Translation - Study Guide PDF

**Study Guide -- Translation** **Corresponds to Lecture Slide Set 22** **I. Translation: RNA to Protein** **A. Overview of Translation** - **Definition**: Translation is the process by which ribosomes synthesize proteins using the nucleotide sequence of mRNA as a guide to determine the...

**Study Guide -- Translation** **Corresponds to Lecture Slide Set 22** **I. Translation: RNA to Protein** **A. Overview of Translation** - **Definition**: Translation is the process by which ribosomes synthesize proteins using the nucleotide sequence of mRNA as a guide to determine the order of amino acids. - Translation occurs in the **cytoplasm** in both prokaryotes and eukaryotes. In eukaryotes, mRNA is transcribed in the nucleus and then transported to the cytoplasm for translation. **B. The Genetic Code** - **Genetic Code**: A set of rules by which the information in the nucleotide sequence of an mRNA molecule is translated into the amino acid sequence of a protein. - The code is nearly **universal** across organisms, from bacteria to humans. - **Codons**: Groups of three nucleotides in mRNA that specify a particular amino acid or a stop signal. - **Start Codon**: AUG, which codes for methionine and signals the start of protein synthesis. - **Stop Codons**: UAA, UAG, UGA, which signal the termination of translation. **II. The Structure and Function of Ribosomes** - **Ribosomes**: Cellular structures composed of ribosomal RNA (rRNA) and proteins that facilitate the translation process. - Ribosomes consist of two subunits: 1. **Small Subunit**: Binds to mRNA and initiates translation. 2. **Large Subunit**: Catalyzes the formation of peptide bonds between amino acids. - Ribosomes move along the mRNA strand in the **5\' to 3\' direction**, reading codons and assembling the corresponding amino acids into a polypeptide chain​. **IV. Key Components of Translation** **A. Types of RNA Involved** 1. **mRNA (Messenger RNA)**: Carries the genetic information from DNA to ribosomes for protein synthesis. 2. **tRNA (Transfer RNA)**: Delivers specific amino acids to the ribosome during translation. It has an anticodon that pairs with the mRNA codon to ensure the correct amino acid is added to the growing polypeptide chain. 3. **rRNA (Ribosomal RNA)**: Forms the core structure of the ribosome and catalyzes peptide bond formation. **B. tRNA and the Anticodon** - **tRNA Structure**: Cloverleaf-shaped molecule with an anticodon loop and an amino acid attachment site. - **Anticodon**: A sequence of three nucleotides in tRNA that pairs with a complementary codon on the mRNA. - **Aminoacyl-tRNA Synthetase**: Enzymes that attach the correct amino acid to its corresponding tRNA, ensuring that the anticodon matches the mRNA codon. **V. Stages of Translation** **A. Initiation** 1. The **small ribosomal subunit** binds to the 5\' end of the mRNA, along with the initiator tRNA that carries methionine (start codon AUG). 2. The **large ribosomal subunit** joins to form a complete ribosome, with the initiator tRNA positioned at the P-site. 3. This process requires initiation factors and GTP for energy. **B. Elongation** 1. **tRNAs** bring amino acids to the ribosome according to the sequence of codons in the mRNA. 2. The ribosome has three binding sites: - **A-site (Aminoacyl site)**: Holds the incoming tRNA carrying the next amino acid. - **P-site (Peptidyl site)**: Holds the tRNA with the growing polypeptide chain. - **E-site (Exit site)**: Where discharged tRNAs leave the ribosome. 3. The ribosome catalyzes the formation of **peptide bonds** between amino acids, extending the polypeptide chain. **C. Termination** 1. When a **stop codon** (UAA, UAG, UGA) is reached, no corresponding tRNA binds to it. 2. Instead, **release factors** bind to the stop codon, prompting the ribosome to release the completed polypeptide chain. 3. The ribosome disassembles into its subunits, and translation ends​. **VI. The Genetic Code and Protein Synthesis** **A. Reading the Genetic Code** - Codons are read in the **5\' to 3\' direction** on the mRNA strand. - Each codon corresponds to a specific amino acid, which is added to the growing polypeptide chain in sequence. - The **redundancy** of the genetic code means that multiple codons can code for the same amino acid, providing some tolerance for mutations. **B. Polyribosomes** - In both prokaryotes and eukaryotes, multiple ribosomes can simultaneously translate a single mRNA molecule, forming a structure known as a **polyribosome** or **polysome.** - This increases the efficiency of protein synthesis by producing multiple copies of a protein simultaneously. **Vocabulary List** 1. **A-site (Aminoacyl site)**: The location on the ribosome where the incoming tRNA carrying a new amino acid binds to the mRNA codon during translation. 