Darwin's Children

A SHORT BIOLOGICAL PRIMER

Humans are metazoans, that is, we are made up of many cells. In most of our cells there is a nucleus that contains the “blueprint” for the entire individual. This blueprint is stored in DNA, deoxyribonucleic acid; DNA and its complement of helper proteins and organelles make up the molecular computer that contains the instructions necessary to construct an individual organism.

Proteins are molecular machines that can perform incredibly complicated functions. They are the engines of life; DNA is the template that guides the manufacture of those engines.

DNA in eukaryotic cells is arranged in two interwoven strands—the “double helix”—and packed tightly into a complex structure called chromatin, which is arranged into chromosomes in each cell nucleus. With a few exceptions, such as red blood cells and specialized immune cells, the DNA in each cell of the human body is complete and identical. Researchers currently estimate that the human genome—the complete collection of genetic instructions—consists of approximately thirty thousand genes. Genes are heritable traits; a gene has often been defined as a segment of DNA that contains the code for a protein or proteins. This code can be transcribed to make a strand of RNA, ribonucleic acid; ribosomes then use the RNA to translate the original DNA instructions and synthesize proteins. Some genes perform other functions, such as making the RNA constituents of ribosomes.

Many scientists believe that RNA was the original coding molecule of life, and that DNA is a later elaboration.

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.

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 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 cannot reproduce by themselves, Instead, they hijack a cell’s reproductive machinery to make new viruses. In bacteria, the viruses are called bacteriophages, (“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.