DNA is generally in the form of a double helix. Bases that are opposite each other generally pair - A with T, and G with C. If the two bases do not match, mismatch correction processes attempt to repair the mismatch. Various mechanisms exist to help ensure that the correction goes in the right direction. For instance, methylated cytosine may be deaminated to thymine, giving an G-T pair. Enzymes exist that preferentially remove the T from this pairing, allowing a new C to be inserted. Another mechanism is used in DNA replication. In its natural state, the DNA duplex is methylated at various locations on both strands. Immediately after replication, the newly formed strand in each helix will be unmethylated. Therefore, any mismatch in the new strands will lead to the base on the unmethylated strand being removed as it is likely to be due to a mistake in replication. Without mismatch correction, the mutation rate increases significantly.

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