Genome Copying Errors

Every time a human cell divides, the DNA polymerase enzymes copy over 3 billion base pairs with high fidelity, to be passed on to the daughter cells.

However, due to the sheer volume of information being duplicated, the DNA polymerase sometimes makes errors while copying. For example, an incorrect base may be added to the newly synthesized strand, such as a cytosine instead of thymine.

In some cases, such changes can be harmful, like the single nucleotide change from GAG  to GTG in the beta-hemoglobin gene which causes sickle-shaped red blood cells. This reduces the oxygen-carrying capacity of red blood cells, resulting in sickle cell anemia.

Extra bases may also be added or deleted from the genes. Such mutations are collectively called indels. In cystic fibrosis, a single base added or deleted in the CFTR gene can shift the reading frame such that the gene encodes a defective protein. This protein cannot transport chloride ions out of the lung epithelial cells, resulting in the accumulation of thick, glue-like mucus, which increases the risk of lung infection.

Fortunately, such copying errors occur at low frequency - around 1 error per 100,000 bases.

Additionally, the majority of