DNA to Protein
Inside the cell, proteins are the molecules that get things done. If the cell were a mafia ring, DNA would be the don and proteins would be the hitmen. The formula for each protein is built-in to the DNA sequence in groups called genes. Transcription is the first step in gene expression. During this process, one DNA strand (the template strand) is copied into RNA. Once DNA has been transcribed into messenger RNA (mRNA), the mRNA strand is processed, exported from the nucleus and translated into protein by the ribosome with the help of transfer RNA (tRNA).
The mRNA strand is read in groups of 3 nucleotides, called codons. A codon specifies which amino acid should be added to the peptide chain. Each amino acid has several codons that code for it; this can be a safeguard against mutations. Translation begins at the start codon, AUG (which codes for the amino acid methionine), a proceeds in a triplet pattern until one of three stop codons is encountered.
The newly formed protein is then subject to additional quality-control checks and post-translational modifications in the endoplasmic reticulum and golgi apparatus. The start codon sets the reading frame for translation. If insertion or deletion mutations are created in a multiple of three, translation will proceed "in-frame," although one or several amino acids will be added or deleted depending on the type of mutation.
An insertion or deletion that does not occur in a multiple of three is a mutation that changes the reading frame of a gene. Translation will continue "out of frame" from the point of the mutation on. For example, an extra "a" added to "The cat ate the rat," turns the sentence into "the caa tat eth era t," if we keep the letters grouped in threes. Suddenly nothing makes sense because some words do not appear as they should.
The same thing happens on the peptide level: amino acids are incorporated into the peptide that should not be there. Frameshift mutations can create an early stop codon and result in a truncated protein, or eliminate a stop codon and the result in an abnormally long protein. Either way, the protein will mostly likely be non-functional.