Gene Expression: From Gene to Protein

Gene Expression: From Gene to Protein

Introduction to Gene Expression

Gene expression is the process by which genetic information flows from DNA to functional protein products. This process involves two main stages: transcription and translation. The central dogma of molecular biology describes this directional flow of information as DNA → RNA → Protein.

The Flow of Genetic Information

Beadle and Tatum's Experiments

Beadle and Tatum demonstrated that genes act by regulating distinct chemical events. By exposing Neurospora crassa (bread mold) to X-rays, they created mutants that could not survive on minimal medium, revealing that each gene encodes a specific enzyme in a metabolic pathway.

• Wild-type cells can grow on minimal medium.

• Mutant classes require specific supplements (e.g., arginine, ornithine, citrulline) to grow, indicating a block in a particular enzymatic step.

One Gene–One Polypeptide Hypothesis

Each gene encodes a single polypeptide, which may function as an enzyme or a structural protein. This concept has evolved from the original one gene–one enzyme hypothesis.

Transcription and Translation: Basic Principles

Overview of Transcription and Translation

Transcription is the synthesis of RNA from a DNA template, producing messenger RNA (mRNA). Translation is the synthesis of a polypeptide using the information encoded in mRNA. Ribosomes are the molecular machines that facilitate translation.

• Prokaryotes: Transcription and translation occur simultaneously in the cytoplasm.

• Eukaryotes: Transcription occurs in the nucleus; translation occurs in the cytoplasm after RNA processing.

The Central Dogma

The central dogma summarizes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

The Genetic Code

Codons: Triplets of Nucleotides

The genetic code is read in triplets called codons, each specifying an amino acid. There are 64 possible codons, 61 of which code for amino acids and 3 serve as stop signals.

• The code is redundant (multiple codons for one amino acid) but not ambiguous (each codon specifies only one amino acid).

• Codons must be read in the correct reading frame for proper protein synthesis.

Transcription: DNA-Directed Synthesis of RNA

Stages of Transcription

• Initiation: RNA polymerase binds to the promoter region, aided by transcription factors in eukaryotes.

• Elongation: RNA polymerase synthesizes the RNA strand, complementary to the DNA template strand.

• Termination: In prokaryotes, transcription ends at a terminator sequence; in eukaryotes, after the polyadenylation signal.

RNA Processing in Eukaryotes

Primary RNA transcripts (pre-mRNA) undergo several modifications:

• Addition of a 5′ cap (modified guanine nucleotide)

• Addition of a 3′ poly-A tail (50–250 adenine nucleotides)

• RNA splicing: removal of noncoding introns and joining of coding exons

RNA Splicing and Spliceosomes

Spliceosomes, composed of small RNAs and proteins, catalyze the removal of introns and ligation of exons. Some RNA molecules (ribozymes) can catalyze their own splicing.

Alternative RNA Splicing

Alternative splicing allows a single gene to code for multiple proteins by varying which exons are included in the final mRNA.

Translation: RNA-Directed Synthesis of Polypeptides

Transfer RNA (tRNA) Structure and Function

tRNAs are adaptor molecules that match amino acids to codons in mRNA. Each tRNA has an anticodon and an amino acid attachment site.

Aminoacyl-tRNA Synthetases

These enzymes attach the correct amino acid to its corresponding tRNA, ensuring translation fidelity.

Ribosome Structure and Function

Ribosomes consist of large and small subunits, each composed of rRNA and proteins. They have three binding sites for tRNA:

• P site: Holds the tRNA with the growing polypeptide chain

• A site: Holds the tRNA with the next amino acid

• E site: Exit site for discharged tRNAs

Stages of Translation

• Initiation: Assembly of the translation initiation complex at the start codon (AUG).

• Elongation: Amino acids are added one by one to the growing chain through codon recognition, peptide bond formation, and translocation.

• Termination: Occurs when a stop codon is reached; the completed polypeptide is released.

Polyribosomes and Protein Targeting

Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome. Proteins may be targeted to specific cellular locations, such as the endoplasmic reticulum for secretion.

Mutations and Their Effects on Protein Structure

Point Mutations

Point mutations are changes in a single nucleotide pair. They can be classified as:

• Silent mutations: No effect on the amino acid sequence

• Missense mutations: Change one amino acid to another

• Nonsense mutations: Change an amino acid codon to a stop codon, truncating the protein

• Insertions/Deletions: Addition or loss of nucleotides can cause frameshift mutations, altering the reading frame

Summary Table: Key Steps in Gene Expression

Step	Location (Eukaryotes)	Main Events
Transcription	Nucleus	DNA → pre-mRNA
RNA Processing	Nucleus	5′ cap, poly-A tail, splicing
Translation	Cytoplasm	mRNA → polypeptide