Translation and Proteins: Mechanisms and Structure

Translation and Proteins

Revision: Key Concepts in Transcription and Translation

This section reviews foundational questions about transcription and translation, essential for understanding gene expression and protein synthesis.

• Product of Transcription: The product is messenger RNA (mRNA), which carries genetic information from DNA to the ribosome.

• Enzyme for RNA Synthesis: RNA polymerase synthesizes RNA from a DNA template.

• Location of Transcription in Eukaryotes: Transcription occurs in the nucleus.

• Codon Structure: A codon consists of three nucleotides.

• Wobble Hypothesis: The hypothesis that the third base of a codon can pair less specifically, allowing some tRNAs to recognize multiple codons.

• Start Codon and Amino Acid: The start codon is AUG, which codes for methionine.

• Stop Codons: The three stop codons are UAA, UAG, and UGA.

• mRNA and DNA Template Relationship: mRNA is complementary and antiparallel to the DNA template strand.

• Example: For DNA sequence ATG CCG TTA, the complementary mRNA sequence is UAC GGC AAU.

• Post-Transcriptional Processing: In eukaryotes, mRNA undergoes modifications such as 5' capping, polyadenylation, and splicing before translation.

Translation: Decoding Genetic Information

Overview of Translation

Translation is the process by which the genetic code carried by mRNA is decoded to produce a specific sequence of amino acids, forming a polypeptide chain. This process occurs in the cytoplasm at the ribosomes.

• Location: Cytoplasm of eukaryotic cells, specifically at ribosomes.

• Function: Ribosomes translate the genetic message of mRNA into proteins.

• Directionality: mRNA is translated from the 5' to 3' end, producing an N-terminal to C-terminal polypeptide.

• Specificity: Amino acids are inserted in the correct sequence due to specific binding of each amino acid to its tRNA and base-pairing between mRNA codons and tRNA anticodons.

Components of Translation

Several molecular components are required for translation to occur efficiently and accurately.

• mRNA: Carries the genetic code from DNA; in eukaryotes, made in the nucleus and transported to the cytoplasm. In prokaryotes, transcription and translation are coupled.

• tRNA: Adaptor molecules that transfer specific amino acids to the ribosome, matching mRNA codons via their anticodon region.

• Ribosomes: Complexes of ribosomal RNA (rRNA) and proteins; the site of protein synthesis.

• Enzymes: Aminoacyl-tRNA synthetases attach amino acids to their corresponding tRNAs; peptidyl transferase (a ribozyme activity of rRNA) forms peptide bonds.

Ribosomes: Structure and Function

Ribosomes are composed of two subunits, each with distinct roles in translation. Their size is measured in Svedberg (S) units, which reflect sedimentation rates during centrifugation.

• Prokaryotic Ribosomes: 50S (large) + 30S (small) = 70S

• Eukaryotic Ribosomes: 60S (large) + 40S (small) = 80S

• Svedberg Units: Not directly additive; reflect shape and density, not just mass.

Organism	Large Subunit	Small Subunit	Total Ribosome
Prokaryotes	50S	30S	70S
Eukaryotes	60S	40S	80S

Types of RNA in Translation

Two major types of RNA are essential for protein synthesis: ribosomal RNA (rRNA) and transfer RNA (tRNA).

• rRNA: Forms the structural and catalytic core of ribosomes; catalyzes peptide bond formation.

• tRNA: Adaptor molecules with a cloverleaf structure; each tRNA carries a specific amino acid and has an anticodon that pairs with mRNA codons.

• Attachment Sites: tRNA has an amino acid attachment site at the 3' end and an anticodon loop at the opposite end.

Stages of Translation

Translation proceeds through three main stages: initiation, elongation, and termination.

• Initiation:

  1. Small ribosomal subunit binds mRNA near the start codon (AUG).

  2. Initiator tRNA binds to AUG.

  3. Large subunit joins; initiation factors are released.

• Elongation:

  1. Charged tRNA enters the A site.

  2. Peptide bond formation occurs (peptidyl transferase activity).

  3. Ribosome translocates three bases; empty tRNA exits via the E site.

• Termination:

  1. Stop codon enters the A site.

  2. Release factors bind and hydrolyze the polypeptide.

  3. Ribosome dissociates.

Initiation in Prokaryotes and Eukaryotes

Initiation mechanisms differ between prokaryotes and eukaryotes.

• Prokaryotes: The Shine-Dalgarno sequence is a ribosomal binding site upstream of the start codon, facilitating ribosome recruitment. Initiation factors IF1, IF2, and IF3 are required.

• Eukaryotes: Initiation factors (eIF4A, eIF4B, eIF4G) associate with the 5' cap of mRNA, remove secondary structures, and help the ribosome scan for the AUG start codon.

Elongation and Termination

Elongation involves the sequential addition of amino acids to the growing polypeptide chain, while termination releases the completed protein.

• Elongation: Methionine-carrying tRNA starts in the P site; the next tRNA enters the A site, and a peptide bond forms. The ribosome moves along the mRNA, and the cycle repeats.

• Termination: Release factors recognize stop codons (UAA, UAG, UGA), bind to the ribosome, and hydrolyze the polypeptide from the tRNA, ending translation.

Protein Structure and Post-Translational Modifications

Proteins are composed of amino acid monomers linked into polypeptide chains, which fold and may undergo chemical modifications after translation.

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Local folding into alpha helices and beta sheets.

• Tertiary Structure: Overall 3D structure of a single polypeptide chain.

• Quaternary Structure: Assembly of multiple polypeptide chains.

• N-terminus and C-terminus: The ends of a protein chain; N-terminus has a free amino group (NH2), C-terminus has a free carboxyl group (COOH).

• Post-Translational Modifications (PTMs): Chemical changes such as phosphorylation, glycosylation, and proteolysis increase protein diversity and regulate function.

• Protein Folding: Chaperones assist in proper folding; misfolded proteins can lead to diseases such as Alzheimer's, Parkinson's, and prion diseases.

Key Equations and Concepts

• Peptide Bond Formation: The peptide bond forms between the carboxyl group of one amino acid and the amino group of the next: $\text{Amino acid}_1 - \text{COOH} + \text{Amino acid}_2 - \text{NH}_2 \rightarrow \text{Amino acid}_1 - \text{CO}-\text{NH}-\text{Amino acid}_2 + H_2O$

• Genetic Code: Each codon (three nucleotides) specifies one amino acid or a stop signal.

Summary Table: Stages of Translation

Stage	Main Events
Initiation	Ribosome assembly, start codon recognition, initiator tRNA binding
Elongation	Amino acid addition, peptide bond formation, ribosome translocation
Termination	Stop codon recognition, release factor binding, polypeptide release

Additional info: Some context and explanations have been expanded for clarity and completeness, including details on post-transcriptional modifications and protein folding diseases.