Question

-What 2 parts does every virus need? What is one optional part?

-Would a virus that infects plants have an envelope? Why or why not?

Answer 1

1.

All viruses contain the following two components: 1) a nucleic acid genome and 2) a protein capsid that covers the genome. Together this is called the nucleocapsid. In addition, many animal viruses contain a 3) lipid envelope. The entire intact virus is called the virion. The structure and composition of these components can vary widely.

A: Viral Genomes: While the genomes of all known cells are comprised of double stranded DNA, the genomes of viruses can be comprised of single or double-stranded DNA or RNA. They can vary greatly in size, from approximately 5-10 kb (Papovaviridae, Parvoviridae, etc.) to greater than 100-200 kb (Herpesviridae, Poxviridae). The known structures of viral genomes are summarized below.

DNA: Double-Stranded - linear or circular

          Single-Stranded - linear or circular

          Other Structures - gapped circles

RNA: Double-Stranded - linear

          Single-Stranded - linear: These single-stranded genomes can be either + sense, - sense, or ambisense The sense strand is the one that can serve directly as mRNA and code for protein, so for these viruses, the viral RNA is infectious. The viral mRNA from - strand viruses is not infectious since it needs to be copied into the + strand before it can be translated. In an ambisense virus, part of the genome is the sense strand, and part is the antisense.

The genome of some RNA viruses is segmented, meaning that a virus particle contains several different molecules of RNA, like different chromosomes.

B: Protein Capsid

Viral genomes are surrounded by protein shells known as capsids. One interesting question is how capsid proteins recognize viral, but not cellular RNA or DNA. The answer is that there is often some type of "packaging" signal (sequence) on the viral genome that is recognized by the capsid proteins. A capsid is almost always made up of repeating structural subunits that are arranged in one of two symmetrical structures, a helix or an icosahedron. In the simplest case, these "subunits" consist of a single polypeptide. In many cases, however, these structural subunits (also called protomers) are made up of several polypeptides.

2. Many viruses that infect humans and other animals have a lipid membrane or envelope that surrounds the protein coat. Such a virus, called an enveloped virus, is rarely found among phage or plant viruses. The envelope is usually acquired from the cytoplasmic membrane of the infected cell during viral release from the cell. Thus, the structure of the viral envelope is similar to the membrane of the cell, a lipid bilayer containing various proteins. In certain virus families, a matrix protein is found just inside the lipid envelope. The attachment proteins, or spikes, that bind the virus to the cell project from the envelope or the capsid. Plant viruses, such as tobacco mosaic virus, do not bind to specific sites on the plant cell wall; rather, they enter through wounds and have no protruding attachment proteins.