Nucleic acids. Nucleic acids are polymeric molecules in which the repeating units are nucleotides. Cells contain two kinds of nucleic acids - deoxyribonucleic acids (DNA) and ribonucleic acids (RNA). The major biological functions of DNA and RNA are, respectively, transfer of genetic information and synthesis of proteins.

Nucleotides. Nucleotides, the monomers of nucleic acid polymers, are molecules composed of a pentose sugar bonded to both a phosphate group and a nitrogen-containing heterocyclic base. The pentose sugar must be either ribose or deoxyribose. Five nitrogen-containing bases are found in nucleotides: adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U).

Primary structure of nucleic acids. The "backbone" of a nucleic acid molecule is a constant alternating sequence of sugar and phosphate groups. Each sugar unit has a nitrogen-containing base attached to it.

Secondary structure of DNA. A DNA molecule exists as two polynucleotide chains coiled around each other in a double helix arrangement. The double helix is held together by hydrogen bonding between complementary pairs of bases. Only two base-pairing combinations occur: A with T, and C with G.

DNA replication. DNA replication occurs when the two strands of a parent DNA double helix separate and act as templates for the synthesis of new chains using the principle of complementary base pairing.

Chromosome. A chromosome is a cell structure that consists of an individual DNA molecule bound to a group of proteins.

RNA molecules. Four important types of RNA molecules, distinguished by their function, are ribosomal RNA (rRNA), messenger RNA (mRNA), primary transcript RNA (ptRNA), and transfer RNA (tRNA).

Transcription. Transcription is the process in which the genetic information encoded in the base sequence of DNA is copied into RNA molecules.

Complementary bases. Complementary bases are specific pairs of bases in nucleic acid structures that hydrogen-bond to each other.

Gene. A gene is a segment of a DNA molecule that contains the base sequence for the production of specific RNA molecules, which in turn guide the production of specific protein molecules. Genes are segmented, with portions called exons that contain genetic information and portions called introns that do not convey genetic information.

Codon. A codon is a three-nucleotide sequence in mRNA that codes for a specific amino acid needed during the process of protein synthesis.

Genetic code. The genetic code consists of all the mRNA codons that specify either a particular amino acid or the termination of protein synthesis.

Anticodon. An anticodon is a three-nucleotide sequence in tRNA that binds to a complementary sequence (a codon) in mRNA.

Translation. Translation is the stage of protein synthesis in which the codons in mRNA are translated into amino acid sequences of new proteins. Translation involves interactions between the codons of mRNA and the anticodons of tRNA.

Recombinant DNA. Recombinant DNA molecules are synthesized by splicing a segment of DNA, usually a gene, from one organism into the DNA of another organism.