Physically, a gene is usually a specific protein-encoding segment of. Dna in a discrete region of a chromosome. Some genes encode various kinds of rna that don"t get translated into protein: genes are divisible, and each gene"s subunits individual nucleotide pairs of dna can mutate independently and can recombine with each other. Everything is in the dna sequence, determining the expression of genes and products that are translated from those genes: any changes to the dna code can affect expression, structure and function, etc. If mutations occur in a somatic cell, it won"t be transmitted to the next generation: only those that occur in the germline will pass on from one generation to the next. Classification of mutations: we can classify mutations according to the way they change dna. Substitution: base is replaced by one of the other 3 bases. If a base is substituted by one of the other 3 bases, it would be a base substitution point mutation.