Translation
Translation is the process in which genetic information, carried by messenger RNA (mRNA), directs the synthesis of proteins from amino acids, whereby the primary structure of the protein is determined by the nucleotide sequence in the mRNA. Although there are some important differences between translation in bacteria and translation in eukaryotic cells the overall process is similar. Essentially, the same type of translational control mechanisms that exist in eukaryotic cells do not exist in bacteria.
A molecule known as the ribosome is the site of the protein synthesis. The ribosome is protein bound to a second species of RNA known as ribosomal RNA (rRNA). Several ribosomes may attach to a single mRNA molecule, so that many polypeptide chains are synthesized from the same mRNA. The ribosome binds to a very specific region of the mRNA called the promoter region. The promoter is upstream of the sequence that will be translated into protein.
The nucleotide sequence on the mRNA is translated into the amino acid sequence of a protein by adaptor molecules composed of a third type of RNA known as transfer RNAs (tRNAs). There are many different species of tRNAs, with each species binding a particular type of amino acid. In protein synthesis, the nucleotide sequence on the mRNA does not specify an amino acid directly, rather, it specifies a particular species of tRNA. Complementary tRNAs match up on the strand of mRNA every three bases and add an amino acid onto the lengthening protein chain. The three base sequence on the mRNA are known as "codons," while the complementary sequence on the tRNA are the "anti-codons."
The ribosomal RNA has two subunits, a large subunit and a small subunit. When the small subunit encounters the mRNA, the process of translation to protein begins. There are two sites in the large subunit, an "A" site, and a "P" site. The start signal for translation is the codon ATG that codes for methionine. A tRNA charged with methionine binds to the translation start signal. After the first tRNA bearing the amino acid appears in the "A" site, the ribosome shifts so that the tRNA is now in the "P" site. A new tRNA molecule corresponding to the codon of the mRNA enters the "A" site. A peptide bond is formed between the amino acid brought in by the second tRNA and the amino acid carried by the first tRNA. The first tRNA is now released and the ribosome again shifts. The second tRNA bearing two amino acids is now in the "P" site, and a third tRNA can now bind to the "A" site. The process of the tRNA binding to the mRNA aligns the amino acids in a specific order. This long chain of amino acids constitutes a protein. Therefore, the sequence of nucleotides on the mRNA molecule directs the order of the amino acids in a given protein. The process of adding amino acids to the growing chain occurs along the length of the mRNA until the ribosome comes to a sequence of bases that is known as a "stop codon." When that happens, no tRNA binds to the empty "A" site. This is the signal for the ribosome to release the polypeptide chain and the mRNA.
Bacterial ribosomes are smaller than eukaryotic ribosomes. In some cases, bacterial ribosomes contain less than have the total protein found in eukaryotic ribosomes. Bacteria also respond to fewer initiation factors than do eukaryotic cells.
After being released from the tRNA, some proteins may undergo post-translational modifications. They may be cleaved by a proteolytic (protein cutting) enzyme at a specific site. Alternatively, they may have some of their amino acids biochemically modified. After such modifications, the polypeptide forms into its native shape and starts acting as a functional protein in the cell.
There are four different nucleotides, A, U, G and T. If they are taken three at a time (to specify a codon, and thus, indirectly specify an amino acid), 64 codons could be specified. However, there are only 20 different amino acids. Therefore, several triplets code for the same amino acid; for example UAU and UAC both code for the amino acid tyrosine. In addition, some codons do not code for amino acids, but code for polypeptide chain initiation and termination. The genetic code is non-overlapping, i.e., the nucleotide in one codon is never part of the adjacent codon. The code also seems to be universal in all living organisms.
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