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Protein synthesis is a very similar process for both prokaryotes and eukaryotes but there are some distinct differences. [1] Protein synthesis can be divided broadly into two phases: transcription and translation. During transcription, a section of DNA encoding a protein, known as a gene, is converted into a molecule called messenger RNA (mRNA).
Ribosomes are the macromolecular machines that are responsible for mRNA translation into proteins. The eukaryotic ribosome, also called the 80S ribosome, is made up of two subunits – the large 60S subunit (which contains the 25S [in plants] or 28S [in mammals], 5.8S, and 5S rRNA and 46 ribosomal proteins) and a small 40S subunit (which contains the 18S rRNA and 33 ribosomal proteins). [6]
Eukaryotic 3-D polyribosomes are similar to prokaryotic 3-D polyribosomes in that they are “densely packed left-handed helices with four ribosomes per turn”. This dense packing can determine their function as regulators of translation, with 3-D polyribosomes being found in sarcoma cells using fluorescence microscopy.
Prokaryotic ribosomes are around 20 nm (200 Å) in diameter and are composed of 65% rRNA and 35% ribosomal proteins. [13] Eukaryotic ribosomes are between 25 and 30 nm (250–300 Å) in diameter with an rRNA-to-protein ratio that is close to 1. [14]
[1] [2] Eukaryotic ribosomes are also known as 80S ribosomes, referring to their sedimentation coefficients in Svedberg units, because they sediment faster than the prokaryotic ribosomes. Eukaryotic ribosomes have two unequal subunits, designated small subunit (40S) and large subunit (60S) according to their sedimentation coefficients.
Prokaryotic ribosomes begin translation of the mRNA transcript while DNA is still being transcribed. Thus translation and transcription are parallel processes. Bacterial mRNA are usually polycistronic and contain multiple ribosome binding sites. Translation initiation is the most highly regulated step of protein synthesis in prokaryotes. [5]
Protein synthesis occurs in three phases: initiation, elongation, and termination. [13] Prokaryotic (archaeal and bacterial) translation differs from eukaryotic translation; however, this section will mostly focus on the commonalities between the two organisms.
The structure of eukaryotic genes includes features not found in prokaryotes. Most of these relate to post-transcriptional modification of pre-mRNAs to produce mature mRNA ready for translation into protein. Eukaryotic genes typically have more regulatory elements to control gene expression compared to prokaryotes. [5]