Principles of Ribosome Structure and Function
Ribosome Morphology and Chemistry Provide Basic Function Insights
Oct 9, 2009
Ribosomes – there are 60,000 or more in every prokaryotic cell. In living cells, they are packed throughout the cytoplasm, and perform their singular and most critical function -– protein synthesis. Ribosomes are small 200 nanometer cell organelles that are barely visible in the best light microscopes, but clearly evident in the electron microscope. Ribosomes have characteristic shapes, sizes and very important functions.
The Universal Theme of Life for Prokaryotes and Eukaryotes
Typically for prokaryotes and eukaryotes the following is the sequence of cellular events:
DNA -----> RNA ----> protein
Despite the simplicity of the formula, there is tremendous complexity involved, as will be seen in part of the following discussion. Remember DNA produces mRNA messages (transcription) to effect protein synthesis (translation).
Basic Ribosome Types, What They are and What They Do
There are two types of living cells: prokaryotic and eukaryotic. The prokaryotic cells are the most primitive cells, and they are called bacteria. The eukaryotic cells are larger and more complex cells and include cells of organisms such as fungi, protozoa, plants and animals.
The prokaryotic cell has the following basic features:
- cell membrane, or plasmalemma,
- cytoplasm with water, metal and non metal atoms, enzymes and other compounds,
- nuclear material,
- ribosomes, 70S (Svedberg sedimentation mass value)
The ribosomes of prokaryotes are 70S, while those of eukaryotes are 80S. These ribosomes each have the same function in their respective cell types – protein synthesis.
Ribosomes Structure is Related Directly to Ribosome Function
The 70S and 80S ribosome types are directly related to the cells they are found in. Prokaryotic ribosomes are composed of 2 subunits, a 50S and a 30S, and when combined (although the math seems illogical) the sedimentation value is a true 70S. In eukaryotes, the 2 subunits are 60S and 40S and the mass of the ribosome is 80S (even though mathematically, again, this seems illogical).
The ribosomes are composed of ribosomal RNA (rRNA) complexed with protein. Ribosomes attach and position messenger RNA (mRNA) to interact and align with transfer RNA (tRNA) to enable the synthesis of protein.
Under the right conditions – even in a test tube – it is possible to synthesize complete and functional proteins, such as hormones and enzymes. This extracellular, in vitro, protein synthesis requires:
- ribosomes
- complete supply of amino acids
- mRNA, tRNA
- GTP (an energy source molecule similar to ATP)
- ribosome protein "enzyme" (for amino acid linkage and peptide bond formation)
The two 70S and 80S ribosomes have different weights, but they also have different proteins and rRNA, one from the other. Therefore, there are multiple differences between the two types of ribosomes. These differences have important consequences for antibiotic activity because prokayotic ribosomes are more susceptible than eukaryotic ribosomes to the effects of many antibiotics.
Ribosomes and Protein Synthesis Activities
All active and living cells produce large amounts of many different proteins. Newly-synthesized proteins are needed to: replace damaged or degraded proteins, build new materials for growing and dividing cells and, in certain cells, produce exportable hormones, enzymes or other necessary proteins.
The dynamics of protein synthesis include the following steps:
- ribosome binds mRNA which contains codons (nucleotide trios) that specify amino acid kinds and order,
- tRNAs each transport one specific amino acid. Each tRNA has an anticodon (nucleotide trio) that can appropriately match to a codon on the mRNA, and react appropriately, so that one amino acid is connected to another assisted by enzyme and ATP input (an endothermic reaction).
- the start codon is AUG which specifies a methionine. Three different stop codons exist: UGA, UAG, and UAA.
- at the end of the protein synthesis, the protein peals off and away from the ribosomes. Ribosomes may act in clusters sharing a single message, and recycling the mRNA to continue to peal off multiple copies of proteins. Eventually, the mRNA degrades.
In October, 2009 the Nobel Chemistry Prize was awarded to three senior scientists for ribosome crystallography studies.
Resources
Lodish, H. et al. 2000. Molecular Cell Biology. Fourth Ed., W. H. Freeman and Co., New York, N.Y.
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