Transitions vs Transversions

Transitions_&_Transversions

DNA sub­sti­tu­tion muta­tions are of two types. Tran­si­tions are inter­changes of two-ring puri­nes (A <-> G) or of one-ring pyrim­idi­nes (C <-> T): they there­fore involve bases of sim­i­lar shape. Trans­ver­sions are inter­changes of purine for pyrim­idine bases, which there­fore involve exchange of one-ring and two-ring struc­tures.

Trans­ver­sions are inter­changes of purine for pyrim­idine bases, which there­fore involve exchange of one-ring and two-ring struc­tures.

Although there are twice as many pos­si­ble trans­ver­sions, because of the mol­e­c­u­lar mech­a­nisms by which they are gen­er­at­ed, tran­si­tion muta­tions are gen­er­at­ed at high­er fre­quen­cy  than trans­ver­sions. As well, tran­si­tions are less like­ly to result in amino acid sub­sti­tu­tions (due to “wob­ble”), and are there­fore more like­ly to per­sist as “silent sub­sti­tu­tions” in pop­u­la­tions as sin­gle nucleotide poly­mor­phisms (SNPs).

Transition_mutation

Induc­tion of tran­si­tion muta­tions by spon­ta­neous tau­tomer­ic shifts

In the orig­i­nal dou­ble-strand­ed DNA mol­e­cule, A in the stan­dard (amino) form pairs with T. Dur­ing repli­ca­tion, the two strands sep­a­rate. In the upper dia­gram, T pairs with A as usu­al, which repli­cates the wild-type sequence. In the low­er dia­gram, A has under­gone a tau­tomer­ic shift to the non-stan­dard (imi­no) form A’, which pairs with C. In the next round of repli­ca­tion, the imi­no A’ shifts back to the amino A form, which pairs with T, which again repro­duces the wild-type sequence. Repli­ca­tion of the oth­er strand pairs C with G. By com­par­ison with the orig­i­nal mol­e­cule, the result is a  T -> C muta­tion. A tau­tomer­ic shift in one strand has pro­duced a  tran­si­tion muta­tion in the com­ple­men­tary strand. If the muta­tion occurs in the germline, it will be trans­mit­ted to future gen­er­a­tions.

IMPORTANT: Note that a tau­tomer­ic shift is not itself a muta­tion, but a tran­sient change to an alter­na­tive form of the mol­e­cule.

https://www.mun.ca/biology/scarr/Transitions_vs_Transversions.html

https://www.mun.ca/biology/scarr/Transition_mutations.html

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