Gene Expression: From Gene to Protein

The Central Dogma of Molecular Biology

Gene expression is the process by which the information encoded in DNA is used to produce a functional product, typically a protein or RNA. The central dogma describes the directional flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• Gene Expression: The process of converting genetic information into functional molecules (RNA or protein).

• Central Dogma: DNA → RNA → Protein

• Transcription: Synthesis of RNA from a DNA template (occurs in the nucleus).

• Translation: Synthesis of a polypeptide (protein) from an mRNA template (occurs at ribosomes in the cytoplasm).

Beadle and Tatum’s Experiment: One Gene, One Protein Hypothesis

Beadle and Tatum used the bread mold Neurospora crassa to demonstrate that genes act by regulating distinct chemical events. Their experiments led to the one gene, one enzyme (later, one protein) hypothesis.

• Model Organism: Neurospora crassa (haploid, easy to mutate and analyze).

• Method: Spores were exposed to X-rays to induce mutations, then tested for growth on minimal and complete media.

• Findings: Mutants unable to grow on minimal media but able to grow on complete media had lost the ability to synthesize certain amino acids, linking specific genes to specific enzymes in metabolic pathways.

• Conclusion: Each gene encodes a single enzyme (or protein), supporting the one gene, one protein hypothesis.

Flow of Genetic Information

The flow of genetic information in cells follows the central dogma: DNA is transcribed into RNA, which is then translated into protein. This process is fundamental to all living organisms.

• Transcription: Occurs in the nucleus; produces pre-mRNA (primary transcript).

• Translation: Occurs in the cytoplasm at ribosomes; produces a polypeptide chain.

Types of RNA

Several types of RNA are involved in gene expression, each with a specific role:

• mRNA (messenger RNA): Carries genetic information from DNA to the ribosome for protein synthesis.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes, the site of translation.

• tRNA (transfer RNA): Brings amino acids to the ribosome and matches them to the mRNA codon via its anticodon.

The Genetic Code

Triplet Code and Codons

The genetic code is read in sets of three nucleotides called codons. Each codon specifies a particular amino acid or a stop signal during translation.

• Template Strand: The DNA strand used to synthesize mRNA (read 3' to 5').

• Coding Strand: The DNA strand with the same sequence as the mRNA (except T for U).

• Codon: A sequence of three mRNA nucleotides that codes for a specific amino acid.

• Start Codon: AUG (codes for methionine; signals the start of translation).

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

• Reading Frame: The way nucleotides are grouped into codons for translation.

Example:

• DNA: TAC-TTT-GGG-ACC-ACT

• RNA: AUG-AAA-CCC-UGG-UGA

• Polypeptide: Methionine-Lysine-Proline-Tryptophan-STOP

Gene Expression in Action: GFP and Fluorescent Proteins

Green Fluorescent Protein (GFP)

GFP is a small protein (238 amino acids) originally found in the jellyfish Aequorea victoria. It fluoresces green under UV light and is widely used as a reporter in molecular biology to study gene expression and protein localization.

• Application: GFP can be fused to other proteins to visualize their location in cells without disrupting function.

Variants and Applications of Fluorescent Proteins

Scientists have engineered GFP variants that fluoresce in different colors, allowing simultaneous tracking of multiple proteins in the same cell.

• Multiple Colors: Enables tagging and visualization of several proteins at once.

• GloFish: Genetically engineered fish expressing fluorescent proteins, sold as novelty pets.

Transcription: DNA-Directed Synthesis of RNA

Stages of Transcription

Transcription is the process of synthesizing RNA from a DNA template. It involves three main stages:

• Initiation: RNA polymerase binds to the promoter region (often containing a TATA box) and begins RNA synthesis.

• Elongation: RNA polymerase moves along the DNA, unwinding the strands and synthesizing the RNA transcript.

• Termination: RNA polymerase releases the completed RNA transcript at the end of the gene.

