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Protein Synthesis Knowledge
1. What is the genetic code? Genetic code is the key for the conversion of DNA nucleotide sequences (and thus RNA nucleotide sequences) into amino acids sequences that will compose proteins. Protein Synthesis - Image Diversity: genetic code 2. Which is the biological molecule that contains the genetic information that is transmitted hereditarily and controls the cellular functioning? The hereditary molecule that controls the cellular functioning is the DNA (deoxyribonucleic acid). The DNA contains information for protein synthesis in cells. 3. How are the concepts of DNA, gene, proteins and characteristics of living beings related? Characteristics of organisms depend on chemical reactions that occur in them. These reactions are catalyzed by enzymes, highly specific proteins. Every protein of an organism is made from information contained in RNA molecules that are made according to a template sequence of nucleotides of a DNA chain. A gene is a DNA polynucleotide sequence that contains information for the production of a protein. 4. What is the role of messenger RNA and ribosomes for the protein synthesis? The mRNA is produced within the cellular nucleus and migrates to the cytoplasm where associated to ribosomes it guides the building of amino acid sequences that will compose proteins. Ribosomes are sites for the meeting and binding of mRNA and transfer RNA (tRNA), they are the structures where amino acids transported by tRNA are united by peptide bonds forming polypeptide chains (proteins). Protein Synthesis - Image Diversity: messenger RNA 5. Of what subunits are ribosomes are made? Ribosomes are made of two subunits, the small subunit and the large subunit. These subunits are made of ribosomic RNA (rRNA) and proteins. Ribosomes have three binding sites, one for mRNA and two for tRNA. Protein Synthesis - Image Diversity: ribosome structure 6. How different are the location of ribosomes in eukaryotic and in prokaryotic cells? In prokaryotes ribosomes are found free in cytoplasm. In eukaryotic cells they can also be found free in cytoplasm and mainly adhered to the external membrane of the karyotheca and of the rough endoplasmic reticulum. 7. How is the finding of ribosomes inside mitochondria and chloroplasts explained? It is a strong hypothesis that mitochondria and chloroplasts were prokaryotes that associated to primitive eukaryotic cells under mutualism (gaining protection and offering energy). This explains why within those organelles there are DNA and protein synthesis machinery, including ribosomes. This hypothesis is known as the endosymbiotic hypothesis on the origin of mitochondria and chloroplasts. 8. What are some examples of human cells that produce proteins for exportation? Which cytoplasmic organelle is expected to be well-developed and abundant in those cells? Specialized cells of the glands, like the Langerhans cells of the pancreas (that produce insulin) or the saliva-producing cells, are examples of secretory cells. In cells specialized in secretion, the endoplasmic reticulum and the Golgi apparatus are well-developed since they participate in the storage and processing of proteins for exportation. Protein Synthesis - Image Diversity: secretory cells 9. Which are the more abundant ribosomes in secretory cells - the free cytoplasmic ribosomes or those associated with the rough endoplasmic reticulum? Free cytoplasmic ribosomes are more related to protein production for internal cellular consumption while those adhered to the rough endoplasmic reticulum are more important in protein synthesis for exportation. Proteins made by adhered ribosomes enter the rough endoplasmic reticulum and are later transferred to the Golgi apparatus. So in secretory cells ribosomes adhered to the endoplasmic reticulum are more notable. 10. Where in eukaryotic cells does mRNA synthesis occur? To where do these molecules migrate? Messenger RNA molecules are synthesized within the nucleus, pass through pores of the nuclear membrane and gain the cytoplasm to reach the ribosomes where protein synthesis occurs. 11. After the fact that it is based on information from mRNA what is the process of protein synthesis called? Protein synthesis is called translation (of genetic information into proteins). Protein Synthesis - Image Diversity: protein translation 12. What is the difference between transcription and translation? Transcription is the name given to the formation of DNA molecules from an open DNA chain used as a template. Translation is the making of polypeptides (amino acids bound in sequence) and thus of proteins based on information encoded in the mRNA molecule. In eukaryotic cells transcription occurs in the nucleus and translation occurs in ribosomes. Transcription precedes translation. 13. How do nucleotides of mRNA chains encode information for the formation of the amino acids sequences of a protein? There are only four types of nitrogen-containing bases that can compose RNA nucleotides: adenine (A), uracil (U), guanine (G) and cytosine (C). Amino acids however are 20 different ones. Considering only one nucleotide (a 1:1 coding) it would be impossible to codify all amino acids. Considering two nucleotides there would be an arrangement of 4 elements, 2 x 2, resulting in a total of only 16 possible codifier units (4 x 4). Nature may know combinatory analysis since it makes a genetic code by arrangement of the 4 RNA bases, 3 x 3, providing 64 different triplets (4 x 4 x 4). So each triplet of nitrogen-containing bases of RNA codifies one amino acid of a protein. As these triplets appear in sequence in the RNA molecule, sequential amino acids codified by them are bound together to make polypeptide chains. For example, a UUU sequence codifies the amino acid phenylalanine, as well the UUC sequence; the ACU, ACC, ACA and ACG sequences codify the amino acid threonine; and so on for all possible triplet sequences and all other amino acids. 