U (Noun)

A heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons.

Nouns denoting substances.

The nuclear power plant used enriched uranium as fuel for its reactors.
Uranium is a highly toxic and radioactive element that requires special handling and storage.
The scientist carefully extracted the uranium from the ore using a complex chemical process.
The country's nuclear arsenal was built using highly enriched uranium as the primary fissile material.
The mining company extracted uranium from the depths of the earth, but the process was fraught with environmental concerns.

A base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine.

Nouns denoting substances.

Uracil is a nucleobase that is commonly found in RNA molecules.
The base pairing rules in RNA state that uracil pairs with adenine.
Uracil is a pyrimidine base that is used in the synthesis of RNA.
In RNA, uracil takes the place of thymine, which is found in DNA.
The molecular structure of uracil consists of a six-membered ring with nitrogen and oxygen atoms.

The 21st letter of the Roman alphabet.

Nouns denoting communicative processes and contents.

The teacher wrote the letter u on the blackboard for the students to practice.
The alphabet song helps children remember the order of letters, including the letter u.
In phonetics, the letter u represents a close back rounded vowel sound.
The letter u is the 21st letter of the modern English alphabet.
Learning the uppercase and lowercase forms of the letter u is an essential skill for young readers.