A New History of Life Read Free Page B

information of any cell. They are giant molecules that combine sugars to nitrogen-containing compounds called nucleotides, themselves formed from subunits called bases, phosphorus, and more sugars. In this arrangement the bases are crucial, for they become the “letters” in the genetic code.
    DNA and RNA are sugars that are among the most important of all molecules of life. DNA, composed of two backbones (the famous double helix described by its discoverers, James Watson and Francis Crick), is the information storage system of life itself. These two spirals are bound together by a series of projections, like steps on ladder, made up of the distinctive DNA bases, or base pairs: adenine, cytosine, guanine, and thymine. The term “base pair” comes from the fact that the bases always join up: cytosine always pairs with guanine, and thymine always joins with adenine. The order of base pairs suppliesthe language of life: these are the genes that code for all information about a particular life form.
    If DNA is the information carrier, a single-stranded variant called RNA is its slave, a molecule that translates information into action—or in life’s case, into the actual production of proteins. RNA molecules are similar to DNA in having a helix and bases. But they differ in usually (but not always) having but a single strand, or helix, rather than the double helix of DNA.
    Why the enormous complexity of DNA and RNA? The answer lies in the need for information to first build (blueprints) and then maintain the many tasks that staying alive requires. DNA is the blueprint, instruction manual, repair manual, and directions for building copies of itself and all that it codes for. In computer terms, DNA is the software, in that it carries information but cannot itself act on the information. Proteins can be thought of as the computer’s hardware, needing the DNA software to provide information of when and where specific chemical changes should occur in time and space, and to produce material necessary for life. RNA has the interesting characteristic of being either hardware or software, and in some cases both at the same time.
    Proteins, the last building blocks, perform four functions in Earth life: building other large molecules, repairing other molecules, transporting material about, and securing energy supplies. Proteins also modify both large and small molecules for a variety of purposes and are involved in cell signaling. There are a huge number of different proteins, and we are only now learning how these work and what they do. A new insight is that their topology, or folding pattern, is as important to their function as their chemical makeup.
    All proteins used in Earth life are formed from the assembly of the same twenty amino acids. A new twenty-first-century area of research is asking an old problem: are these same twenty used because they are the best building blocks out there—or because they were common where life was first forming and then became permanently “coded” into life? In fact it looks like it is the former; they work thebest, at least according to research in 2010. 11 This group is specific to Earth, and perhaps diagnostic of Earth life.
    Proteins are constructed in the cell by stringing together the various amino acids in a long, linear chain that folds into its final shape only when all its amino acids have been joined together. Sometimes they fold as they are still being synthesized. Because the assembly of amino acids into a protein is done one at a time in linear and specific order, that protein is often analogized to a written sentence, each amino acid being a word. Within its cell walls, a living cell is packed with molecules, arranged in rods, balls, and sheets, all floating in a salty gel. There are about a thousand nucleic acids and over three thousand different proteins. All of these are going about some sort of chemistry that combined makes up the process we call life. Many chemical