Microbiology

A virus always has at least two parts: an outer capsid composed of protein units, and an inner core of nucleic acid- either DNA or RNA. The viral genome has at most several hundred genes; a human cell contains thousands of genes. The classification of viruses is based on (1) type of nucleic acid, including whether it is single stranded or double stranded, (2) viral size and shape, and (3) the presence or absence of an outer envelope. It is now believed that viruses are derived from the very cell they infect; the nucleic acid of viruses came from their host cell genomes. Therefore, viruses must have evolved after cells came into existence, and new viruses are evolving now. Viruses, like other organisms, can mutate, and this habit can be quite troublesome because a vaccine that is effective today may not be tomorrow.
Viruses are specific to a particular host cell because portions of the capsid bind in the lock-and-key manner with a receptor on the host cell plasma membrane. After viral nucleic acid enters the cell, it takes over the metabolic machinery of the host so that more viruses are produced. The lytic cycle may be divided into five stages: attachment penetration, biosynthesis, maturation, and release. During attachment,

Viruses are best known for causing infectious diseases in plants and animals, including humans. At least a thousand different viruses cause diseases in plants. About a dozen crop diseases have been attributed not to viruses but to viroids, which are naked strands of RNA not covered by a capsid. Like viruses, though, viroids direct the cell to produce more viroids. Some diseases in humans have been attributed to prions, which are protein particles that possibly can convert other proteins in the cell to become

The singular circular chromosome of bacteria consists only of double-stranded DNA. Bacteria reproduce asexually by means of binary fission. First, the chromosome duplicates; then there are two chromosomes attached to the inside of the plasma membrane. An elongation of the cell, which pushes the chromosomes apart, separates the chromosomes. Then the plasma membrane grows inward and the cell wall forms, dividing the cell into two daughter cells, each of which has its own chromosomes. Sexual exchange of DNA occurs among bacteria in three ways. Conjugation takes place when the so-called male cell passes DNA to the female cell by way of a sex pilus. Transformation occurs when a bacterium binds to and then takes up DNA released into the medium by dead bacteria. During transduction, bacteriophages carry portions of DNA from one bacterium to another.

ent, portions of the capsid combine with a receptor on the rigid bacterial cell wall in the lock-and-key manner. During penetration, a viral enzyme digest away part of the cell wall, and viral DNA is injected into the bacterial cell. Biosynthesis of viral components begins after the virus brings about inactivation of host genes not necessary to viral replication. The virus takes over the machinery of the cell in order to carry out viral DNA replication and production of multiple of the capsid protein subunits. During maturation, viral DNA and capsids are assembled to produce several hundred viral particles. Lysozyme, an enzyme coded for by a viral g