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Baltimore classification groups viruses together based on their manner of mRNA synthesis. Characteristics directly related to this include whether the genome is made of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), the strandedness of the genome, which can be either single- or double-stranded, and the sense of a single-stranded genome, which is either positive or negative.
It is the first step of viral replication. Some viruses attach to the cell membrane of the host cell and inject its DNA or RNA into the host to initiate infection. Attachment to a host cell is often achieved by a virus attachment protein that extends from the protein shell (), of a virus.
Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), sense, and method of replication. [13]
A double-stranded DNA virus; i.e. a virus whose genome is encoded in two complementary strands of DNA, which usually exist as one or more circular molecules. dsDNA viruses constitute Group I in the Baltimore classification system and use methods of replication and transcription that are broadly similar to those of larger organisms such as bacteria.
Zoology mnemonics, which are used to memorize the scientific taxonomic classification system, include: "Despicable King Phillip came over for grandma's specialty." "Despicable King Phillip covets our fancy giant swimming pool." "Dizzy kids puke cereal on Fair Grounds staff." "Do kindly place candy out for good students."
Many viruses replicate this way. The enzymes that copy RNA to new RNA, called RNA-dependent RNA polymerases, are also found in many eukaryotes where they are involved in RNA silencing. [11] RNA editing, in which an RNA sequence is altered by a complex of proteins and a "guide RNA", could also be seen as an RNA-to-RNA transfer.
This complex method of transcription is generally restricted to viruses, especially those of the single-stranded, positive-sense RNA or Class IV viruses using the Baltimore Classification System, e.g. viruses of the order Nidovirales. It is primarily used for compacting more genetic information into a shorter amount of genetic material. [3]
All known RNA viruses, that is viruses that use a homologous RNA-dependent polymerase for replication, are categorized by the International Committee on Taxonomy of Viruses (ICTV) into the realm Riboviria. [3] This includes RNA viruses belonging to Group III, Group IV or Group V of the Baltimore classification system as well as Group VI.