Translation and Gene Expression Notes

Introduction to Translation

• Definition: Translation is the intricate biological process whereby messenger RNA (mRNA), synthesized during transcription, is decoded to synthesize a polypeptide chain, ultimately forming proteins essential for various cellular functions.

• Role of tRNA: Transfer RNA (tRNA) serves as an adaptor molecule that transports specific amino acids to the ribosome, the cellular machinery responsible for protein synthesis, where these amino acids are assembled into polypeptide chains based on the sequence of nucleotides in the mRNA.

Structure of tRNA

• Description: tRNA is a single-stranded RNA molecule composed of approximately 80 nucleotides, featuring a cloverleaf structure when flattened, which is critical for its function in translation.

• Key Features:

  • Amino Acid Attachment Site: One end of the tRNA molecule has an amino acid attachment site that corresponds to a specific amino acid recognized by the enzyme aminoacyl-tRNA synthetase.

  • Anticodon: The opposite end contains an anticodon, a sequence of three nucleotides that is complementary to a specific mRNA codon, ensuring the correct incorporation of amino acids into the growing polypeptide chain.

Enzyme Action in tRNA Activation

• Role of Aminoacyl-tRNA Synthetase: This critical enzyme catalyzes the charging of tRNA with its corresponding amino acid in two main steps:

  1. The tRNA molecule and the amino acid bind to the active site of the enzyme (for instance, tyrosyl tRNA synthetase).

  2. A covalent bond forms between the amino acid and tRNA, with energy derived from ATP. This process releases the charged aminoacyl-tRNA, which is then ready for use in translation.

Structure of Ribosomes

• Function: Ribosomes play a central role in protein synthesis by facilitating the accurate pairing of tRNA anticodons with mRNA codons, ensuring that proteins are synthesized efficiently and correctly.

• Differences in Prokaryotic and Eukaryotic Ribosomes:

  • Eukaryotic ribosomes are larger, containing approximately 80 different proteins and four distinct types of rRNA (18S, 5S, 5.8S, and 28S), reflecting their complexity and functionality.

  • Prokaryotic ribosomes consist of fewer proteins and rRNA components but serve a similar functional role in protein synthesis.

• Ribosome Structure:

  • Ribosomes are composed of two subunits (large and small) that come together during translation.

  • Ribosomal sites include the A (acceptor), P (peptidyl), and E (exit) sites, each playing a critical role in the translation process.

Stages of Translation

1. Initiation:

   • The formation of the translation initiation complex occurs at the P site, consisting of:

     • Initiator tRNA linked to formylmethionine (fMet).

     • The mRNA strand.

     • The small ribosomal subunit.

     • Various initiation factors, which facilitate the assembly of the complex.

   • The Shine-Dalgarno sequence in prokaryotic mRNA plays an essential role in correctly positioning the small subunit of the ribosome.

2. Elongation:

   • Throughout elongation, amino acids are sequentially added to the growing polypeptide chain, involving:

     • Codon Recognition: The tRNA anticodon pairs with the appropriate mRNA codon at the A site.

     • Peptide Bond Formation: A peptide bond forms between the amino acids located at the P and A sites, catalyzed by the ribosomal RNA.

     • Translocation: The ribosome moves along the mRNA by one codon, shifting the tRNA from the A site to the P site, allowing a new tRNA to enter the A site, and continuing the elongation process.

   • Translation proceeds in the 5' to 3' direction along the mRNA strand, ensuring the correct sequence of amino acids.

3. Termination:

   • Termination occurs when a stop codon (UAG, UAA, UGA) is encountered:

     • A release factor binds to the A site, promoting hydrolysis of the polypeptide from the tRNA.

     • This leads to the release of the completed polypeptide and the dissociation of the ribosomal subunits from the mRNA, terminating the translation process.

Post-Translational Modifications

• After translation, newly synthesized polypeptides may undergo essential modifications such as folding, formation of disulfide bonds, and targeting to specific cellular locations such as the endoplasmic reticulum (ER) where they undergo further processing.

• Signal Peptides: These are short sequences of about 16 amino acids that direct the ribosome to the endoplasmic reticulum (ER) during synthesis. Proteins that lack signal peptides often remain in the cytosol, illustrating the importance of these sequences in cellular localization.

Polyribosomes

• In cells, multiple ribosomes can bind to and translate a single mRNA molecule simultaneously, forming structures known as polyribosomes (or polysomes). This arrangement enhances the efficiency of protein production, allowing for the rapid synthesis of proteins when needed.

Coupling of Transcription and Translation

• In bacteria, transcription and translation can occur concurrently within the cytoplasm due to the absence of a nuclear membrane. This allows for immediate translation of mRNA as it is synthesized.

• In eukaryotes, these processes are compartmentalized; transcription occurs in the nucleus, and translation occurs in the cytoplasm, necessitating the export of mRNA from the nucleus to the ribosomes for protein synthesis.

Mutations and Their Effects

• Definition of Mutations: Mutations are defined as changes in the genetic material (DNA or RNA) that can lead to alterations in protein structure and function, potentially resulting in various phenotypic outcomes.

• Point Mutations:

  • These involve changes in a single base pair, which can lead to different consequences for protein function:

  • Missense Mutations: Result in a different amino acid being incorporated into the protein, which can affect its functionality (e.g., sickle cell anemia caused by a single base change in the beta globin gene).

  • Silent Mutations: Do not result in any change to the amino acid sequence due to redundancy in the genetic code, often having no impact on the overall protein function.

  • Nonsense Mutations: Introduce a premature stop codon into the mRNA, leading to truncated proteins that are often nonfunctional and can cause severe diseases.

• Insertions/Deletions: These mutations can lead to frameshift mutations, which alter the reading frame of the mRNA, significantly affecting the resulting protein structure and function.