Molecular and Cellular Basis of Life: Protein Synthesis

The Molecular and Cellular Basis of Life: Replication, Transcription, and Translation

Introduction

  • Genetic information is stored in DNA or RNA (in some viral genomes).

  • Most genetic traits are linked to proteins (including enzymes).

  • Key question: How is genetic information translated from nucleic acids to proteins?

Crick's Adaptor Hypothesis

  • Proposed around 1954 by Francis Crick.

  • Suggests the involvement of an adaptor molecule in protein synthesis.

    • Template Material: RNA acts as a template.

    • Adaptor: Carries amino acids to the template; potentially contains nucleotides.

  • Question Raised: Does such an adaptor exist? What is its nature?

Transfer RNA (tRNA) and its Role

  • Function: tRNA delivers amino acids to mRNA during protein synthesis.

  • Structure:

    • Contains an amino acid binding site.

    • It recognizes codons on mRNA via its anticodons.

Codon and Anticodon Pairing

  • The codon-anticodon pairing is critical for accurate protein synthesis.

  • mRNA codons (from 5' to 3') pair with tRNA anticodons (3' to 5').

    • Example: Codon UAG pairs with anticodon AUC.

Structure of tRNA (Detailed)

  • D arm, T arm, and variable arm present in the structure of tRNA.

  • Anticodon is located at specific positions within tRNA, allowing for flexibility in pairing.

  • Wobble: The first base of the anticodon allows for mispairing, which contributes to genetic code degeneracy.

Properties of the Genetic Code

  1. Composed of Nucleotide Triplets: Codons consist of three nucleotides.

  2. Nonoverlapping: Each nucleotide is part of only one codon.

  3. Comma-Free: No gaps occur between codons in the sequence.

  4. Degenerate: Multiple codons can code for the same amino acid.

  5. Ordered: Codons for similar amino acids are often related.

  6. Start and Stop Signals: Certain codons indicate the beginning (AUG) and end (UAA, UAG, UGA) of translation.

  7. Nearly Universal: With few exceptions, the genetic code is consistent across organisms.

Overlapping vs. Nonoverlapping Code

  • Overlapping Code: Each nucleotide can be part of multiple codons.

    • Example: AUACGAGUC

  • Nonoverlapping Code: Each nucleotide is part of only one codon.

    • Example: AU ACG AGU C

Mutation and Reading Frame

  • Insertion or deletion of one or two nucleotides alters the reading frame, leading to frameshift mutations.

Features of the Genetic Code

  • Written in the 5' to 3' direction.

  • Initiation codon (AUG) establishes the reading frame and codes for methionine.

  • 61 of the 64 codons code for amino acids, while 3 are termination codons.

  • An Open Reading Frame (ORF) is characterized by the presence of an AUG codon followed by a sequence of codons that do not contain STOP codons.

Nucleotide Code Dictionary

  • Similar amino acids are often encoded by similar codons reflecting biochemical similarity.

  • Silent Mutation: Substituting one nucleotide can occur without changing the encoded amino acid.

The Wobble Hypothesis

  • The first base of the anticodon tolerates more mispairing than the other bases.

  • Explains why multiple codons can code for the same amino acid.

  • Example: Anticodon CUA can pair with codons like Leu from different triplets.

Degeneracy of the Genetic Code

  • Several amino acids have multiple corresponding codons.

  • Table of Amino Acid Codons:

    • Methionine (Met): 1 codon

    • Tryptophan (Trp): 1 codon

    • Phenylalanine (Phe): 2 codons

    • Leucine (Leu): 6 codons

    • Serine (Ser): 6 codons

    • Arginine (Arg): 6 codons

Universality of the Genetic Code

  • The genetic code is nearly universal across prokaryotes and eukaryotes with a few exceptions:

    • Mitochondria have a slightly different code and encode their own tRNAs.

    • Rare amino acids are encoded uniquely.

Recap of Key Points

  • 20 amino acids are specified by nucleotide triplets in mRNA.

  • 64 total triplets where 61 code for amino acids and 3 signal termination.

  • Code characteristics include:

    • Nonoverlapping

    • Degenerate

    • Ordered

    • Universal with exceptions.

Translation Overview

  • Main components: mRNA, ribosomes, tRNA.

  • Translation mechanisms involve:

    • Input of ribosomal subunits and initiation factors.

    • Activation of amino acids to tRNA.

Stages of Translation (Protein Synthesis)

  1. Activation of Amino Acids: tRNA aminoacylation.

  2. Initiation of Translation: mRNA and aminoacyl-tRNA bind to ribosome.

  3. Elongation: Successive cycles of aminoacyl-tRNA binding and peptide bond formation until reaching a STOP codon.

  4. Termination and Ribosome Recycling: The mRNA and protein dissociate; the ribosome is recycled.

  5. Folding and Post-Translational Processing: Catalyzed by various enzymes.

Stage 1: Activation of tRNA in E. Coli

  • Aminoacyl-tRNA Synthetases are enzymes that attach specific amino acids to their corresponding tRNA.

    • Each cell contains 20 distinct synthetases, one for each amino acid.

Stage 2: Initiation in E. Coli

Ingredients Needed:

  • 30S ribosomal subunit.

  • Initiator tRNA (tRNA^fMet).

  • mRNA.

  • Initiation Factors (IF-1, IF-2, IF-3).

  • One molecule of GTP.

  • 50S ribosomal subunit.

Initiation in Prokaryotes

  1. Initiation factors and GTP bind to the small ribosomal subunit.

  2. tRNA^fMet and mRNA bind to form an initiation complex.

  3. The large subunit joins, completing the ribosome assembly.

Initiation Complex Formation

  • Requires recognition between the 16S rRNA and the Shine-Dalgarno sequence on mRNA for binding.

Stage 3: Elongation in E. Coli

  • Elongation is cyclical, facilitated by:

    • EF-Tu transporting activated tRNA to the ribosome (A site).

    • Formation of peptide bonds between amino acids.

    • Translocation facilitated by EF-G.

Stage 4: Termination in E. Coli

  • Triggered by a STOP codon (UAA, UAG, UGA) in the A site.

    • Involves release factors (RF-1, RF-2, RF-3) to hydrolyze the terminal peptide-tRNA bond, releasing the peptide and the tRNA.

    • Causes dissociation of the ribosomal subunits for further cycles of initiation.

Key Differences between Prokaryotic and Eukaryotic Translation

Prokaryotes:

  • Transcription and translation occur simultaneously.

  • mRNA is polycistronic.

  • Ribosome size: 70S (50S + 30S).

  • Initiation relies on Shine-Dalgarno sequences.

  • N-Formylmethionine is the initiating amino acid.

Eukaryotes:

  • mRNA is monocistronic and must exit the nucleus before translation.

  • Ribosome size: 80S (60S + 40S).

  • Initiation begins with the 5' end through Kozak sequences.

  • Methionine is the initiating amino acid, with distinct eukaryotic initiation factors.

Conclusion

  • Understanding these fundamental processes in replication, transcription, and translation is crucial for studying molecular biology.

Additional Resources