Converting the information contained on a cassette tape into sound waves requires many different components. Some components are magnetic tape in the cassette tape, instruments inside the tape deck that read the magnetic tape, and the speakers that emit the sound waves. In the translation process of synthesizing a protein, there are also many components. The key components are messenger RNA, transfer RNA, and ribosomes.

We saw before that mRNA is the type of RNA that’s used as a template to make a protein. mRNA contains the codons, read in the 5 prime to 3 prime direction, that will be translated into amino acids. Transfer RNA, or tRNA, transfers an amino acid to the growing polypeptide. Like mRNA, tRNA is a single strand of nucleic acid. But unlike mRNA, tRNA folds back on itself in such a way as to create an L-shaped structure.

There is at least one type of tRNA for each amino acid. A tRNA carries an amino acid at its 3 prime end. In the cell’s cytoplasm, enzymes called aminoacyl-tRNA synthetases help attach amino acids to tRNA. There is at least one type of aminoacyl-tRNA synthetase for each amino acid. The enzyme brings together a specific amino acid and a specific tRNA. The amino acid becomes covalently attached to the 3 prime end of the tRNA molecule.

The opposite end of the tRNA molecule contains an anticodon. An anticodon is a nucleotide triplet that binds to a complementary codon of mRNA. The anticodon is essential in the tRNA because it allows the tRNA to be an “adapter” between the mRNA codons and the amino acids.

Let’s look at the last key component in translation, ribosomes.Ribosomal RNA, or rRNA, is a type of RNA molecule that is one of the components of a ribosome. Ribosomes are composed of rRNA and several proteins. Ribosomes facilitate the pairing of mRNA and tRNA and the synthesis of a polypeptide. Ribosomes consist of two subunits, one small and one large. The two subunits are initially apart in the cell’s cytoplasm. During translation they work together with mRNA and tRNA.

The smaller subunit has a binding site for the mRNA. The larger subunit has two binding sites for two tRNA molecules. One site is called the P site, or peptidyl site. It holds the tRNA that has the growing polypeptide. The other site is called the A site, or aminoacyl site. It holds the tRNA that has the next amino acid that will be added to the polypeptide.

There are two kinds of ribosomes: free and bound. Free ribosomes are located in the cytoplasm. Bound ribosomes are attached to the rough endoplasmic reticulum (rough ER). The bound ribosomes are what makes the rough ER look rough. A ribosome can switch from being a free ribosome to being a bound ribosome. We’ll see later what determines whether a ribosome is free or bound.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education