Translation Elongation: Codon Recognition and tRNA Binding

Transcription Initiation

• The assembly of transcription factors, including the TATA box binding protein (TBP), forms the transcription initiation complex.
• The TATA box, located about 25 nucleotides upstream of the gene, serves as a crucial binding site for these factors.
• TBP is part of a larger protein complex that includes RNA Polymerase II.
• Once assembled, this complex marks the start of transcription.

Initiation of Transcription

• With the transcription initiation complex in place, RNA Polymerase II binds to it.
• Additional transcription factors also join at the promoter region of the gene.
• This binding signifies the beginning of transcription.
• RNA Polymerase II reads the DNA template strand, synthesizing a complementary RNA strand.

Coding and Non-Coding Strands

• The coding strand of DNA has the same sequence as the RNA transcript, except with Thymine (T) instead of Uracil (U).
• The non-coding strand, also known as the template strand, serves as the template for RNA synthesis.

Elongation

• Once initiated, RNA polymerase begins moving downstream along the gene.
• It reads the DNA strand and adds complementary RNA nucleotides to the growing mRNA (messenger RNA) chain.
• During this process, RNA uses Uracil (U) instead of Thymine (T), pairing Adenine (A) with Uracil.

Gene Expression

• Gene expression is the process of going from DNA to a functional product (protein).
• DNA → RNA → Protein is the sequence of gene expression.
• Genotype refers to an organism's hereditary information, while phenotype refers to the actual observable or physiological traits.
• Genotype and its interactions with the environment determine the phenotype.

Transcription Factors

• Transcription factors are proteins that play a pivotal role in regulating gene expression.
• They help determine which genes are turned on or off by binding to nearby DNA.
• Activator transcription factors enhance gene transcription, while repressor factors decrease it.

Processing of Pre-mRNA to Mature mRNA

• Capping involves adding a modified guanine nucleotide to the 5' end of the pre-mRNA transcript, which serves several critical functions, including protection, ribosome binding, and export.
• Tailing involves adding a sequence of 50 to 250 adenine nucleotides (poly-A tail) to the 3' end of the pre-mRNA transcript, which protects the mRNA from degradation, assists in transporting the mature mRNA, and is involved in binding proteins that initiate translation.
• Splicing is the process of removing introns (non-coding regions) from the pre-mRNA transcript, ensuring efficient resource utilization and contributing to cellular specialization and adaptation.

Protein Synthesis and Vesicular Transport

• Proteins destined for the cytosol are continuously produced and function in the cytosol, completing translation on free ribosomes.
• Proteins destined for the endomembrane system complete translation at fixed ribosomes on the rough ER (RER).
• The ribosome's location determines its function, with fixed ribosomes on the RER specialized for proteins destined for the endomembrane system.
• Signal peptide and signal recognition particle (SRP) are involved in the targeting of proteins to the endomembrane system.

Translation Elongation

• The ribosome reads the mRNA codon in the A site, and the anticodon of the incoming tRNA base pairs with the mRNA codon.
• GTP is invested to ensure accuracy and efficiency.
• The large ribosomal subunit catalyzes the formation of a peptide bond, linking the incoming amino acid to the growing polypeptide chain.

Translation Termination

• When a stop codon (UAA, UAG, or UGA) enters the A site, it's recognized by a release factor.
• The release factor fits neatly into the P site, and the bond between the P-site tRNA and the last amino acid is hydrolyzed.
• This marks the end of translation.