2. **Amino Acid**: The building blocks of proteins, each containing an amino group, a carboxyl group, a hydrogen atom, and a unique side chain (R group). Amino acids are linked together by peptide bonds to form polypeptides. 3. **Aminoacyl-tRNA Synthetase**: An enzyme that attaches the correct amino acid to its corresponding tRNA, ensuring that each tRNA carries the appropriate amino acid according to its anticodon. 4. **Anticodon**: A sequence of three nucleotides in a tRNA molecule that is complementary to a specific codon in the mRNA; it ensures the correct amino acid is added to the growing polypeptide chain during translation. 5. **Codon**: A sequence of three nucleotides in mRNA that specifies a particular amino acid or signals a start or stop for protein synthesis. 6. **E-site (Exit site)**: The site on the ribosome where the now-empty tRNA, after transferring its amino acid to the growing polypeptide chain, exits the ribosome. 7. **Elongation (Translation)**: The stage of translation in which amino acids are sequentially added to the growing polypeptide chain as the ribosome moves along the mRNA, reading codons and incorporating the corresponding amino acids. 8. **Exon**: A segment of a gene that is transcribed into RNA and remains in the mature mRNA after introns are removed during RNA splicing; exons are expressed as proteins. 9. **Genetic Code**: The set of rules by which the information encoded in nucleotide sequences of mRNA is translated into the amino acid sequences of proteins. The code is nearly universal and is based on codons, which are triplets of nucleotides. 10. **Initiation (Translation)**: The first stage of translation where the small ribosomal subunit binds to the mRNA, and the initiator tRNA carrying methionine binds to the start codon (AUG), followed by the assembly of the large ribosomal subunit. 11. **Intron**: A non-coding sequence in a gene that is transcribed into pre-mRNA but is removed during RNA splicing before the mRNA is translated into protein. 12. **Messenger RNA (mRNA)**: A type of RNA that carries the genetic information from DNA in the nucleus to the ribosome in the cytoplasm, where it directs protein synthesis. 13. **Nucleotide**: The basic building block of nucleic acids (DNA and RNA), composed of a sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base (adenine, cytosine, guanine, thymine, or uracil). 14. **P-site (Peptidyl site)**: The site on the ribosome that holds the tRNA with the growing polypeptide chain attached during elongation. 15. **Peptide Bond**: A covalent bond formed between the carboxyl group of one amino acid and the amino group of another, linking them together in a polypeptide chain. 16. **Polypeptide**: A chain of amino acids linked by peptide bonds that folds into a functional protein. 17. **Polyribosome (Polysome)**: A cluster of multiple ribosomes simultaneously translating the same mRNA molecule, which increases the efficiency of protein synthesis. 18. **Redundancy**: The characteristic of the genetic code where multiple codons can specify the same amino acid, providing a safeguard against mutations that might alter the protein sequence. 19. **Release Factor**: A protein that binds to the ribosome when a stop codon is reached during translation, causing the release of the newly synthesized polypeptide chain and the disassembly of the ribosomal complex. 20. **Ribosome**: A cellular structure composed of rRNA and proteins that assembles amino acids into proteins by translating the information in mRNA. 21. **Ribosomal RNA (rRNA)**: A type of RNA that combines with proteins to form the structure of ribosomes and catalyzes the formation of peptide bonds during translation. 22. **Start Codon**: The specific codon (AUG) that signals the beginning of translation and the incorporation of methionine as the first amino acid in the polypeptide chain. 23. **Stop Codon**: A sequence of three nucleotides (UAA, UAG, UGA) in mRNA that signals the end of translation, causing the ribosome to release the completed polypeptide chain. 24. **Termination (Translation)**: The final stage of translation where the ribosome encounters a stop codon, triggering the release of the completed polypeptide chain and the disassembly of the ribosome. 25. **Transfer RNA (tRNA)**: A type of RNA that carries amino acids to the ribosome during translation. Each tRNA has an anticodon that pairs with a complementary codon on the mRNA. 26. **Translation**: The process by which ribosomes synthesize proteins by decoding the sequence of codons in mRNA into a specific sequence of amino acids. 27. **Translocation**: The movement of the ribosome along the mRNA strand during translation, shifting tRNAs from the A-site to the P-site, and from the P-site to the E-site, facilitating the addition of new amino acids. 28. **Universal Genetic Code**: The concept that the genetic code is nearly identical across all living organisms, with only a few minor exceptions, allowing genes to be expressed similarly in different species.