DNA, like protein, is a polymer. It is linear, with a double-helix formation, and is constructed from nucleic acid monomers. DNA stands for deoxyribonucleic acid. It is the molecule that can be thought of as the blueprint from which all proteins are assembled. This important molecule is stored in the nucleus of eukaryotic cells (this is a cell type with membrane-bound organelles such as the nucleus, ribosomes, mitochondria, chloroplasts, and, in green plants, chloroplasts). In prokaryotic cell such as bacteria, it is loose in the cytoplasm of the cell.
DNA has several important structural elements. The double strand is held together by two sugar-phosphate backbones. Attached to these backbones are nitrogenous bases. These bases (adenine, guanine, cytosine, and thymine - abbreviated A, T, C, and G) are the language used by the cell to transfer information from one generation to another, as well as provide the instructions for making proteins. The bases pair in a specific order: A with T and C with G.
Although all genes are present in a cell's DNA, only certain genes are expressed in a given cell. The biochemistry of cellular activity determines when protein products are needed, and under these conditions the cell initiates the process of making proteins.
The genes in the DNA can be thought of like the cookbook for living things - it has the recipes for making all proteins. Just like recipes, and proteins, some are long, others short. To carry the analogy further, each sequence of nucleotides is like a recipe, and each three base codon is like an ingredient.
RNA stands for ribonucleic acid, and like DNA, it is a nucleic acid polymer. It differs from DNA in that it is single stranded, has a ribose sugar in the backbone (instead of a deoxyribose sugar) and uses uracil (U) instead of thymine (T). RNA does not remain in the nucleus of the eukaryotic cell; it is a copy of the DNA that leaves the double-membrane bound nucleus and travels to a cellular organelle called the ribosome. The ribosome can be thought of as the cell's protein factory.
When a cell is in need of a particular protein product the DNA is transcribed. Transcription is the process by which the genetic code on DNA is copied to the RNA, specifically messenger RNA (mRNA) that then carries this message to the ribosome.