In biology, a gene is a sequence of nucleotides in DNA or RNA that encodes the synthesis of a gene product, either RNA or protein. During gene expression, the DNA is first copied into RNA. The RNA can be directly functional or be the intermediate template for a protein that performs a function.

A gene is the basic physical and functional unit of heredity. Genes are made up of DNA. Some genes act as instructions to make molecules called proteins. However, many genes do not code for proteins. In humans, genes vary in size from a few hundred DNA bases to more than 2 million bases. The Human Genome Project estimated that humans have between 20,000 and 25,000 genes.

Every person has two copies of each gene, one inherited from each parent. Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

Scientists keep track of genes by giving them unique names. Because gene names can be long, genes are also assigned symbols, which are short combinations of letters (and sometimes numbers) that represent an abbreviated version of the gene name. For example, a gene on chromosome 7 that has been associated with cystic fibrosis is called the cystic fibrosis transmembrane conductance regulator; its symbol is CFTR.

The DNA in each chromosome constitutes many genes. The DNA also contains large sequences that do not code for any protein and their function is not known. The gene of the coding region encodes instructions that allow a cell to produce a specific protein or enzyme. There are nearly 50,000 and 100,000 genes with each being made up of hundreds of thousands of chemical bases.

In order to make proteins, the gene from the DNA is coped by each of the chemical bases into messenger RNA (ribonucleic acid) or mRNA. The mRNA moves out of the nucleus and uses cell organelles in the cytoplasm called ribosomes to form the polypeptide or amino acid that finally folds and configures to form the protein.

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