While DNA strands are largely continuous, different regions of our DNA encodes specific genes. Genes are stretches of DNA sequence that encode instructions for the structure, function, and regulation of the body’s cells, tissues and organs. Genes are generally the most important parts of a DNA sequence.
Most genes encode proteins, which are a group of complex molecules that perform critical roles in our cells and bodies. However, the entire length of a gene does not encode a protein. Rather, proteins are encoded by subsets of a gene’s length; these portions of a gene are called “exons” or the “coding regions” (see What is an exome?).
The current estimate is that the human genome, which is all the DNA sequences in our cells (see What is a genome?), contains approximately 25,000 genes. In humans, genes vary in size from a few hundred DNA bases to more than 2 million base pairs.
Although each cell contains a full complement of DNA, different kinds of cells utilize specific genes. For example, heart cells have different active genes compared with brain cells because each cell type performs different functions and, therefore, require different proteins. Different genes can also be turned on during development or in response to environmental factors such as infection or stress. The interactions between genes and the proteins they encode can be complex and not all genes have a currently known function. Understanding how different genes function and interact is an area of ongoing research.
While most DNA sequences are identical between people, there are some differences (see What is a variant?). Differences in DNA sequences within a gene can result in different versions of the gene, which are called alleles. Generally, each person inherits one allele of every gene from each parent, resulting in each person having two copies of every gene (see How are genes inherited?). Some alleles result in no changes to the function of a gene, some contribute to the variations that make each human unique, and some are linked with disease. It can sometimes be difficult to predict how a particular allele will impact gene function.