The ribosome represents a complex of ribonucleic acid with various proteins. There, protein synthesis takes place according to the nucleotide sequence stored in DNA by translation into a polypeptide chain.
What is the ribosome?
Ribosomes are composed of rRNA and various structural proteins. The rRNA (ribosomal RNA) is transcribed in the DNA. There, in the form of rDNA, are genes for the synthesis of ribosomal RNA. The rDNA is not transcribed into proteins, but only into ribosomal RNA. In this process, rRNA serves as the basic building block of ribosomes. There it catalyzes the translation of the genetic information of the mRNA into proteins. The proteins within the ribosomes are not covalently linked to the rRNA. They hold the structure of the ribosome together, while the actual catalysis of protein synthesis is performed by the rRNA. Ribosomes consist of two subunits that assemble into a ribosome only during protein synthesis. Their building blocks are synthesized in the nucleus at the DNA. There, both rRNA and the proteins are produced, which assemble into the two subunits within the nucleus. They enter the cytoplasm through the nuclear pores. In a eukaryotic cell there are 100,000 to 10,000,000 ribosomes, depending on the protein synthesis activity. There are more ribosomes in cells with very active protein synthesis than in a cell with lower activity. In addition to the cytoplasm, ribosomes are also found in mitochondria or, in plants, in chloroplasts.
Anatomy and structure
As mentioned earlier, ribosomes are composed of rRNA and structural proteins, which are responsible for the proper positioning and cohesion of the structure. After synthesis in the nucleus, two subunits are formed that assemble into a ribosome only during protein synthesis through contact with mRNA. After completion of protein biosynthesis, the corresponding ribosome breaks down into its subunits again. In mammals, the small subunit is composed of 33 proteins and one rRNA, and the large subunit is composed of 49 proteins and three rRNA. Upon contact with the mRNA, which carries the genetic information from the DNA for a particular protein, the two subunits assemble to form the ribosome proper, at which point protein synthesis can begin. The ribosomal proteins tend to sit at its edge. Ribosomes can be found free in the cytoplasm or membrane-bound in the endoplasmic reticulum. They constantly alternate between free and membrane-bound states. The ribosomes located in the free cytoplasm produce proteins, which are also to be released immediately into the cytoplasm. Proteins are formed at the endoplasmic reticulum and enter the lumen of the ER via the cotranslational protein transporter. Mostly, these are proteins formed in secretion-forming cells such as the pancreas.
Function and Tasks
The function of ribosomes is to catalyze protein biosynthesis. The actual genetic information for the proteins is carried by mRNA, which is transcribed at the DNA. After it leaves the nucleus, it immediately binds to a ribosome for protein synthesis. In the process, the two subunits assemble. Furthermore, individual amino acids are transported from the cytoplasm to the ribosomes by means of tRNA. There, there are three tRNA binding sites. These are the aminoacyl (A), peptidyl (P) and exit (E) sites. At the beginning of protein synthesis, two sites, the A and P sites, are initially occupied by amino acid-loaded tRNA. This state is called the pre-translational state. After formation of a peptide bond between the two amino acids, the post-translational state is established, where the A site becomes the E site and the P site becomes the A site, and a new tRNA docks three nucleotides further along at the new P site. The former P-site tRNA, stripped of its amino acid, is now shuttled out of the ribosome. The states oscillate constantly during protein synthesis. A high activation energy is required for each change. The individual tRNA molecules dock to the respective complementary codon of the mRNA. In this process, protein synthesis takes place between the two subunits of the ribosome in a tunnel-shaped structure. The actual biosynthesis is controlled by the large subunit of the ribosome. The small subunit controls the function of the rRNA.Since synthesis takes place in a kind of tunnel, the protein chains that are still unfinished during protein assembly are protected from degradation by repair enzymes. In this form, these proteins would be recognized as defective in the cytoplasm and immediately degraded. After complete protein synthesis, the ribosome breaks down into its subunits again.
Diseases
Disruption of protein synthesis can lead to serious health problems. Orderly progression of this process is essential for life functions. However, there are some mutations that have effects on structural proteins or mRNA. One disease in which the cause is thought to be mutations in ribosomal proteins is known as Diamond-Blackfan anemia. Diamond-Blackfan anemia is a very rare blood disorder in which the synthesis of red blood cells is disturbed. Anemia results, which prevents the organs from receiving an adequate supply of oxygen. Treatment consists of lifelong blood transfusions. In addition, there are other physical malformations. According to one theory, the malfunction of ribosomal proteins leads to increased apoptosis of erythrocyte precursor cells, causing the anemia. Most mutations occur spontaneously. Inheritance of the syndrome can be demonstrated in only 15 percent of all cases.