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The codon table. On the left is the first letter of the codon (from top to bottom – U, C, A, G). On the top is the second letter (left to right U, C, A, G). On the right is the third letter (in each row, this is designated from top to bottom as U, C, A, G. UUU and UUC are Phe. UUA and UUG are Leu. UCU, UCC, UCA and UCG are Ser. UAU and UAC are Tyr. UAA and UAG are stop. UGU and UGC are Cys. UGA is stop. UGG is Trp. CUU, CUC, CUA, and CUG are Leu. CC, CCC, CCA, and CCG are Pro. CAU and CAC are his. CAA and CAG are Gln. CGU, CGC, CGA, CGG are Arg. AUU, AUC, AUA are Ile, AUG is Met and start. ACU, ACC, ACA, ACG is Thr. AAU AAc, is Asn. AAA, AAG is Lys. AGU, AGC is SEr. AGA, AG is ARg. GUU, GUC, GUA, GUG is Val. GCU, GCC, GCA, GCG, is ala. GAU, GAC is Asp. GAA, GAG is Glu. GGU, GGC, GGA, GGG is Gly.
This figure shows the genetic code for translating each nucleotide triplet in mRNA into an amino acid or a termination signal in a nascent protein. The first letter of a codon is shown vertically on the left, the second letter of a codon is shown horizontally across the top, and the third letter of a codon is shown vertically on the right. (credit: modification of work by National Institutes of Health)
  • How many bases are in each codon?
  • What amino acid is coded for by the codon AAU?
  • What happens when a stop codon is reached?

The protein synthesis machinery

In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation . The composition of each component varies across taxa; for instance, ribosomes may consist of different numbers of ribosomal RNAs (rRNAs) and polypeptides depending on the organism. However, the general structures and functions of the protein synthesis machinery are comparable from bacteria to human cells. Translation requires the input of an mRNA template, ribosomes, tRNAs, and various enzymatic factors.

Ribosomes

A ribosome is a complex macromolecule composed of catalytic rRNAs (called ribozyme s) and structural rRNA s, as well as many distinct polypeptides. Mature rRNAs make up approximately 50% of each ribosome. Prokaryotes have 70S ribosomes, whereas eukaryotes have 80S ribosomes in the cytoplasm and rough endoplasmic reticulum, and 70S ribosomes in mitochondria and chloroplasts . Ribosomes dissociate into large and small subunits when they are not synthesizing proteins and reassociate during the initiation of translation . In E. coli , the small subunit is described as 30S (which contains the 16S rRNA subunit), and the large subunit is 50S (which contains the 5S and 23S rRNA subunits), for a total of 70S (Svedberg units are not additive). Eukaryote ribosomes have a small 40S subunit (which contains the 18S rRNA subunit) and a large 60S subunit (which contains the 5S, 5.8S and 28S rRNA subunits), for a total of 80S. The small subunit is responsible for binding the mRNA template, whereas the large subunit binds tRNAs (discussed in the next subsection).

Each mRNA molecule is simultaneously translated by many ribosomes, all synthesizing protein in the same direction: reading the mRNA from 5’ to 3’ and synthesizing the polypeptide from the N terminus to the C terminus. The complete structure containing an mRNA with multiple associated ribosomes is called a polyribosome (or polysome ). In both bacteria and archaea , before transcriptional termination occurs, each protein-encoding transcript is already being used to begin synthesis of numerous copies of the encoded polypeptide(s) because the processes of transcription and translation can occur concurrently, forming polyribosomes ( [link] ). The reason why transcription and translation can occur simultaneously is because both of these processes occur in the same 5’ to 3’ direction, they both occur in the cytoplasm of the cell, and because the RNA transcript is not processed once it is transcribed. This allows a prokaryotic cell to respond to an environmental signal requiring new proteins very quickly. In contrast, in eukaryotic cells, simultaneous transcription and translation is not possible. Although polyribosomes also form in eukaryotes, they cannot do so until RNA synthesis is complete and the RNA molecule has been modified and transported out of the nucleus.

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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