Chapter 17 Biology

Gene Expression: From Gene to Protein

The Flow of Genetic Information

Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically a protein. This process involves two main stages: transcription and translation.

• Transcription: The synthesis of RNA from a DNA template.

• Translation: The synthesis of a polypeptide (protein) from an mRNA template.

• Central Dogma: The directional flow of genetic information: DNA → RNA → Protein.

Experimental Evidence: Beadle and Tatum’s Work

Beadle and Tatum’s experiments with Neurospora crassa demonstrated that genes specify enzymes, supporting the one gene–one enzyme hypothesis. Mutants unable to grow on minimal media were identified, and their growth requirements were mapped to specific metabolic steps.

• Mutants: Created by exposing cells to X-rays.

• Growth Tests: Mutants were tested on media supplemented with different amino acids.

• Conclusion: Each gene encodes a specific enzyme required for a metabolic pathway.

Basic Principles of Transcription and Translation

• RNA: Acts as a bridge between DNA and proteins.

• Transcription: Produces messenger RNA (mRNA).

• Translation: Occurs at ribosomes, where mRNA is decoded to build proteins.

• Prokaryotes: Translation can begin before transcription is finished.

• Eukaryotes: Transcription and translation are separated by the nuclear envelope; RNA processing occurs before translation.

Codons: Triplets of Nucleotides

The genetic code is based on codons, which are three-nucleotide sequences in mRNA that specify amino acids.

• Codon: A sequence of three nucleotides in mRNA.

• 64 Codons: 61 code for amino acids, 3 are stop signals.

• Redundancy: Multiple codons can code for the same amino acid.

• Reading Frame: Codons must be read in the correct grouping.

Transcription: DNA-Directed Synthesis of RNA

Transcription is catalyzed by RNA polymerase and occurs in three stages: initiation, elongation, and termination.

• Initiation: RNA polymerase binds to the promoter region.

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

• Termination: Transcription ends at a terminator sequence (mechanisms differ in prokaryotes and eukaryotes).

RNA Processing in Eukaryotes

Eukaryotic pre-mRNA undergoes several modifications before becoming mature mRNA.

• 5′ Cap: A modified guanine nucleotide added to the 5′ end.

• Poly-A Tail: 50–250 adenine nucleotides added to the 3′ end.

• Functions: Facilitate export, protect from degradation, and help ribosome binding.

Split Genes and RNA Splicing

Most eukaryotic genes contain introns (noncoding regions) and exons (coding regions). RNA splicing removes introns and joins exons.

• Introns: Noncoding regions removed during splicing.

• Exons: Coding regions joined together.

• Spliceosome: Complex that carries out splicing.

Translation: RNA-Directed Synthesis of a Polypeptide

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

• tRNA: Transfers amino acids to the ribosome.

• Aminoacyl-tRNA Synthetase: Enzyme that attaches amino acids to tRNA.

• Ribosome: Has three binding sites for tRNA: A (aminoacyl), P (peptidyl), and E (exit).

Translation Initiation, Elongation, and Termination

• Initiation: Small ribosomal subunit binds to mRNA; initiator tRNA binds to start codon; large subunit joins.

• Elongation: Amino acids are added one by one; involves codon recognition, peptide bond formation, and translocation.

• Termination: Occurs when a stop codon is reached; release factor promotes hydrolysis and ribosome dissociates.

Targeting Polypeptides to Specific Locations

Proteins synthesized by ribosomes may be targeted to specific cellular locations.

• Free Ribosomes: Synthesize proteins for the cytosol.

• Bound Ribosomes: Synthesize proteins for secretion or for the endomembrane system.

• Signal Peptide: Directs ribosome to the ER.

Transcription and Translation in Prokaryotes vs. Eukaryotes

• Prokaryotes: Transcription and translation occur simultaneously.

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

Mutations and Their Effects on Protein Structure and Function

Mutations are changes in the genetic material that can affect protein structure and function.

• Point Mutations: Changes in a single nucleotide pair.

• Types: Substitutions, insertions, deletions.

• Effects: Silent, missense, nonsense, or frameshift mutations.

Summary Table: Types of Point Mutations

Type	Description	Effect
Silent	Substitution that does not change amino acid	No effect on protein
Missense	Substitution that changes one amino acid	May affect protein function
Nonsense	Substitution that creates a stop codon	Premature termination
Frameshift	Insertion or deletion that alters reading frame	Usually severe effect

Key Terms and Concepts

• Gene Expression: The process by which genetic information directs protein synthesis.

• Transcription: DNA-directed synthesis of RNA.

• Translation: RNA-directed synthesis of protein.

• Codon: Three-nucleotide sequence in mRNA.

• Mutation: Change in DNA sequence.

Equations and Formulas

• Central Dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Base Pairing in Transcription: $\text{A} \leftrightarrow \text{U}, \text{T} \leftrightarrow \text{A}, \text{C} \leftrightarrow \text{G}, \text{G} \leftrightarrow \text{C}$

Additional info:

• Alternative RNA splicing increases protein diversity.

• Mutations can be beneficial, neutral, or harmful depending on their effect on protein function.