In the bacterial cell, several related polypeptide sequences are carried on the same mRNA template. Each sequence on the mRNA strand has its own ribosome binding site, start and stop codons. Since transcrpition occurs the the cytoplasm, translation can begin even before transcription is complete.
Eukaryotic mRNA differs from that of prokaryotes in that it (1) undergoes post-transcriptional modification, (2) carries instructions for the synthesis of only one polypeptide and (3) is not translated until after it has been transcribed completely, modified, and has left the nucleus.
Transfer RNA (tRNA) is a sequence of ~75 ribonucleotides that fold to form a three-dimensional structure as seen above. Each tRNA has an acceptor stem that forms a temporary bond with one amino acid and an anticodon that can bind to its complementary codon on mRNA.
Though there are 62 codons on mRNA, there are only about 40 tRNA molecules. Each tRNA anticodon can bind to a mRNA codon provided that at least the first two mRNA nucleotides are complementary to its first two nucleotides. This can occur because of "wobble," a phenomenon associated with the ability of the third nucleotide on an anticodon to hydrogen bond to another nucleotide different from its complement. This redundancy enables the cell to produce the proper polypeptide even if minor mutations (errors) occur in the DNA during replication or the mRNA during transcription.
Though both prokaryote and eukaryote cells contain ribosomes differences do exist:
1. Prokaryote ribosomes are 70S in size, composed of
a 30S small subunit and a 50S
large subunit. Eukaryote ribosomes are 80S, having a 40S small subunit and a 60S
large subunit. The ribosomes in mitochondria and chloroplasts are, however, 70S
like those of bacteria.
2. Prokaryote ribosomes are synthesized in the
cytoplasm, while eukaryote ribosome
subunits are synthesized in the nucleolus and assemble themselves in the cytoplasm
or along the rough endoplasmic reticulum.
The differences in the size and rRNA between the two types serves as a target for selectively toxic antimicrobials such as erythromycin, that binds to the 23S rRNA portion of the bacterial ribosome (the eukaryote rRNA portion is 28S).
Prokaryote translation always begins with the amino acid N-formylmethionine (f-met), while eukaryote translation always begins with methionine (met). In both eukaryotes and prokaryotes, elongation of the polypeptide chain is powered by guanosine triphosphate (GTP) and terminates at a stop codon with the help of release factor proteins that breaks the bonds between the tRNA and final amino acid in the sequence, allowing the ribosome subunits to detach.