Translation and Protein Synthesis

Overview

• Translation is the process of converting messenger RNA (mRNA) into a protein.
• Occurs in the cytoplasm.
• All proteins begin translation in the cytoplasm; some complete it elsewhere or move to other organelles.

Central Dogma Reminder

• DNA → RNA → Protein

Components Needed for Translation

• mRNA: Mature with a 5' cap and poly-A tail.
• Ribosome: Composed of large and small subunits; has three active sites (A, P, E sites).
• tRNA (transfer RNA): One for each amino acid; carries amino acids to the ribosome.
• rRNA (ribosomal RNA): Part of the ribosome; catalyzes peptide bonds.
• Aminoacyl tRNA Synthetase: Enzyme that attaches the correct amino acid to its tRNA.
• Initiation Factors: Proteins that help assemble the ribosome and position the mRNA.
• Elongation Factors: Assist in peptide chain synthesis.
• Release Factors: Help disassemble the translation complex at the end.

Ribosome Structure and Function

• A-site (Aminoacyl site): Where tRNA first binds.
• P-site (Peptidyl site): Where peptide bonds form.
• E-site (Exit site): Where tRNA exits after delivering its amino acid.

Process of Translation

Initiation

• mRNA binds to the small subunit of the ribosome.
• Initiation factors assist in positioning the start codon (AUG).
• Large subunit binds to form the complete initiation complex.

Elongation

• tRNA brings amino acids to the ribosome in sequence dictated by mRNA codons.
• Peptide bonds form between amino acids, elongating the polypeptide chain.
• The ribosome moves along the mRNA from 5' to 3'.

Termination

• Occurs at a stop codon (UAA, UAG, UGA).
• Release factors help disassemble the ribosome and release the completed polypeptide.

Importance of Reading Frame

• Correct reading frame ensures proper protein synthesis.
• Mutations like insertions/deletions can disrupt the reading frame, potentially causing nonfunctional proteins.

mRNA Codon Table

• 64 codons in total; 61 code for amino acids, 3 are stop codons.
• Redundancy in the code provides some tolerance to mutations.

Regulation and Mutations

• Translation and preceding processes (transcription, RNA processing) are tightly regulated.
• Mutations can play a role in diseases like cancer by affecting protein function.

Protein Sorting and Secretion

• Proteins without a signal sequence stay in the cytoplasm.
• Proteins with a signal sequence are directed to the endoplasmic reticulum (ER) and may be secreted.
• Transmembrane proteins have stretches of hydrophobic amino acids to embed in membranes.

Protein Folding

• Happens spontaneously; all information needed is in the amino acid sequence.
• No energy required for folding.

Evolution and Protein Families

• Proteins evolve through mutations that may offer survival advantages.
• Gene duplications contribute to evolution.
• Proteins are grouped by sequence and structure into families.

Visual Aids

• Diagrams of transcription, RNA processing, and translation are useful for visual learners.