Translation of RNA to Protein Practice Flashcards

Comparison of Protein Synthesis in Eukaryotes and Prokaryotes

• Eukaryotic Pathway:

  • Location: Occurs in two distinct compartments. Transcription and RNA processing happen in the nucleus, while translation occurs in the cytoplasm.

  • DNA to pre-mRNA: DNA contains both exons (coding sequences) and introns (non-coding sequences). Transcription produces pre-mRNA.

  • RNA Processing: Before export, pre-mRNA undergoes three major modifications:

    • $5'$ Capping: Addition of an RNA cap to the $5'$ end.

    • RNA Splicing: Removal of introns.

    • $3'$ Polyadenylation: Addition of a poly-A tail to the $3'$ end.

  • Export: The mature mRNA is exported from the nucleus to the cytoplasm through nuclear pores.

  • Translation: In the cytoplasm, mRNA is translated into protein. Eventually, mRNA undergoes degradation.

• Prokaryotic Pathway:

  • Location: Transcription and translation occur in the same compartment (cytoplasm) as there is no nucleus.

  • Process: DNA is transcribed into mRNA, which is immediately available for translation into protein. mRNA degradation follows.

The Genetic Code and Codon Logic

• Definition: The nucleotide sequence of an mRNA is translated into the amino acid sequence of a protein via the genetic code.

  • Codon Structure: Codons consist of three nucleotides. They are typically written with the $5'$ terminal nucleotide to the left.

  • Redundancy: Most amino acids are represented by more than one codon (degeneracy). Codons for the same amino acid often share the same nucleotides at the first and second positions.

  • Stop Codons: There are three specific codons that do not specify an amino acid but serve as termination sites to signal the end of translation.

  • Reading Frames: An mRNA molecule can theoretically be translated in three possible reading frames depending on where the decoding process begins. Only one frame typically produces the functional protein.

• Cracking the Code (Historical Context):

  • Experimental setups used to decipher the code included:

    • Synthetic poly-U mRNAs.

    • Bacterial ribosomes.

    • tRNAs and necessary enzymes.

    • Radioactive amino acids for tracking.

    • Other essential small molecules.

Transfer RNA (tRNA) as Molecular Adaptors

• Function: tRNA molecules act as adaptors that link specific amino acids to their corresponding codons in the mRNA.

• Structural Features:

  • Anticodon: A set of three nucleotides that binds to a complementary mRNA codon via base pairing.

  • $3'$ Attachment Site: The specific amino acid matching the anticodon is covalently attached to the $3'$ end of the tRNA.

• Chemical Modifications: tRNAs contain unusual bases produced by chemical modification of uracil after synthesis:

  • $\psi$ (Pseudouridine): Derived from uracil.

  • $D$ (Dihydrouridine): Derived from uracil.

• Charging mRNA: Specific enzymes called aminoacyl-tRNA synthetases couple each tRNA to its correct amino acid. This process is known as "charging." Each synthetase is specific to one particular amino acid.

Ribosome Structure and the Translation Cycle

• Ribosome Location (Eukaryotes): Found in the cytoplasm. Some are free-floating, while others are bound to the membranes of the endoplasmic reticulum (ER).

• Subunits:

  • Large Subunit: $60S$ in eukaryotes.

  • Small Subunit: $40S$ in eukaryotes.

• Binding Sites: Each ribosome contains one binding site for mRNA and three sites for tRNA:

  • A site (Aminoacyl-tRNA): Where the incoming charged tRNA binds.

  • P site (Peptidyl-tRNA): Where the tRNA carrying the growing polypeptide chain resides.

  • E site (Exit): Where spent tRNAs are released.

• The Four-Step Translation Cycle:

  1. Step 1: A charged tRNA binds to the A site.

  2. Step 2: A new peptide bond is formed between the amino acid in the A site and the growing chain in the P site.

  3. Step 3: The large ribosomal subunit translocates (shifts forward), moving the tRNAs into the E and P sites.

  4. Step 4: The small ribosomal subunit translocates to catch up with the large subunit, and the ejected tRNA leaves the E site. This resets the A site for the next charged tRNA.

Translation Initiation and Termination

• Eukaryotic Initiation:

  • Requires translation initiation factors (specifically eIF1-3).

  • A special initiator tRNA (methionyl-tRNA or Met-tRNAi) is used to locate the start codon ($AUG$).

• Prokaryotic Initiation:

  • Ribosomes initiate translation at specific ribosome-binding sites (e.g., the Shine-Dalgarno sequence).

  • These sites can be located in the interior of the mRNA, allowing one mRNA to encode multiple different proteins (often organized in operons).

  • The $16S$ rRNA in the $30S$ subunit of Escherichia coli is composed of $1540$ nucleotides and contains a sequence ($5'-GAUCACCUCCUUA-3'$) that interacts with the binding site.

• Termination:

  • Translation stops when the ribosome reaches a stop codon ($UAG$, $UAA$, or $UGA$).

  • A release factor binds to the A site instead of a tRNA.

  • The polypeptide chain is released, and the ribosome dissociates into its two subunits.

Polyribosomes and Post-Translational Regulation

• Polyribosomes (Polysomes): Multiple ribosomes can translate a single mRNA molecule simultaneously to increase protein production efficiency.

  • Eukaryotic Specialization: mRNA often loops into a circular shape. This is mediated by PABPI (poly-A binding protein) which binds to the $3'$ poly-A tail.

• Protein Regulation and Degradation:

  • The amount of protein in a cell is regulated by controlled breakdown.

  • Polyubiquitin Chains: Proteins destined for destruction are tagged with a chain of ubiquitin molecules.

  • Proteasome: A large protein complex lined with proteases that recognizes ubiquitin tags and degrades the protein into small peptide fragments.

• Post-Translational Modifications: To become fully functional, polypeptides must:

  • Fold correctly into a three-dimensional conformation.

  • Bind required cofactors or other protein subunits.

  • Undergo covalent modifications (more than $100$ types exist), including Phosphorylation and Glycosylation.

Antibiotics Targeting Translation

Many antibiotics specifically target prokaryotic protein or RNA synthesis without affecting eukaryotic cells:

• Tetracycline: Blocks the binding of aminoacyl-tRNA to the A site of the ribosome (Step 1).

• Streptomycin: Prevents the transition from the initiation complex to chain elongation; also induces miscoding.

• Chloramphenicol: Inhibits the peptidyl transferase reaction on ribosomes (Step 2).

• Cycloheximide: Blocks the translocation step in translation (Step 3).

• Rifamycin: Inhibits RNA polymerase, thereby blocking the initiation of transcription.

RNA and the Origins of Life

• Autocatalysis: Life requires the ability to catalyze reactions and store information.

• RNA World Hypothesis: Suggests that life on Earth began with RNA molecules that could both store genetic information and catalyze their own replication.

• Ribozymes: RNA molecules with catalytic activity.

  • The ribosome itself is a ribozyme, meaning the RNA components, not the proteins, catalyze the formation of peptide bonds.

  • Ribozymes are also found in viroids and large RNA genomes.