Darwin's children

While most cells in the body of an individual carry identical DNA, as the person grows and develops, that DNA is expressed in different ways within each cell. This is how identical embryonic cells become different tissues.

When DNA is transcribed to RNA, many lengths of nucleotides 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.

Bacteriaare 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 biofilms; you may be familiar with these bacterial “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 eukaryotic 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 plasmids.Plasmids supplement the bacterial genome and allow them to respond quickly to threats such as antibiotics. Plasmids 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 ca

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

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:prokaryotes, tiny living cells whose genetic material is not enclosed in a nucleus. Bacteria perform important work in nature and are the base of all food chains.

Bacteriophage:see phage.

Chromosome:arrangement of tightly packed and coiled DNA. Diploid cells such as body cells in humans have two sets of twenty-two autosomesas well as two sex chromosomes; haploid cells such as gametes—sperm or ova—have only a single set of chromosomes. The total number of chromosomes varies between apes and humans. Chromosome numbers for so-called ancestral species such as Homo sapiens neandertalensisand Homo erectusare not known; any DNA extracted from even relatively recent (~20,000 years) fossil specimens is generally limited to mitochondrial DNA. Polyploidy—having extra sets of chromosomes—results in infertile offspring or totally precludes reproduction between organisms and can often define a barrier between species. This should prevent successful mating between SHEVA individuals and older variety humans. Apparently, it does not. This puzzles scientists, and further research is in order.

Cro-Magnon:early variety of modern human, Homo sapiens sapiens,from Cro-Magnon in France. Homois the genus, sapiensthe species, sapiensthe subspecies.

DNA:Deoxyribonucleic acid, the famous double-helix molecule that codes for the proteins and other elements that help construct the phenotypeor body structure of an organism.

ERV or endogenous retrovirus:virus that inserts its genetic material into the DNA of a host. The integrated proviruslies dormant for a time. ERVs may be quite ancient and fragmentary and no longer capable of producing infectious viruses.

Exogenous virus:virus that does not insert its genes into host DNA on a long-term basis. Some viruses, such as MMTV or mouse mammary tumor virus, seem to be able to choose whether to insert or not insert their genetic code into host DNA. See ERV.

Exon:region of DNA that codes for proteins or RNA.

Frithing:also, flehman. Sucking air over the vomeronasal organ to detect pheromones. See vomeronasal organ.

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 one or more proteins or parts of proteins. Besides the nucleotides 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. In humans, the genome appears to consist of approximately thirty thousand genes—half to one-third the number predicted at the time of the publication of Darwin's Radio.

Genotype:the genetic character of an organism or distinctive group of organisms.

Glycome:the total complement of sugars and related compounds in a cell. Sugars can form links with proteins and lipids to make glycoproteins and glycolipids.

Herpes:HSV-1 or -2. Herpes simplex virus types responsible for cold sores and genital herpes. Though herpes viruses are not retroviruses they can lie dormant in nerve cells for years, and often reactivate in response to stress. Chicken pox and its recurrent form, shingles, or herpes zoster, are also related to herpes.

HERV: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 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 host to host, in lateralor horizontal transmission.Many HERV do produce viruslike particles within the cells and body, 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 verticallywhen 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 retroviruses, are found in many other organisms, as well.

Homo erectus:general classification for fossils of the genus Homodated chronologically and evolutionarily prior to Homo sapiens. Homo erectuswas a very successful human species, surviving for at least a million years. Calling any of these fossils “ancestral” is problematic both scientifically and philosophically, but it's a simple and easily understood description of a complex relationship. There are many interpretations of these relationships in the literature, but growing sophistication in genetics will probably lead to a general shaking out and clarification over the next ten to twenty years.