Ribosomes are the protein factories of cells. They function by reading messenger RNA and translating the code to build proteins out of amino acid, which are the building blocks.
Ribosomes have a flattened spherical shape with a diameter ranging between 15 to 25 nm. They are made of two major pieces or subunits. The smaller subunit reads the mRNA code, whereas the larger assembles the polypeptide chain from amino acids. Each subunit also contains ribosomal RNA (rRNA). The two subunits assemble to translate mRNA and disassemble when translation is complete.
Parts of the ribosome
Ribosomal structure is highly conserved between all species, but its exact composition depends on the organism. There are about 80 proteins in the eukaryotic ribosome, as well as some nucleic acids.
The parts of a ribosome are named according to their sedimentation coefficients or Svedberg units, abbreviated S. The eukaryotic ribosome is known as an 80S ribosome. Inside the smaller subunit or 40S RNA, are 18S RNA, which is 1900 nucleotides long, and 33 proteins. The larger eukaryotic ribosomal subunit is referred to as 60S. The components of the 60S RNA are 46 proteins as well as 28S RNA, 5S RNA and 5.8S RNA, which are 4700, 120 and 160 nucleotides in length, respectively.
The RNA within ribosomes is organized by tertiary structural motifs, such as pseudoknots, and is responsible for carrying out the catalytic activity of the ribosome. The proteins surround the ribosome on the outside and are believed to stabilize its structure.
Prokaryotic ribosomes are smaller in size than eukaryotic ribosomes. Known as 70S ribosomes, prokaryotic ribosomes have a 30S subunit and a 50S subunit. The smaller subunit is made of 16S RNA and 21 proteins. The 50S subunit’s components are 23S RNA (2900 nucleotides long), 5S RNA (120 nucleotides long) and 31 proteins.
Chloroplasts and mitochondria also have 70S ribosomes and are otherwise quite similar, suggesting a common evolutionary ancestor. Many antibiotics target prokaryotic RNA to remove bacterial infections without harming host ribosomes, which in the case of eukaryotes are different.
Free and membrane bound ribosomes
The location of protein synthesis depends on the ultimate purpose and destination of the protein. If the protein has a signal directing it to the endoplasmic reticulum (ER), then the ribosome will locate itself to the ER and become membrane bound. The same ribosome may disassemble and become a free ribosome when it begins translating its next protein. Free ribosomes synthesize proteins mainly for use within the cell.
Ribosomes bound to a membrane synthesize proteins for export outside the cell or for use in lysosomes. Membrane bound ribosomes are found in the ER. In fact, the presence of ribosomes gives the ER its signature “rough” appearance in electron micrographs.
Assembly of ribosomes is a complex and precisely coordinated process that is carried out in the nucleoplasm, nucleolus, and cytoplasm in eukaryotic cells. It involves over 170 factors that participate in bringing all of the components together. Transcription occurs in the nucleus. In order to mature, the transcripts are exported to the cytoplasm with the help of specific receptors and factors. The entire process is so tightly regulated that there is also a process for recognizing and destroying incorrectly synthesized proteins. This apparatus can remove defective pre-ribosomes in the nucleolus and the cytoplasm.
Reviewed by Jonas Wilson, Ing. Med.
- British Society for Cell Biology-Ribosome, http://bscb.org/learning-resources/softcell-e-learning/ribosome/
- Ben-Shem A, Garreau de Loubresse N, Melnikov S, Jenner L, Yusupova G, Yusupov M (February 2011). "The structure of the eukaryotic ribosome at 3.0 Å resolution". Science. 334 (6062): 1524–1529. doi:10.1126/science.1212642.PMID 22096102
- Rabl, Leibundgut, Ataide, Haag, Ban (February 2010). "Crystal Structure of the Eukaryotic 40S Ribosomal Subunit in Complex with Initiation Factor 1".Science. 331 (6018): 730–736. doi:10.1126/science.1198308.PMID 21205638
- Klinge, Voigts-Hoffmann, Leibundgut, Arpagaus, Ban (November 2011). "Crystal Structure of the Eukaryotic 60S Ribosomal Subunit in Complex with Initiation Factor 6". Science. 334 (6058): 941–948.
- Recht MI, Douthwaite S, Puglisi JD (1999). "Basis for bacterial specificity of action of aminoglycoside antibiotics". EMBO J. 18 (11): 3133–8.doi:10.1093/emboj/18.11.3133. PMC 1171394
- Ribosome assembly in eukaryotes, http://www.ncbi.nlm.nih.gov/pubmed/12957375
- Molecular Biology of the Cell. 4th Ed., The Endoplasmic Reticulum, http://www.ncbi.nlm.nih.gov/books/NBK26841/