14. What is the name of an RNA sequence that codifies one amino acid? Each sequence of three nitrogen-containing bases of RNA that codifies one amino acid is called a codon. The codon is the codifier unit of the genetic code. 15. Since among the 64 codons of mRNA 61 codify amino acids that form polypeptide chains what are the functions of the three remaining codons? Since there are 20 amino acids and 64 possibilities of mRNA codons, it is expected some amino acids to be codified by more than one codon. And that really happens. Not all 64 codons however codify amino acids. Three of them, UAA, UGA and UAG, work on information that the last amino acid of a polypeptide chain under productions was already bound, i.e., they signal the end of the polypeptide synthesis. These codons are called terminal codons. The codon AUG codifies the amino acid methionine and at the same time it signals the beginning of the synthesis of a polypeptide chain (it is an initialization codon). In prokaryotic cells there is a sequence called Shine-Dalgarno sequence (in general AGGAGG) in the position that antecedes the initialization codon AUG. The function of this sequence is distinctness between the initialization AUG and other AUG codons of the RNA. 16. What is the cellular structure to which mRNA molecules bind to start the protein synthesis? To make proteins mRNA molecules necessarily associate to ribosomes. Ribosomes have two sites for the binding of two neighboring mRNA codons and where anticodons of tRNA bind by hydrogen bond. Thus ribosomes are the structure responsible for the positioning and exposure of mRNA codons to be translated. In ribosomes the peptide bond between two amino acids brought by tRNA molecules also occurs. The peptide bond happens when tRNAs carrying amino acids are bound to exposed mRNA codons. 17. How are amino acids brought to the cellular site where translation takes place? What is an anticodon? Amino acids are brought to ribosomes by RNA molecules known as transfer RNA, or tRNA. One tRNA associated to its specific amino acid binds by a special sequence of three nucleotides to a mRNA codon exposed in the ribosome. This sequence in the tRNA is known as anticodon. The tRNA anticodon must be complementary to the mRNA codon to which it binds, according to the rule A-U, CG. The ribosome then slides along the mRNA molecule (a process called translocation) to expose the following codon to the binding of other tRNA. When amino acids corresponding to neighboring codons bind by peptide bond the first tRNa is liberated. Protein Synthesis - Image Diversity: transfer RNA 18. Why is the proximity between ribosomes and amino acids important for the protein formation? What is the enzyme that catalyzes that reaction? The proximity between ribosomes and amino acids is important because the enzyme that catalyzes the peptide bond resides in ribosomes. As substrates of these enzymes, amino acids need to bind to the enzyme activation centers. The enzyme that catalyzes the peptide bond is the peptidyl transferase. 19. Why do ribosomes move along mRNA during translation? During translation the ribosome always exposes two mRNA codons to be translated by moving along the mRNA. When a peptide bond is made the ribosome moves to expose the next codon. This moving is called ribosomal translocation. (In the rough endoplasmic reticulum ribosomes are attached outside the membrane and mRNA molecules rather moving through them). Protein Synthesis - Image Diversity: ribosomal translocation 20. How many of the same proteins are made at the same time by each ribosome in the translation of one mRNA molecule? How does consecutive protein production occur in translation? Ribosomes do not make several different proteins simultaneously. They make them one after another. Along one single mRNA molecule however many ribosomes may move in a real mass manufacturing of the same protein. The unit made of many ribosomes working upon the same mRNA molecule is called polysome. Protein Synthesis - Image Diversity: polysome 21. An mRNA molecule codifies only one type of protein? Eukaryotic cells have monocistronic mRNA, i.e., each mRNA codifies only one polypeptide chain. Prokaryotes can present polycistronic mRNA. At the end of the assembling of amino acids into a polypeptide chain, the mRNA, by one of its terminal codons, signals to the ribosome that the polypeptide is complete. The ribosome then liberates the produced protein. In prokaryotes after this conclusion the information for the beginning of the synthesis of another different protein may follow in the same mRNA. 22. If a tRNA anticodon is CAA what is its corresponding mRNA codon? For the genetic code which amino acid does this codon codify? According to the A-U , C-G rule, the corresponding codon to the CAA anticodon is GUU. The genetic code table for translation is related to codons and not to anticodons. The amino acid codified by GUU, according to the genetic code, is valine. 23. If a fragment of nucleic acid has a nucleotide sequence TAC can one assert that it is a codon or an anticodon? A nucleic acid having a TAC sequence surely is not tRNA, it is DNA since RNA does not present the nitrogen-containing base thymine. Since it is not RNA it cannot be a codon or an anticodon. 24. Why can the genetic code be qualified as a “degenerate code”? The genetic code is a degenerate code because there are amino acids codified by more than one type of codon. It is not a system in which each element is codified by only one codifying unit. For example, the amino acid arginine is codified by six codons: CGU, CGC, CGA, CGG, AGA and AGG. 25. What is the concept of universality of the genetic code? What are the exceptions to this universality? The genetic code is universal because the rules of protein codification based on mRNA codons are practically the same for all known living beings. For example, the genetic code is the same for humans, for bacteria and for invertebrates. The protein synthesis in mitochondria, chloroplasts and some protozoans however are accomplished by different genetic codification. 26. How does the universality of the genetic code make the recombinant DNA technology possible? The universality of the genetic code refers to the fact that all living beings have their protein synthesis machinery functioning according to the same principles of storage, transmission and recognition of information, including translation of mRNA codons. This fact makes possible the exchanging of genes or gene fragments between different organisms and secures that these genes continue to command protein synthesis. This universality, for example, makes feasible the insertion of a fragment of human DNA containing a gene for the production of a given protein into the genetic material of bacteria. Since the bacterial transcription and translation systems work in the same manner as the correspondent human systems do, the bacteria will begin to synthesize the human protein related to the inserted DNA fragment. There are industries that produce human insulin (for use by diabetic patients) in this way, synthesized by bacteria with modified DNA. If the genetic code was not universal this kind of genetic manipulation would be impossible or very difficult to accomplish without new technological progresses. |