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DNA replication occurs constantly in humans, however sometimes errors occur. Small errors can lead to huge consequences due to the many different possible altered structures of proteins. RNA is used to make proteins from the DNA code. Each nucleotide in DNA is replicated by the mRNA. All errors in the sequencing occur in the DNA. Errors include insertion or deletion and base pair substitution. Insertion or deletion is the inserting or deleting of a nucleotide in the sequence resulting in the codons to be matched up in completely different groups. In this case, the end amino acid may occur early or may occur late or not at all causing useless proteins to form. Sometimes 3 codons will be inserted or deleted, resulting in the rest of the strand being preserved. This may or may not result in a different protein being made. Base pair substitution is when one nucleotide is instead replaced by another, causing a different codon sequence. The wobble effect utilizes the fact that different codons may form the same amino acid; thus, sometimes changing the nucleotide results in the exact same amino acid. This is called a silent replication error, since though the error occurred, it would not be visible because the same amino acid was formed causing no disruption in the forming of the polypeptide. In other cases, one nucleotide or more is changed causing a different amino acid or amino acids to form which may or may not result in visible effects.
DNA polymerase is generally very good at catching errors with its 3'-to-5' exonuclease but sometimes errors slip through. These errors are generally because of tautomers of the nucleotide slipping into the active site of the polymerase. The tautomers have a different major groove/minor groove binding pattern so the polymerase identifies it as a base different than what it actually is. Once incorporated, the bases commonly spontaneously reverse tautomerization. In other cases, RNA will slip into the active site or other bases will spontaneously deaminate.