U vs uracil

U and uracil both are nouns.

Word	Noun	Adjective	Verb	Adverb
U	Yes	No	No	No
uracil	Yes	No	No	No

As nouns, U and uracil are synonyms defined as:

U and uracil: a base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine

U (noun)	uracil (noun)
the 21st letter of the Roman alphabet	a base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine
a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons
a base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine