A gene is a hereditary unit consisting of DNA made up of nucleotides, complementary to each other on opposite sides of the DNA molecule, occupying a specific position (loci) on a chromosome that specifies for a specific characteristic within an organism. Genes are transcribed into mRNA within the cell nucleus and then translated to create a polypeptide chain of a specific sequence that go on to form a specific protein. Alleles are different forms of a gene, and are present in a diploid cell on both of the homologous chromosomes at the exact same loci. They are variants because of mutations; random nitrogenous base sequence changes that lead to different nucleotide sequences. A different base sequence can therefore code for a different codon on the mRNA transcribing molecule, and therefore lead to the binding of a different anticodon on a tRNA molecule, producing an alternate sequence of amino acids during the process of translation on the ribosome. This alternate sequence of amino acids may produce a different protein to the non-mutated form of the gene. In this way, genes are vital coding units for the proteins of the cell.