TRANSLATION AND PROTEIN SYNTHESIS The process of protein synthesis is called translation. There are 20 amino acids that together make all the proteins of life. Each is associated with a triplet of base pairs, called a codon. There are sixty-four possible combinations of base pairs so there are 64 codon types. A total of 61 of them code for amino acids, while the rest are either stop codons or start codons. The codon AUG is a start codon. Because there are 20 amino acids, there is more than one codon for the same amino acid. Essentially all forms of life have the same codons matching for the same amino acids. Ribosomes are made from ribosomal RNA and protein material. The different organisms have structurally different ribosomes. Prokaryotes are said to have a 70S ribosome, while eukaryotes are said to have an 80S ribosome. There are two subunits. The small subunit binds messenger RNA, while the large subunit binds transfer RNA. The messenger RNA is read from the five-prime end to the three-prime end, making a protein that starts with an amino group and ends with a carboxyl group. A polyribosome is a ribosome plus a messenger RNA segment. In prokaryotes, translation and transcription happen at the same time. This means that prokaryotes can respond quickly to environmental signals. Transfer RNA is a special type of RNA that specifically helps to extend and elongate the polypeptide chain. It has a unique three-dimensional shape. Figure 46 shows what transfer RNA looks like:
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