The continuity of life is based on heritable information in the form of DNA

Inside the dividing cell in Figure 1.7 (on the previous page), you can see structures called chromosomes, which are stained with a blue-glowing dye. The chromosomes have almost all of the cell's genetic material, its DNA (short for deoxyribonucleic acid). DNA is the substance of genes, the units of inheritance that transmit information from parents to offspring.Your blood group (A, B, AB, or 0), for example, is the result of certain genes that you inherited from your parents.
DNA Structure and Function.
Each chromosome has one very long DNA molecule, with hundreds or thousands of genes arranged along its length. The DNA of chromosomes replicates as a cell prepares to divide, and each ofthe two cellular offspring inherits a complete set of genes.
Each of us began life as a single cell stocked with DNA inherited from our parents. Replication of that DNA with each round of cell division transmitted copies of it to our trillions of cells. In each cell, the genes along the length of the DNA molecules encode the information for building the cell's other molecules. In this way, DNA controls the development and maintenance ofthe entire organism and, indirectly, everything it does (Figure 1). The DNA serves as a central database.
The molecular structure ofDNA accounts for its ability to store information. Each DNA molecule is made up of two long chains arranged in a double helix. Each chain link is one of four kinds of chemical building blocks called nucleotides (Figure 2). The way DNA encodes information is analogous to the waywe arrange the letters ofthe alphabet into precise sequences with specific meanings. The word rat, for example, evokes a rodent; the words tar and art, which contain the same letters, mean very different things. Libraries are filled with books containing information encoded in varying sequences of only 26 letters. We can think of nucleotides as the alphabet of inheritance. Specific sequential arrangements of these four chemical letters encode the precise information in genes, which are typically hundreds or thousands of nucleotides long. One gene in a bacterial cell may be translated as "Build a certain component of the cell membrane. A particular human gene may mean "Make growth hormone".
More generally, genes like those just mentioned program the cell's production oflarge molecules called proteins. Other human proteins include a muscle cell's contraction proteins and the defensive proteins called antibodies. A class of proteins crucial to all cells are enzymes, which catalyze (speed up) specific chemical reactions. Thus, DNA provides the blueprints, and proteins serve as the tools that actually build and maintain the cell and carry out its activities.
The DNA of genes controls protein production indirectly, using a related kind ofmolecule called RNA as an intermediary.
The sequence of nucleotides along a gene is transcribed into RNA, which is then translated into a specific protein with a unique shape and function. In the translation process, all forms of life employ essentially the same genetic code. A particular sequence of nucleotides says the same thing to one organism as it does to another. Differences between organisms reflect differences between their nucleotide sequences.
Not all RNA in the cell is translated into protein. We have known for decades that some types ofRNA molecules are actually components of the cellular machinery that manufactures proteins. Recently, scientists have discovered whole new classes of RNA that play other roles in the cell, such as regulating the functioning of protein-coding genes.
The entire "library" of genetic instructions that an organism inherits is called its genome. A typical human cell has two similar sets of chromosomes, and each set has DNA totaling about 3 billion nucleotides. If the one-letter symbols for these nucleotides were written in letters the size of those you are now reading, the genetic text would fill about 600 books the size of this one. Within this genomic library of nucleotide sequences are genes for about 75,000 kinds of proteins and an as yet unknown number of RNA molecules.


Inherited DNA directs development of an organism.


(a)DNA double helix. This mcxlel shows each atom in a segment of DNA. Made up of two long chains of bUilding blocks called nucleotides, a DNA molecule takes the three-dimensional form of adouble helix.
(b)Single strand of DNA. These geometric shapes and letters are simple symbols for the nucleotides in asmall section of one chain of a DNA molecule Genetic information is encoded in specific sequences of the four types of nucleotides. (Their names are abbreviated here as A. T, C, and G.)