Final answer:
Ribosomes have three tRNA binding sites: the A site for incoming charged tRNA, the P site holding the growing polypeptide, and the E site for exiting uncharged tRNA. Three nucleotides of RNA, called a codon, code for a single amino acid. An mRNA sequence with four codons translates into a peptide that is 4 amino acids long.
Step-by-step explanation:
Ribosomes, the cellular machines responsible for protein synthesis, have three tRNA binding sites. During the elongation phase of translation, a charged tRNA molecule binds to the A site (aminoacyl site). The next two sites are the P site (peptidyl site), where the growing polypeptide chain is held, and the E site (exit site), where the uncharged tRNA exits the ribosome after its amino acid has been added to the chain. Regarding the genetic code, every three letters (nucleotides) in an RNA sequence provide the code for a single amino acid, referred to as a codon.
Looking at a specific mRNA sequence, such as 5'-AUGGGCUACCGA-3', the peptide translated from it would be 4 amino acids long, given that the sequence contains four codons, each corresponding to one amino acid, starting with the start codon AUG.
In terms of tRNA variety, bacteria can have 60 to 90 species of tRNA. Each one has a specific three-nucleotide anticodon and carries the corresponding amino acid. Finally, each aminoacyl tRNA synthetase is an enzyme responsible for attaching amino acids to their respective tRNA, and there are at least 20 different types, one for each amino acid.