RNA that carries amino acids to the ribosome where proteins are made is called tRNA or transfer RNA. It is the smallest RNA and has a molecular weight of 25,000 daltons. About 15% of the cell is tRNA. Each tRNA molecule is made up of 90 nucleotides. It is produced in the nucleolus of the cell. tRNA molecules contain some unusual bases. Such as inosinic acid, thiamine etc. They are highly stable and make proteins for cells. There are about 100 types of tRNA in living organisms. A cell contains 31-42 types of tRNA.