love with a cryptologist?...

Don't ask anyone, solve the problem yourself. I know those stories: she will go with the guy who really solves it, I'll guarantee that.

well ... DNA has correspondences like

A <-> C
C <-> A
G <-> T
T <-> G

Probably there are pairs of characters that need to be swapped!
Probaly

E <-> I
r <-> t
u <-> h
z <-> e
...

hum ... there is the risk deciphering to let everyone read a confidential message !

You might read
Douglas [^]

I read an edition of the book and learned the techniques to break those simple ciphers 10 years ago as a sophomore in college.

Well, I think you are on the right track with E=I, but it could also be A

Lets start with the simple one: "adta'y", how many letters do you think Y could be? S, T, perhaps D


if you look at the character distribution, there are:
14 e
13 u
11 n
10 a
8 x
8 y
7 b
...

So, E, U and N are common letters, something like A, I, E, T or M

We have 3 2 letter words starting with E: "eb", "ea", "ey", Assuming that E remains as I, we are looking for 3 2 letter words starting with I. This gives the possibilities of "if", "im", "in", "is", "it".
B and A both recur quite often, and Y is pretty common too.

Now if we look at the words "ub" "eb" "ubb ub", "ubb ub" is probably "off of" (could also be "as ass", "ha haa", "we wee") making U=O and B=F.
This helps support the fact that E=I, as this would make "eb" "if", as opposed to E=A.

Now analyse "ea ey": this could be "if im", "if it", "im in", "in it", "is it", "it is". Since we have B=F, we can rule out "if im" and "if it", leaving A=[MNST] and Y=[NST], from the earlier analysis of "adta'y", Y cant be N, so Y=[ST]

We now have:
A=[MNST]
B=F
E=I
U=O
Y=[ST]

because this is DNA coding, the translations work both ways, so we can also get:
F=B
I=E
O=U

Taking a look at the message now, we have:
I pOjmnx IF rOz gtj jIczxn [MNST]dI[ST] Oz[MNST]... I pIhh kn [ST]U vxOzmIF rOz cn[MNST] [MNST]dI[ST]. [ST]vxxr FOx [ST]OEn OF [MNST]dn pnIxm nqgnv[MNST]IOj[ST], kzq I[MNST] I[ST] kt[ST]nm OFF OF mjt gOmIjc, [MNST]dt[MNST]'[ST] pdr. pI[MNST]d hOUn, IhtxIt
(capitals are what we have broken)

Now we get into a little guess work... what 3 letter words would follow the word "if" having an o in the middle. This would most likely be "you", and considering this word is recurring, this would be a good guess, however mean that our implication of O=U earlier is incorrect or a double up.
We now have:
O=[U?]
R=Y
Z=U

Applying our rule of DNA to R=Y and Z=U, we get
Y=[STR] (leave this option open because it was not in our earlier possibilities)
U=[Z?] (leave this option open because of the earlier difference)

Another good thing to guess would be the last sentance. This is most likely a sign-off (i.e. the girls name) with the word after the comma been the girls name.

I will leave you to figure out the rest as I have spent enough time on this answer already. Hopefully this gives you some idea into basic code breaking.