There are many different definitions of a gene but when discussing genomes and molecular biology the best definition is:
A gene is a DNA sequence that is transcribed to produce a functional product.
The key parts of this definition are that a molecular gene is a transcription unit so introns are part of the gene but regulatory sequences are not. The functional product of the gene is an RNA molecule and that can be a messenger RNA (mRNA), in which case the gene is a protein-coding gene, or it can be a functional RNA such as ribosomal RNA or miRNA, in which case the gene is a noncoding gene.
The human genome contains somewhere between 19,000 and 20,000 protein coding genes and an unknown number of noncoding genes. At the present time there are less than 5,000 noncoding genes with a known biological function but many sources claim that there are tens of thousands of noncoding genes.
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