Translation PDF

Translation Objectives At the end of the lesson, the learners’ should have: -discussed steps in protein synthesis Translation Translation is the final process of protein synthesis that takes place in the cytoplasm. The genetic information of the DNA is used as the origin to for...

Translation Objectives At the end of the lesson, the learners’ should have: -discussed steps in protein synthesis Translation Translation is the final process of protein synthesis that takes place in the cytoplasm. The genetic information of the DNA is used as the origin to form messenger RNA (mRNA) by the transcription process. The single stranded mRNA then serves as a template during translation.  During translation, the mRNA sequence is decoded to produce a specific amino acid chain called the polypeptide. Folding of the polypeptide produces an active protein which is able to perform important functions within the cell.  Amino Acids Most amino acids have a structural template where an alpha carbon is bonded to the following forms: A hydrogen atom (H) A carboxyl group (-COOH) An amino group (-NH2) A “variable” group/R group/ Side chain The “variable” group is most responsible for difference as all of them have hydrogen, carboxyl group, and amino group bonds. Amino acids combine through a dehydration link called a peptide bond.  When several groups of amino acids are joined together, a protein macromolecule is formed. This is why proteins are considered as polymers of amino acids. Proteins are typically made of a chain of 20 amino acids. The human body makes any protein it needs by using a combination of these 20 amino acids. Protein Structure Proteins may generally have globular or fibrous structure depending on its particular role in the bodily functions. Globular proteins are spherical, compact and soluble. Fibrous proteins are elongated and insoluble. However, these two structure types may exhibit one or more types of protein structures.  These amino acids are grouped as: essential and non- essential. Non-essential amino acids are those which the human body is capable of synthesizing, whereas essential amino acids must be obtained from the diet.  The key components required for translation are: mRNA tRNA ribosomes aminoacyl tRNA synthetases Ribosome The ribosome is a complex organelle, present in the cytoplasm, which serves as the site of action for protein synthesis. It provides the enzymes needed for peptide bond formation. The nucleotide sequence in mRNA is recognized in triplets, called codons.  The ribosome moves along the single strand mRNA, and when a complimentary codon sequence belonging to amino acid bearing tRNA bonds with the mRNA, the amino acid is added to the chain. The mRNA possesses a stop codon, a sequence of three nucleotides that indicates that translation is complete. Upon reaching the stop codon, the ribosome ceases translation and releases the mRNA and newly generated polypeptide. Messenger RNA (mRNA) mRNA is used to convey information from DNA to the ribosome. It is a single strand molecule, complimentary to the DNA template, and is generated through transcription. Strands of mRNA are made up of codons, each of which signifies a particular amino acid to be added to the polypeptide in a certain order.  mRNA must interact with ribosomal RNA (rRNA), the central component of ribosomal machinery that recognizes the start and stop codons of mRNA, and transfer RNA (tRNA), which provides the amino acid once bound with a complimentary mRNA codon. Transfer RNA (tRNA) This is a single strand of RNA composed of approximately 80 ribonucleotides. Each tRNA is read as a ribonucleotide triplet called an anticodon that is complementary to an mRNA codon. tRNA carry a particular amino acid, which is added to the growing polypeptide chain if complimentary codons bond. Aminoacyl tRNA synthetases Also called tRNA-ligase These are enzymes that link each amino acid to their corresponding tRNA. Each amino acid has a unique synthetase and the active site of each enzyme fits only one specific combination of the amino acid and tRNA. 3 Major Steps in Translation There are three major steps in translation: initiation elongation termination  Initiation After mRNA is formed in the nucleus, it leaves and moves to the cytoplasm where it finds the ribosome. Small ribosomal subunits then bind to mRNA. The initiator tRNA which is equipped with the anticodon (UAC) also binds to the start codon (AUG) of the mRNA. Let us say we have the mRNA codon AUG-UGC- AAG-UCC-GGA-CAG, the tRNA anticodon would be UAC-ACG-UUC-AGG-CCU-GUC. The resulting large complex forms a complete ribosome and initiates protein synthesis. Each different tRNA is covalently linked to a particular amino acid. Elongation Following initiation, a new tRNA-amino acid complex enters the codon next to the AUG codon. If the anticodon of the new tRNA matches the mRNA codon, base pairing occurs and the two amino acids are linked by the ribosome through a peptide bond. If the anticodon does not match the codon, base pairing cannot happen and the tRNA is rejected. Then, the ribosome moves one codon forward making space for a new tRNA-amino acid complex to enter. This process is repeated several times until the entire polypeptide has been translated. Termination As the ribosome moves along the mRNA, it encounters one of the three stop codons (UAA, UAG and UGA) for which there is no corresponding tRNA. Terminator proteins present at the stop codon bind to the ribosome and trigger the release of the newly synthesized polypeptide chain. The ribosome then disengages from the mRNA. On release from the mRNA, the small and large subunits of the ribosome dissociate and prepare for the next round of translation. The polypeptide chains produced during translation undergo some post-translational modifications, such as folding, before becoming a fully active protein.  Codon Charts These charts are for mRNA codons, NOT tRNA!  Start at the center of the chart for the first letter. Move to the outside next ring for the second letter and finally, find the final letter among the smallest set of letters in the third ring. Then you can read the amino acid in that sector.  To decode the codon for CAC, find the first letter C in the set of bases at the center of the circle. Then find the letter A in the second ring, then C in the third ring. There, you will read the amino acid in this sector as Histidine. Some of these codons are special. AUG is the start codon which initiates translation by coding for Methionine. And these three are stop codons: UAA, UAG and UGA. These are the ones that terminate translation.  Answers mRNA AUG ACU AGC UGG GGG UAU UAC UUU UAG tRNA UAC UGA UCG ACC CCC AUA AUG AAA AUC Amino Acid Methionine Threonine Serine Tryptophan Glycine Tyrosine Tyrosine Phenylalan Stop /start ine mRNA AUG GCG AGG CGG CAG CUG UUA UGG UGA tRNA UAC CGC UCC GCC GUC GAC AAU ACC ACU Amino Acid Methionine Alanine Arginine Arginine Glutamine Leucine Leucine Tryptophan Stop /start mRNA AUG GUG GGG GCA UAC CGA CCC UUA UAG tRNA UAC CAC CCC CGU AUG GCU GGG AAU AUC Amino Acid Methionine Valine Glycine Alanine Tyrosine Arginine Proline Leucine Stop /start  Answers mRNA AUG AGA GGG UUU UUU AUG GUG GGG UAG tRNA UAC UCU CCC AAA AAA UAC CAC CCC AUC Amino Acid Methionine Arginine Glycine Phenylalan Phenylalan Methionine Valine Glycine Stop /start ine ine mRNA AUG GAG UGU GAU GCG UAC AAC CCC UAA tRNA UAC CUC ACA CUA CGC AUG UUG GGG AUU Amino Acid Methionine Glutamic Cysteine Aspartic Alanine Tyrosine Asparagine Proline Stop /start Acid Acid