Genes are the fundamental units of inheritance in living organisms. Together, they hold all the information necessary to reproduce a given organism and to pass on genetic traits to its offspring.
Biologists have long debated what constitutes a gene in molecular terms but one useful definition is a region of DNA that carries that code necessary to make a molecular chain called a polypeptide. These chains link together to form proteins and so are the bricks and mortar out of which all organism are constructed.
Given this crucial role, it is no surprise that an ongoing goal in biology is to work out the total number of protein-coding genes necessary to construct a given organism. Biologists think the yeast genome contains about 5300 coding genes and a nematode worm genome contains about 20,470.
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