When stained with chemical dyes for examination under a microscope, cancer cells usually show enlarged nuclei and often more than one nucleolus, a spherical body within the nucleus. The cytoplasm, the major portion of a cell, is the protoplasm that lies outside the nucleus. Normal cells from a given tissue tend to be uniform in size and shape, whereas cancer cells are variable in these characteristics. Most cancer cells tend to resemble their tissue of origin at first; eventually the descendants of these cells may lose their original cytoplasmic markings and functions. The cells are then said to be "dedifferentiated," or more primitive than those in the original tissue. In general, the less differentiated the cancer cells, the greater is their malignancy.

Located in the nucleus is a network of chromatin granules, which in preparation for cell division arrange themselves into discrete structural units called chromosomes; in cells prepared for microscopic examination the chromatin is stained more deeply in cancer cells than in normal cells. Functional segments of the chromosomes, numbering 46 per body cell in man, are the genes, of which there are millions in an animal or human cell.

Genes, the biologic units of heredity, are composed of deoxyribonucleic acid (DNA), which contains genetic information coded into the chemical structure. DNA has the ability to reproduce itself; thus, the genetic information is passed from one generation to the next, and it determines exactly what an individual organism will be like. The DNA also directs the life processes of a cell; the most important product is proteins, the basic structural and functional matter of living things. This function is accomplished through the intermediate action of another nucleic acid, ribonucleic acid (RNA). Most drugs effective against cancer act on sequences concerned with nucleic acid synthesis and metabolism, the sum of all the life processes, including growth and repair of tissues and release of energy for maintenance and function of an organism.

The muter molecule of life, DNA

The muter molecule of life, DNA, is the chemical of the chromosomes, which transmit hereditary characteristics. The molecule consists of 2 intertwining spiral strands of sugar and phosphate connected by 4 paired bases. The DNA bases are adenine <A) paired with thymine T . and guanine (G) paired with cytosine (C).

Coded instructions in the master nucleic acid

Coded instructions in the "master" nucleic acid, DNA, direct synthesis of a companion nucleic acid, RNA, or ribonucleic acid. Formed against the DNA template, RNA is a single-stranded molecule with its own four-base code. RNA has a ribose sugar component and the bases adenine (A), guanine (G), cytosine (C), and uracil (U).

The DNA molecule, the "master" molecule of life, is a double-stranded chain arranged in a helical structure and consisting of hundreds of thousands of paired units, or nucleotides. Each nucleotide contains a sugar component (de-oxyribose), a phosphoric acid component, and either a purine base (adenine or guanine) or a pyrimidine base (thymine or cytosine). The four units of DNA are always paired in the same way: nucleotides with the adenine base with those containing the thymine base; guanine with cytosine. Each strand of DNA is complementary with the other in the double molecule.

Just before cell division, the double molecule "unzips," that is, the two chains separate, and each strand serves as a template for the synthesis of its complementary mate from free nucleotide units present in the cell, creating two double molecules. At cell division, each daughter cell receives one of the original strands of DNA and one that is newly synthesized. The genetic code of the cell is determined by the sequence arrangements of the four bases in its DNA.

Proteins

Proteins (compounds consisting of amino acids ) are needed for the body's growth and functioning. They are made primarily in the cell's cytoplasm at protein-manufacturing centers called ribosomes. It is believed that, as a ribosome travels along a strand of messenger RNA, each "triplet" of RNA bases signals transfer-RNA to bring a specific amino acid to the ribosome. There the amino acids are joined to form protein.

The DNA molecule also directs the synthesis of the other known type of nucleic acid, RNA, which is a single-stranded molecule with its own four-base code. The nucleotides of RNA have a ribose sugar component instead of the deoxy-ribose of DNA and a pyrimidine base, uracil, in place of thymine.

The sequence of nucleotides in messenger RNA (mRNA) is determined by one of the two chains of DNA acting as a template in much the same manner in which a new DNA chain is constructed alongside an old one. Each segment of DNA that can be defined as a gene gives rise to its own specific mRNA. In a cell, mRNA moves from the nucleus to the cytoplasm where each molecule becomes associated with a number of ribosomes, preparatory to protein synthesis.

Ribosomes are submicroscopic structures containing protein and nonspecific structural RNA.

In combination with ribosomal RNA, mRNA molecules serve as templates against which amino acids are lined up in a sequence corresponding to the coded instructions in the mRNA. Amino acids in the cell are transported to the ribosomes by a third type of RNA, transfer RNA (tRNA), and are aligned at their proper sites in a protein chain under construction at the surface of a ribosome. The protein is then stripped off the ribosome and released into the cell. The biological properties of the cell are determined by the amino acid sequences of its proteins.

Some genes act as regulators that turn the protein manufacturing genes "on" or "off." This is part of a complex system of control that enables the cell to carry out life processes in an orderly fashion.