Variety of Subcellular Genetic Entities Exist

A whole range of entities exist that have genetic information, but do not themselves possess the machinery of life and cannot exist without a host cell to parasitize (Fig. 2.30).Viruses are the most complex of these subcellular genetic elements. In this book, these elements will sometimes be collectively referred to as "gene creatures" to emphasize that they possess genetic information, but have no cell structure or metabolism of their own. Gene creatures may be thought of as inhabiting cells, much as living cells live in their own, larger-scale environment. The term gene creatures is intended to focus attention on the properties of these genetic elements in contrast to the traditional viewpoint, which regards them merely as parasites or accessories to "real cells". These assorted genetic elements will be dealt with in subsequent chapters. Here they will just be introduced, to give some idea of the range of gene creatures that share the biosphere with the more traditional life forms (Fig. 2.31).

As discussed above, viruses carry their genes inside a protective shell of protein. DNA viruses have their genes in the form of DNA, and RNA viruses contain genes as RNA. Retroviruses have RNA copies of their genes inside the virus particle, but once inside the host cell, they make a DNA copy of their genome (see Ch. 17).

Viroids and plasmids are self-replicating molecules of nucleic acid that lack the protein coat characteristic of a virus. Viroids are naked molecules of RNA that infect plants and trick the infected plant cell into replicating more viroid RNA (see Ch. 17). Like a virus, they are released into the environment and must find a new cell to infect.

DNA virus A virus whose genome consists of DNA

plasmid Self-replicating genetic elements that are sometimes found in both prokaryotic and eukaryotic cells. They are not chromosomes nor part of the host cell's permanent genome. Most plasmids are circular molecules of double stranded DNA although rare linear plasmids and RNA plasmids are known retrovirus Type of virus which has its genes as RNA in the virus particle but converts this to a DNA copy inside the host cell by using reverse transcriptase

RNA virus A virus whose genome consists of RNA

viroid Naked single-stranded circular RNA that forms a stable highly base-paired rod-like structure and replicates inside infected plant cells. Viroids do not encode any proteins but possess self-cleaving ribozyme activity

FIGURE 2.31 The Molecular Biologist's "Tree of Life"

This tree of life includes both the traditional living creatures, such as plants and animals, as well as the two genetically distinct types of prokaryotic cell (eubacteria and archaebacteria). At the bottom are shown a variety of gene creatures, whose relationships are still mostly uncertain.

Mammals

Birds, Reptiles, & Dinosaurs

Mammals

Birds, Reptiles, & Dinosaurs

Eubacteria

Archaebacteria

DNA viruses Plasmids \ \

DNA gene creatures

RNA viruses Retrons

RNA gene creatures \ \

Viroids Retrotransposons

Eubacteria

Eukaryotes

Archaebacteria

Living cells

DNA viruses Plasmids \ \

DNA gene creatures

Transposons

RNA viruses Retrons

RNA gene creatures \ \

Viroids Retrotransposons

Gene creatures

Unlike a virus, their extra-cellular phase lacks a protective protein shell. Plasmids are self-replicating molecules of DNA that live permanently inside host cells (see Ch. 16). Although some plasmids can be transferred from one host cell to another, they have no extra-cellular phase and so unlike viruses or viroids, they do not destroy their host cell. Plasmids are widely used to carry genes during many genetic engineering procedures.

Transposable elements, or transposons, are simpler still. They are nucleic acid molecules, usually DNA, that lack the ability to self-replicate. In order to get replicated, they must insert themselves into other molecules of DNA that are capable of replicating themselves. Thus transposable elements require a host DNA molecule, such as the chromosome of a cell, a virus genome or a plasmid. Transposable refers to the fact that these elements possess the ability of jumping from one host DNA molecule to another, a property that is essential for their survival and distribution (see Ch. 15).

Prions are infectious protein molecules, the ultimate parasites. They contain no nucleic acid and possess genetic information only in the sense of being gene products. Prions infect cells in the nervous systems of animals and cause diseases, the most famous of which is bovine spongiform encephalopathy, better known as mad cow disease. The prion protein is actually a misfolded version of a normal protein found in nerve cells, especially in the brain. When the prion infects a nerve cell, it promotes the misfolding of the corresponding normal proteins, which causes the cell to die. The prion protein is actually encoded by a gene belonging to the host animal that it infects.

prion Distorted, disease-causing form of a normal brain protein which can transmit an infection transposable element or transposon Segment of DNA that can move as a unit from one location to another, but which always remains part of another DNA molecule

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