In addition to the different translational factors and mechanisms used by eukaryotic organisms as described earlier, an organizational difference exists in eukaryotes that contrasts with prokaryotic protein synthesis. In organisms lacking a nucleus, transcription of the genetic message from DNA to RNA occurs in the same location as translation. In fact, bacterial ribosomes typically begin translating an mRNA as it emerges from RNA polymerase. Eukaryotic protein synthesis is uncoupled from transcription, as mRNAs are synthesized and processed in the nucleus and then transported to the ribosomes in the cytoplasm.
While most protein synthesis occurs in the cytoplasm, mitochondria also have small chromosomes and synthesize some of their own proteins. For example, the human mitochondrial chromosome is composed of less than 17,000 bp and encodes 13 proteins, 22 tRNAs, and 2 rRNAs. Mitochondria use a genetic code that is slightly different from the “standard” code shown in Table I. Codon degeneracy is greater in mitochondria; for example, UGA is not used as a termination signal, but instead codes for tryptophan. Likewise, the trinucleotide AUA codes for methionine (and is also an initiation signal) in mammalian mitochondria, while in the standard code AUA specifies isoleucine. Last, only 22 tRNAs are used to decode all 61 sense codons, in contrast to the 60 or so that are used in prokaryotes or the cytoplasm of eukaryotes.
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