In eukaryotes, transcription initiation involves activators binding to enhancers, DNA looping, and assembly of a transcription initiation complex at the promoter.

RNA Processing in Eukaryotes

Modifications to Pre-mRNA

In eukaryotic cells, the primary RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA ready for translation:

• 5' Cap: Addition of a modified guanine nucleotide to the 5' end (aids in ribosome binding and protects from degradation).

• Poly-A Tail: Addition of 50-250 adenine nucleotides to the 3' end (protects mRNA and aids export from the nucleus).

• RNA Splicing: Removal of non-coding introns and joining of coding exons.

Introns, Exons, and RNA Splicing

• Introns: Non-coding sequences removed from pre-mRNA during splicing.

• Exons: Coding sequences that remain in mRNA and are expressed as protein.

• Spliceosome: A complex of proteins and small nuclear RNAs (snRNAs) that catalyzes the removal of introns.

• Ribozyme: An RNA molecule with catalytic activity (e.g., snRNA in the spliceosome).

• Alternative Splicing: Allows a single gene to code for multiple proteins by varying the combination of exons included in the final mRNA.

Translation: RNA-Directed Synthesis of Polypeptides

Stages of Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. It occurs in three stages:

• Initiation: Ribosome assembles around the start codon of the mRNA.

• Elongation: Amino acids are added one by one to the growing polypeptide chain.

• Termination: The ribosome encounters a stop codon, releases the completed polypeptide, and disassembles.

Transfer RNA (tRNA) and the Ribosome

• tRNA: Adaptor molecule that brings specific amino acids to the ribosome, matching its anticodon to the mRNA codon.

• Aminoacyl-tRNA Synthetase: Enzyme that attaches the correct amino acid to its corresponding tRNA.

• Wobble: Flexibility in base pairing at the third position of the codon, allowing some tRNAs to pair with multiple codons (often results in silent mutations).

• Ribosome: Composed of large and small subunits made of rRNA and proteins; catalyzes peptide bond formation.

Ribosome Sites

• A site (Aminoacyl site): Entry point for tRNA carrying the next amino acid.

• P site (Peptidyl site): Holds the tRNA with the growing polypeptide chain.

• E site (Exit site): Where empty tRNAs exit the ribosome.

Termination and Post-Translational Modifications

• Termination: When a stop codon is reached, a release factor binds, releasing the polypeptide and disassembling the ribosome.

• Post-Translational Modifications: Proteins may be folded, chemically modified, or targeted to specific cellular locations after translation.

• Signal Peptide: Directs proteins to the endomembrane system via the signal recognition particle (SRP).

• Polyribosomes: Multiple ribosomes translating a single mRNA simultaneously, increasing protein production.

Mutations and Their Effects on Proteins

Types of Mutations

• Mutation: Any change in the DNA sequence.

• Nucleotide-Pair Substitution: Replacement of one nucleotide pair with another.

• Point Mutation: Change in a single nucleotide pair.

• Silent Mutation: No effect on the amino acid sequence.

• Missense Mutation: Changes one amino acid in the protein.

• Nonsense Mutation: Changes a codon to a stop codon, resulting in a truncated protein.

• Base Addition/Deletion: Addition or removal of nucleotides, which can cause a frameshift mutation.

• Frameshift Mutation: Alters the reading frame, affecting all downstream codons.

Diseases Caused by Point Mutations

• Cystic Fibrosis: Mutation in the CFTR gene leads to thick mucus in the lungs.

• Sickle Cell Anemia: Mutation in the hemoglobin gene causes abnormal, sickle-shaped red blood cells that block blood flow.

• Tay Sachs Disease: Mutation causes accumulation of fat in the brain.

Causes of Mutations

• Mutagens: Physical or chemical agents that cause mutations (e.g., radiation, chemicals).

• Carcinogens: Mutagens that cause cancer.

Additional info: The above notes expand on the provided material with definitions, examples, and context to ensure a comprehensive, self-contained study guide for college-level General Biology students.