small sections of dna are called what
1 A gene is a small section of DNA that contains the instructions for a specific molecule, usually a protein. 2  The purpose of genes is to store information. 3  Each gene contains the information required to build specific proteins needed in an organism. In humans, alleles of particular genes come in pairs, one on each chromosome (we have 23 pairs of chromosomes). 2  If the alleles of a particular gene are the same, the organism is described as homozygous for that gene. 3  If they are different the organism is described as heterozygous for that gene.