Translation Ch 7

Chapter 7, Part 2: Translation

Translation Overview

• Translation Definition: The process through which RNA nucleotides are translated into amino acids, which are the subunits of proteins.

• Coding Problem: This describes the challenge of how the genetic code is deciphered, established over 3 billion years of evolution, ultimately solved by the human brain.

DNA to Protein Process

• Transcription Process: The process of converting the DNA sequence into RNA.

• Post-Transcription Processing: Modifications made to RNA after transcription, which may include adding a 5’ cap (a modified guanine nucleotide) and a Poly A tail (a chain of adenine nucleotides) that:

  • Protects 3’ end: Stabilizes the RNA molecule.

  • Protects 5’ end: Facilitates ribosome binding during translation.

Genetic Code

• Genetic Code Definition: A set of rules that defines how the nucleotide sequence of a gene is translated into an amino acid sequence of a protein.

• Translation: The conversion of information from mRNA into protein.

• Key Features:

  • The code cannot be a simple 1:1 substitution because there are 4 different RNA nucleotides and 20 different amino acids, necessitating a more complex coding scheme.

Codon Structure

• Reading mRNA:

  • mRNA is read in groups of three nucleotides, known as codons.

  • Each codon corresponds to one of the 20 amino acids.

  • Since there are four different nucleotides (A, U, G, C):
    4 imes 4 imes 4 = 64 possible codons exist, but only 20 amino acids are coded for, leading to redundancy: multiple codons may code for the same amino acid.

Reading Frames

• Reading Frames:

  • There are 3 possible reading frames in any mRNA sequence depending on where translation begins.

  • Exercise: Translate the given mRNA sequences based on positional starting points.

  • Example sequences:

  • Sequence 1: 5’ CACGUUCACGGUCA 3’

  • Sequence 2: 5’ CACGUUAACGGUCA 3’

Transfer RNA (tRNA)

• tRNA Function: Acts as an adaptor molecule; capable of pairing specific codons with the corresponding amino acids.

  • The 3’ end of tRNA is linked to an amino acid.

  • Each tRNA molecule contains an anticodon that base-pairs with the mRNA codon, allowing for precise decoding.

  • Hydrogen bonds between the anticodon and codons create a folded structure of the tRNA molecule.

Aminoacyl-tRNA Synthetases

• Function: Enzymes that attach the correct amino acid to its corresponding tRNA.

  • They ensure the appropriate pairing between the tRNA anticodon and the amino acid it transports.

  • Each amino acid typically has a specific synthetase enzyme; for example,

    • Tryptophanyl-tRNA Synthetase: Pairs the amino acid tryptophan with its respective tRNA (tRNASER).

Ribosome Structure and Function

• Ribosomes: The cellular machinery for protein synthesis, made of two subunits.

  • Binding Sites:

  • 1 mRNA Binding Site: Located on the small subunit.

  • 3 tRNA Binding Sites:

    • Site A (Aminoacyl site): Entry point for tRNA.

    • Site P (Peptidyl site): Binds tRNA attached to the growing peptide chain.

    • Site E (Exit site): Where tRNA exits after the amino acid is added to the chain.

Polypeptide Elongation

• Visual Representation: Similar to a choreographed dance.

• Elongation Steps: Newly charged tRNAs enter the ribosome, aligning codons with the correctly assembled peptide bonds, often visually represented with labeled steps for clarity.

Initiation of Protein Synthesis

• Initiation: Begins with the start codon (AUG), which signifies where translation starts.

  • The initiation tRNA, carrying methionine, binds to the P site of the ribosome.

  • Translation Initiation Factors: These proteins facilitate the assembly of the ribosome components at the start codon.

Termination of Protein Synthesis

• Stop Codons:

  • Codons UAA, UAG, and UGA signal termination of translation and do not specify amino acids.

  • Release Factors: Proteins that recognize stop codons and bind to the A site, facilitating the release of the newly synthesized polypeptide chain.

Polyribosomes and Efficacy

• Polyribosomes: Structures that consist of multiple ribosomes translating a single mRNA strand simultaneously, which enhances synthesis efficiency.

• The process from initiation to completion can take approximately 20 seconds to several minutes.

Post-Translation Modifications

• Chaperone Proteins: Assist polypeptide chains in folding into their functional conformation post-translation, ensuring proper protein functionality.

Protein Degradation

• Proteosomes: Cellular complexes that degrade proteins, balancing protein synthesis with degradation rates.

• Proteolysis: The breakdown of proteins, a mechanism used for recycling amino acids.

• Ubiquitin: A small protein that serves as a marker for proteins destined for degradation, tagging them for entry into proteosome pathways.

Summary of Transcription and Translation

• Gene Expression Regulation: This process is predominantly controlled at the transcription initiation site, determining how genes are expressed into functional proteins.