Biology Chapter: Overview of Translation

Overview of Translation

• Translation is the process where mRNA codons specify the sequence of amino acids to create a polypeptide.
• One or more polypeptides fold and assemble to form a functional protein.
• Proteins are important for cell structure and function.
• DNA's primary role is storing the information for protein synthesis.
• Genes encoding amino acid sequences are called protein-encoding genes or structural genes.
• mRNA is the RNA transcribed from protein-encoding genes.

The Genetic Basis For Protein Synthesis

• The genetic material's main function is producing proteins accurately, precisely and in correct amounts.
• Cells make thousands of different proteins.
• Metabolic pathways are a series of metabolic conversions of molecules, each step catalyzed by a specific enzyme.
• An enzyme is a specific protein that catalyzes a particular reaction.
• All proteins are encoded by genes; many function as enzymes.
• One gene can code for multiple polypeptides due to alternative splicing.

The Genetic Code

• The genetic code is composed of 64 codons.
• The genetic code has degeneracy, meaning multiple codons can specify the same amino acid.
• Codons are groups of three nucleotides that specify an amino acid.
• Example: GGU, GGC, GGA, and GGG all specify glycine.
• The codon AUG usually starts polypeptide synthesis and is used to specify additional methionine.
• Three codons (UAA, UAG and UGA) are stop codons (termination or nonsense codons) marking the end of polypeptide production.

The Polypeptide Chain

• Polypeptide synthesis is directional, matching the order of codons.
• Peptide bonds form between the carboxyl group of one amino acid and the amino group of the next.
• The newest amino acid has a free carboxyl group.
• The first amino acid is at the amino-terminal end (N-terminus).
• The last amino acid is at the carboxyl-terminal end (C-terminus).

Amino Acid Side Chains

• Amino acids have unique side chains (R-groups) with distinct chemical properties.
• Nonpolar amino acids are hydrophobic, usually found within folded proteins.
• Polar amino acids are hydrophilic, often found on the protein surface interacting with water.

Ribosome Structure and Assembly

• Ribosomes are macromolecular complexes with a large and a small subunit.
• Ribosomal subunits are composed of rRNA and proteins.
• Bacteria have 70S ribosomes, while eukaryotes have 80S ribosomes.
• The ribosome is the site of protein synthesis, with three binding sites for tRNA: A, P, and E.

Stages of Translation

• Initiation: Ribosomal subunits, mRNA, and initiator tRNA combine.
• Elongation: tRNA molecules bring amino acids to the A site, peptide bonds form, and the ribosome moves along mRNA.
• Termination: A stop codon signals release factor binding and polypeptide release.
• Aminoacyl-tRNA synthetases: Enzymes that attach the correct amino acid to its corresponding tRNA.

The Initiation Stage

• The initiator tRNA binds to the mRNA start codon (AUG in eukaryotes and prokaryotes).
• In bacteria, the initiator tRNA carries formyl methionine.
• In eukaryotes, initiator tRNA carries methionine.
• The Shine-Dalgarno sequence (in prokaryotes) and the Kozak sequence (in eukaryotes) facilitate mRNA binding to the ribosome.

The Elongation Stage

• A charged tRNA binds to the A site.
• Peptide bond formation between amino acids.
• The ribosome translocates one codon, moving the tRNA from A to P to E site.
• The process repeats until a stop codon is reached.

The Termination Stage

• A release factor binds to the A site.
• The polypeptide is released from the tRNA.
• The ribosome subunits dissociate from mRNA.

Antibiotics and Translation

• Some antibiotics inhibit bacterial translation by targeting ribosomes.
• Different antibiotics affect different steps of translation.

This quiz covers the process of translation, detailing how mRNA codons specify amino acid sequences to produce polypeptides. It also explores the significance of proteins in cell structure and function and the role of genes in protein synthesis. Test your understanding of the genetic basis for protein synthesis and the functions of enzymes.