Simplified diagram of the structure of a virus A virus particle, also called a virion, consists of genes made from DNA or RNA which are surrounded by a protective coat of protein called a capsid. The capsid is made of many smaller, identical protein molecules called capsomers. The arrangement of the capsomers can either be icosahedral (20-sided), helical, or more complex. There is an inner shell around the DNA or RNA called the nucleocapsid, made out of proteins. Some<unk>  are surrounded by a bubble of lipid (fat) called an envelope, which makes them vulnerable to soap and alcohol. Size Virions of some of the most common human<unk>  with their relative size. The nucleic acids are not to scale.<unk>  are among the smallest infectious agents, and are too small to be seen by light microscopy; most of them can only be seen by electron microscopy. Their sizes range from 20 to 300 nanometres; it would take 30,000 to 500,000 of them, side by side, to stretch to one centimetre (0.4 in). In comparison, bacteria are typically around 1000 nanometres (1 micrometer) in diameter, and host cells of higher organisms are typically a few tens of micrometers. Some<unk>  such as megaviruses and pandoraviruses are relatively large<unk> . At around 1000 nanometres, these<unk> , which infect amoebae, were discovered in 2003 and 2013. They are around ten times wider (and thus a thousand times larger in volume) than influenza<unk> , and the discovery of these "giant"<unk>  astonished scientists. Genes The genes of<unk>  are made from DNA (deoxyribonucleic acid) and, in many<unk> , RNA (ribonucleic acid). The biological information contained in an organism is encoded in its DNA or RNA. Most organisms use DNA, but many<unk>  have RNA as their genetic material. The DNA or RNA of<unk>  consists of either a single strand or a double helix.<unk>  can reproduce rapidly because they have relatively few genes. For example, influenza virus has only eight genes and rotavirus has eleven. In comparison, humans have 20,000–25,000. Some viral genes contain the code to make the structural proteins that form the virus particle. Other genes make non-structural proteins found only in the cells the virus infects. All cells, and many<unk> , produce proteins that are enzymes that drive chemical reactions. Some of these enzymes, called DNA polymerase and RNA polymerase, make new copies of DNA and RNA. A<unk>  polymerase enzymes are often much more efficient at making DNA and RNA than the equivalent enzymes of the host cells, but viral RNA polymerase enzymes are error-prone, causing RNA<unk>  to mutate and form new strains. In some species of RNA virus, the genes are not on a continuous molecule of RNA, but are separated. The influenza virus, for example, has eight separate genes made of RNA. When two different strains of influenza virus infect the same cell, these genes can mix and produce new strains of the virus in a process called reassortment. Protein synthesis Diagram of a typical eukaryotic cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles within centrosome (14) a<unk> hown to approximate scale Proteins are essential to life. Cells produce new protein molecules from amino acid building blocks based on information coded in DNA. Each type of protein is a specialist that usually only performs one function, so if a cell needs to do something new, it must make a new protein.<unk>  force the cell to make new proteins that the cell does not need, but are needed for the virus to reproduce. Protein synthesis consists of two major steps: transcription and translation. Transcription is the process where information in DNA, called the genetic code, is used to produce RNA copies called messenger RNA (mRNA). These migrate through the cell and carry the code to ribosomes where it is used to make proteins. This is called translation because the protein's amino acid structure is determined by the mRNA's code. Information is hence translated from the language of nucleic acids to the language of amino acids. Some nucleic acids of RNA<unk>  function directly as mRNA without further modification. For this reason, these<unk>  are called positive-sense RNA<unk> . In other RNA<unk> , the RNA is a complementary copy of mRNA and these<unk>  rely on the cell's or their own enzyme to make mRNA. These are called negative-sense RNA<unk> . In<unk>  made from DNA, the method of mRNA production is similar to that of the cell. The species of<unk>  called retroviruses behave completely differently: they have RNA, but inside the host cell a DNA copy of their RNA is made with the help of the enzyme reverse transcriptase. This DNA is then incorporated into the host's own DNA, and copied into mRNA by the cell's normal pathways.
Introduction to viruses
Viral pathogenesis
Human genetic enhancement
Xenobiotic
Human reproduction
Toxicant
Tumor initiation
Social immunity