The total amount of DNA in an organism is termed its genome. The genome of each species contains a unique arrangement of genes which define its appearance (phenotype), metabolic activities, and pattern of reproduction.

The genome of procaryotes is quite small compared to eucaryotes. Bacterial DNA consists of a few thousand genes in one circular chromosome. Eucaryotic genomes range from thousands to hundreds of thousands of genes. Their DNA is packaged in tightly wound spirals arranged in discrete chromosomes.

DNA copies itself just prior to cellular division by the process of semiconservative. Semiconservative replication means that each ``old'' DNA strand is the template upon which each ``new'' strand is synthesized.

The circular bacterial chromosome is replicated at two forks as directed by DNA polymerase. At each fork, two new strands are synthesized; one continuously and one in short fragments.

Application of the DNA Code: Transcription and Translation
Information in DNA genes is converted to actual substances by the process of transcription and translation. The substances produced are usually, but not always, enzymes. Enzymes, in turn, control an organism's metabolic activities.

DNA triplets specify the same mRNA codons, regardless of the species in which they occur. The language of DNA and mRNA extends into the noncellular world of viruses as well.

The processes of transcription and translation are similar but not identical for procaryotes and eucaryotes. Eucaryotes transcribe DNA in the nucleus, remove its introns, and translate it in the rough ER. Bacteria transcribe and translate simultaneously because there are no introns present. Polyribosomes attach themselves directly to the mRNA as it is released by its DNA template.

Viruses replicate by utilizing the transcription and translation processes of cellular organisms. They invade host cells and ``force'' them to transcribe and translate viral genes into new virus particles. In some cases, viruses connect themselves to the host genome and are passed on to its progeny, some of which will become virus factories many generations later.

The genetic material of viruses can be either DNA or RNA occurring in single, double, sense, or anti-sense strands. Viral DNA contains just enough information to force a cell to become a virus factory.

Genetic Regulation of Protein Synthesis and Metabolism
Gene expression must be orchestrated to coordinate the organism's needs with nutritional resources. Genes can be turned ``on'' and ``off'' by specific molecules, which expose or hide their nucleotide codes for transcribing proteins. Most, but not all, of these proteins are enzymes.

Operons are the gene sequences in bacteria which code for specific proteins (usually enzymes). They are controlled by regulator genes. Nutrients may combine with regulator gene products to turn a set of structural genes on (inducible genes) or off (repressible genes). The LAC operon is an example of an inducible operon. The ARG operon is an example of a repressible operon.

The rifamycins, tetracyclines, and aminoglycosides are classes of antibiotics that are effective because they interfere with transcription and translation processes in microorganisms.

Changes in the Genetic Code: Mutations, Intermicrobial Exchange, and Recombination
Changes in the genetic code can occur by two means: mutation and recombination. Mutation means a change in the nucleotide sequence of the organism's genome. Recombination means the addition of genes from an outside source, such as a virus or another cell.

Mutations can be either spontaneous or induced by exposure to some external mutagenic agent.

All cells have enzymes which repair damaged DNA. When the degree of damage exceeds the ability of the enzymes to make repairs, mutations occur.

Mutation-induced changes in DNA nucleotide sequencing range from a single nucleotide to addition or deletion of large sections of genetic material.

The Ames test is used to identify potential carcinogens based on their ability to cause back mutations in bacteria.

Genetic recombination occurs in eucaryotes through sexual reproduction. In bacteria, recombination occurs through the processes of translation, conjugation, transduction, and the incorporation of lysogenic viruses into the genome.