Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Nucleic Acids

Nucleic Acids, extremely complex molecules produced by living cells and viruses. Their name comes from their initial isolation from the nuclei of living cells. Certain nucleic acids, however, are found not in the cell nucleus but in cell cytoplasm. Nucleic acids have at least two functions: to pass on hereditary characteristics from one generation to the next, and to trigger the manufacture of specific proteins. How nucleic acids accomplish these functions is the object of some of the most intense and promising research currently under way. The nucleic acids are the fundamental substances of living things, believed by researchers to have first been formed about 3 billion years ago, when the most elementary forms of life began on earth. The origin of the so-called genetic code they carry has been accepted by researchers as being very close in time to the origin of life itself (see Evolution; Genetics). Biochemists have succeeded in deciphering the code, that is, determining how the sequence of nucleic acids dictates the structure of proteins.

The two classes of nucleic acids are the deoxyribonucleic acids (DNA) and the ribonucleic acids (RNA). The backbones of both DNA and RNA molecules are shaped like helical strands. Their molecular weights are in the millions. To the backbones are connected a great number of smaller molecules (side groups) of four different types (see Amino Acids). The sequence of these molecules on the strand determines the code of the particular nucleic acid. This code, in turn, signals the cell how to reproduce either a duplicate of itself or the proteins it requires for survival.

All living cells contain the genetic material DNA. The cells of bacteria may have but one strand of DNA, but such a strand contains all the information needed by the cell in order to reproduce an identical offspring. The cells of mammals contain scores of DNA strands grouped together in chromosomes. In short, the structure of a DNA molecule or combination of DNA molecules determines the shape, form, and function of the offspring. Some viruses, called retroviruses, contain only RNA rather than DNA, but viruses in themselves are generally not considered true living organisms (see Virus).

Certain kinds of RNA have a slightly different function from that of DNA. They take part in the actual synthesis of the proteins a cell produces. This is of particular interest to virologists because many viruses reproduce by “forcing” the host cells to manufacture more viruses. The virus injects its own RNA into the host cell, and the host cell obeys the code of the invading RNA rather than that of its own. Thus the cell produces proteins that are, in fact, viruses instead of the proteins required for cell function. The host cell is destroyed, and the newly formed viruses are free to inject their RNA into other host cells.