Translation - Proteins are Synthesized

Once in the cytoplasm, messenger RNA acts as a template for the ordering of amino acids in proteins. A template is a pattern. The actual process of bonding amino acids to form proteins is called translation. This is because the original order of bases in a portion of DNA is translated into the order of amino acids in a particular protein. In this way, the instruction in the DNA are followed to synthesize a protein.

The next typr of RNA involved in protein synthesis is what like a coveleaf. One end of the molecule has an attachment site for an amino acid. The other end has an exposed base triplet called an anticodon. An anticodon has bases that are complementary to the bases in its corresponding codon. If a codon is AUC, its anticodon is UAG. One codon for Valine is GUU. Valine will attach to tRNA molecules with the anticodon CAA.

The amino acids needed for protein synthesis are spread throughout the cytoplasm. In your body cells, for example, these amino acids have come from the food you have eaten. Remember, it is the combination of the 20 different amino acids that make up different proteins. During translation, individual amino acids in the cytoplasm are arranged in the right order to make a specific protein.

The third type of RNA is ribosomal RNA (rRNA). Ribosomes are made of two unequal subunits. Each subunit is composed of Ribosomal RNA and protein. One of the subunits is larger than the other. The units are usually separate in the cytoplasm. They are joined during protein synthesis.

Protein Synthesis - the process

First, an initiation codon (usually AUG, which codes for methionine) on an mRNA molecule binds to a special site on a small ribosome subunit. There are two such special sites on ribosomes. After the mRNA is attached, a large ribosome subunit attaches to the mRNA. Transfer RNA with initiation anticodon UAC bind to the mRNA on the ribosome. The amino acid methionine is attached to this tRNA. THis signals the start of the amino acid chain. The second special site on the ribosome is lined up with the next codon on the mRNA molecule. A tRNA molecule with the correct anticodon binds to the codon at the second site. This tRNA has an amino acid attached to it. Methionine now has another amino acid next to it. Enzymes from the ribosome form a bond between the two amino acids. The tRNA that had been attached to the methionine moves away from the ribosome. The ribosome now shifts so that the second tRNA moves to the first site. Another tRNA, with the correct anticodon, binds to the mRNA codon that is now at the second site. This means that a third amino acid is now present. Enzymes join it to the second amino acid. The second tRNA leaves, and the ribosome shifts again.

In this way a chain of amino acids is built up. The sequence is determined by the codons on the mRNA. An mRNA unit can have many ribosomes attached to it. As each ribosome moves along the mRNA, it builds up proteins. The chains usually start with methionine and end when a "stop" codon on the mRNA is reached.

A single mRNA molecule may have up to 100 ribosomes attached to it. Therefore, as the ribosomes move along, many molecules of the same protein are being synthesized at the same time.