DNA is made up of the nucleotide bases guanine (G), adenine (A), thymine (T), and cytosine (C). These four nucleotides occur in pairs, called base pairs, along the DNA strand. When DNA is replicated, copy errors can occur in these bases. For example, C might be inadvertently substituted for T. These small copy errors are called mutations. Depending on the type of mutation, these mutations are either transversions, or transitions.
DNA is made of two parallel polymer chains that wind around each other to form a double helix. These two strands are connected to each other like rungs on a ladder by base pairs, and each rung is designated by a position like a coordinate on a map. For example, Position 1234 might have a pair of AT, with A on the forward strand and T on the reverse strand.
To make things easier, DNA tests report each location with the nucleotide attached to the forward strand only. For example, if a pair of AT has A on the forward strand it will be listed as A. This is called the value of that position. We can then compare the value on your strand to the reference value to track mutations.
On a chemical level, Adenine and Guanine are double-ringed shapes called purines, whereas Cytosine and Thymine are single-ringed shapes called pyrimidines. Because of their shapes, purines pair with pyrimidines. Specifically, the purine adenine (A) pairs with the pyrimidine thymine (T) and the purine guanine (G) pairs with the pyrimidine cytosine ( C ).
When a purine is exchanged for a pyrimidine, (A to T for example) it is called a transversion. This will result in a change from an AT pair (with A on the forward strand) to a TA pair (with T on the forward strand). Because the nucleotide found on the forward strand is the value for each position, this will change the value from A to T. This is typically called an A to T mutation.
The other types of transversions are a change of A <-> C, G <-> T, G <-> C and T <-> A
When a purine is exchanged for another purine, this is called a transition. This also applies when a pyrimidine is exchanged for another pyrimidine. For example, the purine A could be substituted with the purine G or vice versa, or the pyrimidine C could be substituted with the pyrimidine T. Other possible substitutions are T to C and G to A.
Although there are more potential transversion combinations than transitions, transitions are more likely because of their shape. The double-ring structure of A is more easily substituted by the double-ring structure of G. The same is true for C and T.
For mtDNA, the value for each mutation is reported in the format of what the ancestral (original) value was, the location, and the derived (mutated) value. For example, A1234G represents an A to G mutation at location 1234.
Other types of mutations
*Decimal points - eg. 315.1C - This means additional base pairs have been added in adjacent to the original. In other words the .1C is an extra C in between positions 315 and 316. .2 is two base pairs, etc.