Children resemble their parents because of inherited traits, but what actually passes from parents to a child is genetic information, in the form of DNA molecules from the parents' sex cells. As an offspring develops, mitosis passes the information on to new cells. Genetic information "tells" cells how to construct a great variety of protein molecules, each with a specific function. The portion of DNA molecule that contains the genetic information for making a particular protein is called a gene.
All of the DNA in a cell constitutes the genome. Researchers began to decipher genome sequences in 1995, and added humans to the list in 2000. Not all of the human genome encodes protein-functions of many DNA sequences are not known.
Nucleotides are the building blocks of nucleic acids. A nucleotide consists of a 5-carbon sugar (ribose or deoxyribose), a phosphate group, and one of several organic, nitrogen-containing (nitrogenous) bases. DNA and RNA nucleotides form long strands (polynucleotide chains) by alternately joining their sugar and phosphate portions, which provides a "backbone" structure.
A DNA molecule consists of two polynucleotide chains. The nitrogenous bases project from the sugar-phosphate backbone of one strand and bind, or pair, by hydrogen bonds to the nitrogenous bases of the second strand. The resulting structure is somewhat like a ladder, in which the uprights represent the sugar and phosphate backbones of the two strands and the rungs represent the paired nitrogenous bases. Notice that the sugars forming the two backbones point in opposite directions. For this reason, the two strands are called antiparallel.
A DNA molecule is sleek and symmetrical because the bases pair in only two combinations, maintaining a constant width of the overall structure. In a DNA nucleotide, the base may be one of four types: adenine, thymine, cytosine, or guanine. Adenine, a two-ring structure, binds to thymine, a single-ring structure. Guanine, a two-ring structure, binds to cytosine, a single-ring structure. These pairs-A with T, and G with C-are called complementary base pairs. Because of this phenomenon, the sequence of one DNA strand can always be derived form the other by following the "base pairing rules." For example, if the sequence of one strand of the DNA molecule is G, A, C, T, then the complementary strand's sequence is C, T, G, A. (The sequence of bases in one of these strands encodes the instructions for making a protein.
The double-stranded DNA molecule twists to form a double helix, resembling a spiral staircase. An individual DNA molecule may be several million base pairs long. Investigators can use DNA sequences to identify individuals.