated.
Viruses: Composition, Structure, and Function
A virus is a small, acellular parasite that cannot reproduce extracellularly. Upon infecting a susceptible cell, it hijacks the cellular machinery to produce more viruses. Viruses are nucleic acid molecules encased in protein capsules, enabling them to penetrate cells. Ranging from 20 to 300 nm in size, they are visible only with high-magnification electron microscopy.
Key Characteristics of Viruses
- Viruses can only replicate inside living cells, relying on the cell’s structures and energy.
- They are obligate parasites of organisms from all five kingdoms.
- Viruses contain only one type of nucleic acid (either DNA or RNA, never both) and have an eclipse stage where they are undetectable within the host cell.
Virion Structure
A complete virus particle is called a virion. Simple virions consist of nucleic acid enclosed within a protein capsid made of capsomeres. Enveloped viruses have an additional outer layer of lipoproteins and glycoproteins, essential for infectivity.
Viral Nucleic Acid
Viral nucleic acids can be DNA or RNA, single or double-stranded, and circular or linear. Some viruses have segmented genomes, meaning their genetic information is distributed across multiple independent nucleic acid chains. Segmented genomes are found in all double-stranded RNA viruses and some single-stranded RNA and DNA viruses. Each segment typically encodes one or two viral proteins. Segmented DNA genomes and circular RNA genomes have not been observed. Most DNA viruses replicate in the nucleus using cellular enzymes like DNA polymerase, while most RNA viruses replicate in the cytoplasm.
Viral Proteins
The simplest viruses encode only a few proteins, while more complex ones can encode many more. Viral proteins can be structural (forming the virion), enzymatic (involved in nucleic acid synthesis), or functional, such as those that interact with cell receptors and facilitate infection spread. DNA viruses utilize cellular enzymes for genome synthesis, while most viral enzymes are involved in mRNA and protein synthesis.