About 75 to 80 ribonucelotides, with the bottom three called an anticodon, and one amino acid form tRNA. The anticodon of tRNA reads the mRNA codons (sets of three ribonucleotides). The ribonucleotide sequence of mRNA comes from DNA strands through transcription. The ribonucleotides in tRNA and mRNA---G, A, U and C---are opposites of each other. For instance, if an mRNA codon is GUA, the corresponding tRNA anticodon would be CAU, with C and G matching and A and U matching. Each mRNA codon codes for a specific amino acid. mRNA also contains codons that signal translation to stop and start.
mRNA takes genetic coding from DNA, a process called transcription, and carries it to the ribosome of the cell, where tRNA can read the codons and begin translation.
tRNA reads the mRNA codon and carries the corresponding amino acid to a growing protein. Each different tRNA strand codes for a different amino acid. This process of an mRNA codon coding for a specific amino acid, read by tRNA, is called translation.