Nucleotides are the building blocks of DNA and RNA. They consist of a sugar molecule, a phosphate group, and a nitrogenous base. The nitrogenous bases are the molecules that are held together by weak hydrogen bonds.

There are four main nitrogenous bases in DNA:

* Adenine (A)

* Thymine (T)

* Guanine (G)

* Cytosine (C)

In RNA, uracil (U) replaces thymine.

Hydrogen bonding between nucleotides:

* Adenine (A) forms two hydrogen bonds with thymine (T) or uracil (U).

* Guanine (G) forms three hydrogen bonds with cytosine (C).

Therefore, the nucleotides that can be held together by weak hydrogen bonds are:

* Adenine (A) and thymine (T) or uracil (U)

* Guanine (G) and cytosine (C)

These specific pairings are essential for the structure and function of DNA and RNA, as they ensure the correct base pairing and maintain the integrity of the genetic code.