The direct templates for protein synthesis are RNA molecules, which are transcribed intermediates of DNA. RNA is transcribed from DNA. The sugar units in RNA are ribose rather than deoxyribose and one of the four major bases in RNA is uracil rather than thymine as in DNA. The different forms of RNA perform different functions in gene expression, messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). All cellular RNA is synthesized from a DNA template by RNA polymerases.