Gene Expression: Transcription and Protein Synthesis Study Notes

Gene Expression I: The Genetic Code and Transcription

Introduction to Transcription

Transcription is the process by which genetic information encoded in DNA is copied into messenger RNA (mRNA) for protein synthesis. This process is fundamental to gene expression and occurs in the nucleus of eukaryotic cells.

• Genetic Code: The set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins by living cells.

• Transcription Initiation: Begins at promoters, specific DNA sequences recognized by RNA polymerase and transcription factors.

• RNA Polymerases: Enzymes responsible for synthesizing RNA from a DNA template. In eukaryotes, there are three main types: RNA polymerase I, II, and III.

• Promoters: DNA sequences that define where transcription of a gene by RNA polymerase begins.

• Transcription Factors: Proteins that help regulate the transcription process by assisting RNA polymerase binding and initiation.

• Termination: The process by which transcription ends, releasing the newly synthesized RNA molecule.

Example: In eukaryotes, the TATA box is a common promoter element that helps position RNA polymerase II for transcription initiation.

Additional info: Transcription also involves elongation (RNA chain growth) and post-transcriptional processing such as splicing, capping, and polyadenylation.

Prokaryotic vs. Eukaryotic Transcription

Transcription mechanisms differ between prokaryotes and eukaryotes, primarily in the complexity of regulatory elements and RNA processing.

• Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm; RNA polymerase recognizes simple promoter sequences.

• Eukaryotes: Transcription occurs in the nucleus; involves multiple RNA polymerases and extensive RNA processing.

Example: Eukaryotic mRNA undergoes splicing to remove introns, while prokaryotic mRNA does not.

Gene Expression II: Protein Synthesis

The Genetic Code and Translation

Translation is the process by which mRNA is decoded to produce a specific polypeptide, or protein. This occurs in the ribosome and involves tRNA molecules that bring amino acids corresponding to codons in the mRNA.

• Codons: Triplets of nucleotides in mRNA that specify particular amino acids.

• Start Codon: AUG, which codes for methionine and signals the start of translation.

• Stop Codons: UAA, UAG, UGA; signal the end of translation.

• tRNA: Transfer RNA molecules that carry amino acids to the ribosome and match them to the mRNA codon via their anticodon.

• Ribosome: The molecular machine that facilitates the assembly of amino acids into polypeptides.

Example: The codon UUU codes for the amino acid phenylalanine.

Stages of Translation

Translation occurs in three main stages: initiation, elongation, and termination.

• Initiation: The ribosome assembles around the target mRNA. The first tRNA is attached at the start codon.

• Elongation: tRNAs bring amino acids to the ribosome, and the polypeptide chain is formed.

• Termination: When a stop codon is reached, the ribosome releases the polypeptide.

Equation:

$\text{Protein Synthesis Rate} = k[\text{Ribosome}][\text{mRNA}]$

Post-Translational Modifications

After translation, proteins may undergo modifications that affect their function, localization, or stability.

• Phosphorylation: Addition of phosphate groups to proteins, often regulating activity.

• Glycosylation: Addition of carbohydrate groups, important for protein folding and stability.

• Proteolytic Cleavage: Removal of specific peptide segments to activate or mature the protein.

Example: Insulin is produced as a precursor (proinsulin) and activated by proteolytic cleavage.

Table: Comparison of Prokaryotic and Eukaryotic Translation

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Cytoplasm (after mRNA export from nucleus)
Initiation Factors	Fewer, simpler	More, complex
mRNA Processing	None	Splicing, capping, polyadenylation
Ribosome Size	70S	80S

Additional info: Eukaryotic translation is regulated by various signaling pathways and can be modulated in response to cellular conditions.