a

a

adenine

g

g

guanine

c

c

cytosine

t

t

thymine

u

u

uracil

r

purine

y

t/u or c

pyrimidine

m

a or c

amino

k

keto

s

g or c

strong interactions
3H-bonds

w

a or t/u

weak interactions
2H-bonds

b

g or c
or t/u

not a

d

a or g
or t/u

not c

h

a or c
or t/u

not g

v

a or g
or c

not t, not u

n

a or g or c
or t/u, unknown, or other

any