When DNA is transcribed to RNA, many lengths of nu-cleotides that do not code for proteins, called introns, are snipped out of the RNA segments. The segments that remain are spliced together; they code for proteins and are called exons. On a length of freshly transcribed RNA, these exons can be spliced together in different ways to make different proteins. Thus, a single gene can produce a number of products at different times.
Bacteria are tiny single-celled organisms. Their DNA is not stored in a nucleus but is spread around within the cell. Their genome contains no introns, only exons, making them very sleek and compact little critters. Bacteria can behave like social organisms; different varieties both cooperate and compete with each other to find and use resources in their environment. In the wild, bacteria frequently come together to create biofilm “cities”; you may be familiar with these cities from the slime on spoiled vegetables in your refrigerator. Biofilms can also exist in your intestines, your urinary tract, and on your teeth, where they sometimes cause problems, and specialized ecologies of bacteria protect your skin, your mouth, and other areas of your body. Bacteria are extremely important and though some cause disease, many others are necessary to our existence. Some biologists believe that bacteria lie at the root of all life-forms, and that eu-caryotic cells — our own cells, for example — derive from ancient colonies of bacteria. In this sense, we may simply be spaceships for bacteria.
Bacteria swap small circular loops of DNA called plas-mids. Plasmids supplement the bacterial genome and allow them to respond quickly to threats such as antibiotics. Plasraids make up a universal library that bacteria of many different types can use to live more efficiently.
Bacteria and nearly all other organisms can be attacked by viruses. Viruses are very small, generally encapsulated bits of DNA or RNA that cannot reproduce by themselves, Instead, they hijack a cell’s reproductive machinery to make new viruses. In bacteria, the viruses are called bacterio-phages (“eaters of bacteria”) or just phages. Many phages carry genetic material between bacterial hosts, as do some viruses in animals and plants.
It is possible that viruses originally came from segments of DNA within cells that can move around, both inside and between chromosomes. Viruses are essentially roving segments of genetic material that have learned how to “put on space suits” and leave the cell.
SHORT GLOSSARY OF SCIENTIFIC TERMS
Amino acid: building block for proteins. Most living things use only twenty amino acids. Antibody: molecule that attaches to an antigen, inactivates it, and attracts other defenses to the intruder.
Antibiotics: a large class of substances manufactured by many different kinds of organisms that can kill bacteria. Antibiotics have no effect on viruses.
Antigen: intruding substance or part of an organism that provokes the creation of antibodies as part of an immune response.
Bacteria: procaryotes, tiny living cells whose genetic material is not enclosed in a nucleus. Bacteria perform much important work in nature and are the base of all food-chains.
Bacteriophage: seephage. Bacteriocin: one of many substances created by bacteria that can kill other bacteria.
Chromosome: arrangement of tightly packed and coiled DNA. Diploid cells such as body cells in humans have two sets of twenty-three chromosomes; haploid cells such as gametes — sperm or ova — have only a single set of chromosomes.
Cro-Magnon: early variety of modern human, Homo sapiens sapiens, from Cro-Magnon in France. Homo is the genus, sapiens the species, sapiens the subspecies.
DNA: deoxyribonucleic acid, the famous double-helix molecule that codes for the proteins and other elements that help construct the phenotype or body structure of an organism.
ERV or endogenous retrovirus: virus that inserts its genetic material into the DNA of a host. The integrated provirus lies dormant for a time. ERVs may be quite ancient and fragmentary and no longer capable of producing infectious viruses.
Exon: regions of DNA that code for proteins or RNA.
Gamete: a sex cell, such as egg or sperm, capable of joining with an opposite gamete — egg plus sperm — to make a zygote.
Gene: The definition of a gene is changing. A recent text defines a gene as “a segment of DNA or RNA that performs a specific function.” More particularly, a gene can be thought of as a segment of DNA that codes for some molecular product, very often a protein. Besides the nucleo-tides that code for the protein, the gene also consists of segments that determine how much and what kind of protein is expressed, and when. Genes can produce different combinations of proteins under different stimuli. In a very real sense, a gene is a tiny factory and computer within a much larger factory-computer, the genome.
Genome: sum total of genetic material in an individual organism.
Genotype: the genetic character of an organism or distinctive group of organisms.
HERV or human endogenous retrovirus: Within our genetic material are many remnants of past infections by retroviruses. Some researchers estimate that as much as one third of the sum total of our genetic material may consist of old retroviruses. No instance is yet known of these ancient viral genes producing infectious particles (virions) that can move from cell to cell, in lateral or horizontal transmission. Many HERV do produce viruslike particles within the cell, however, and whether these particles serve a function or cause problems is not yet known. All HERV are part of our genome and are transmitted vertically when we reproduce, from parent to offspring. Infection of gametes by retroviruses is the best explanation so far for the presence of HERV in our genome. (ERV, endogenous retrovirus, are found in many other organisms, as well.)
Homosome: the complete complement of usable genetic material both inside and outside a cell or organism. Bacteria exchange circular loops of DNA called plasmids and may have some genes carried by lysogenic phages; this total pool of genetic material constitutes the bacterial homosome.
Immune response (immunity, immunization): the provoking and marshaling of defensive cells within an organism to ward off and destroy pathogens, disease-causing organisms such as viruses or bacteria. Immune response may also identify nonpathogenic cells as foreign, not part of the normal body complement of tissues; transplanted organs cause an immune response and may be rejected.
Intron: regions of DNA that do not generally code for proteins. In most eucaryotic cells, genes consist of mingled exons and introns. Introns are clipped out of transcribed messenger RNA (mRNA) before it is processed by ribo-somes; ribosomes use the code contained in lengths of mRNA to assemble specific proteins out of amino acids. Bacteria lack introns.
Lysogenic phage: phage that attaches to a bacterial capsule and inserts genetic material into the bacterial host, where it then forms a circular loop, integrates with the host DNA, and lies dormant for a time. During this stage, the host bacterium reproduces the prophage or integrated phage genome with its own. Damage or “stress” to a host bacterium may result in the transcription of the phage genes, which then replicate new phages, releasing them by lysing or breaking open the host. In this stage, they are called lytic phage. Lysogenic/lytic phages may also transcribe and carry host genes, along with their own, from one bacterium to another. Many bacteria that cause severe disease in humans, such as cholera, can have their toxicity triggered by the transfer of genetic material by lysogenic phages. Such phages, understandably, are dangerous in their natural form and useless in controlling bacterial pathogens.