To continue with our house example, once the working copy of the blueprint has reached the site, the workers must assemble the materials according to the instructions; this process is called translation. In the case of a protein, the workers are the ribosomes and special RNA molecules called transfer RNA (tRNA). The construction materials are the amino acids.
First, let's look at the ribosome. The ribosome is made of RNA called ribosomal RNA (rRNA). In prokaryotes, rRNA is made in the cytoplasm; in eukaryotes, rRNA is made in the nucleolus. The ribosome has two parts, which bind on either side of the mRNA. Within the large part are two "rooms" (P and A sites) that will fit two adjacent codons of the mRNA, two tRNA molecules and two amino acids. At first, the P site holds the first codon in the mRNA and A site holds the next codon.
Next, let's look at the tRNA molecules. Each tRNA has a binding site for an amino acid. Because each tRNA is specific for a single amino acid, it must be able to recognize the codon on the mRNA that codes for that particular amino acid. Therefore, each tRNA has a specific three-nucleotide sequence called an anti-codon that matches up with the appropriate mRNA codon, like a lock and key. For example, if a codon on mRNA has the sequence ...-uracil-uracil-uracil-... (UUU) which codes for the amino acid phenylalanine, then the anti-codon on the phenylalanine tRNA will be adenine-adenine-adenine (AAA); remember that A binds with U in RNA. The tRNA molecules float in the cytoplasm and bind free amino acids. Once bound to amino acids, the tRNAs (also called amino-acyl tRNAs) will seek out ribosomes.
Finally, let's look at the events in the synthesis of new proteins. For example, let's consider a small mRNA molecule with the following sequence:
All mRNA molecules begin with AUG (the start codon). UGA, UAA, and UAG are stop codons; stop codons have no corresponding tRNA molecules (Actual mRNA molecules have hundreds of codons).
The corresponding sequence of tRNA anti-codons will be:
There is no tRNA corresponding to the stop codons.
The amino acid sequence specified by this small mRNA is:
We know this sequence of amino acids by using a table of the genetic code. The genetic code table below is for mRNA and specifies the bases in the first, second and third positions of the codon with their corresponding amino acids.
Let's read the amino acid specified by the mRNA codon, AUG. First, place your left finger on the first position codon (A), in the first column of the table. Move your left finger across the row under the second position codon (U) in the first row. Now, place your right finger over the third position codon (G) in the same row of the last column (G). Move your right finger across the row until it meets your left finger and read the amino acid (methionine).
In the next section we'll look at the protein synthesis process.