Gene Expression: From Gene to Protein

Concept 17.1: Genes Specify Proteins via Transcription and Translation

This section explores how genetic information flows from DNA to protein, highlighting key experiments and concepts that shaped our understanding of gene expression.

• Beadle and Tatum’s Experiments: George Beadle and Edward Tatum used the bread mold Neurospora crassa to demonstrate that genes control metabolic pathways by encoding enzymes. By inducing mutations and observing growth on minimal media, they established the "one gene—one enzyme" hypothesis.

• One Gene—One Enzyme vs. One Gene—One Polypeptide: The original hypothesis stated that each gene encodes a single enzyme. Later discoveries showed that some proteins are made of multiple polypeptides, each encoded by a separate gene, leading to the "one gene—one polypeptide" hypothesis.

• Differences Between RNA and DNA:

  • RNA contains ribose sugar; DNA contains deoxyribose.

  • RNA uses uracil (U) instead of thymine (T).

  • RNA is usually single-stranded; DNA is double-stranded.

• Flow of Genetic Information (Central Dogma):

  • DNA --(transcription)--> mRNA --(translation)--> Protein

  • Transcription: Synthesis of RNA from DNA template.

  • Translation: Synthesis of polypeptide using mRNA as a template.

• Transcription vs. Translation:

  • Transcription: DNA is used as a template to make RNA.

  • Translation: mRNA is decoded to build a polypeptide chain (protein).

• Location in Prokaryotes vs. Eukaryotes:

  • Prokaryotes: Both processes occur in the cytoplasm.

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

• Codons: A codon is a sequence of three nucleotides on mRNA that specifies a particular amino acid.

• Genetic Code:

  • Redundant: More than one codon can code for the same amino acid.

  • Unambiguous: Each codon specifies only one amino acid.

• Reading Frame: The way nucleotides are grouped into codons. Shifting the reading frame changes the resulting amino acid sequence.

Concept 17.2: Transcription is the DNA-Directed Synthesis of RNA

Transcription is the process by which RNA is synthesized from a DNA template, involving several key steps and components.

• RNA Polymerase Recognition: RNA polymerase binds to a specific DNA sequence called the promoter to initiate transcription. The terminator signals the end of transcription.

• Transcription Complex: Includes RNA polymerase and various transcription factors that help initiate and regulate transcription.

• Steps of Transcription:

  1. Initiation: RNA polymerase binds to the promoter and unwinds DNA.

  2. Elongation: RNA nucleotides are added to the growing RNA strand.

  3. Termination: RNA polymerase reaches the terminator sequence and releases the RNA transcript.

Concept 17.3: Eukaryotic Cells Modify RNA After Transcription

In eukaryotes, the primary RNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA.

• RNA Modifications:

  • 5' Cap: Addition of a modified guanine nucleotide to the 5' end for stability and ribosome binding.

  • Poly-A Tail: Addition of a string of adenine nucleotides to the 3' end for stability and export from the nucleus.

  • Splicing: Removal of non-coding sequences (introns) and joining of coding sequences (exons).

• Ribozymes: RNA molecules with catalytic activity, capable of self-splicing or catalyzing other reactions.

• Significance of Introns:

  • Functional: Allow for alternative splicing, increasing protein diversity.

  • Evolutionary: May facilitate evolution of new proteins by exon shuffling.

Concept 17.4: Translation is the RNA-Directed Synthesis of a Polypeptide

Translation is the process by which the information in mRNA is used to assemble a chain of amino acids, forming a polypeptide.

• tRNA Structure and Function: Transfer RNA (tRNA) has a cloverleaf structure with an anticodon that pairs with mRNA codons and an attached amino acid.

• Aminoacyl-tRNA Synthetase: Enzyme that joins each tRNA to its correct amino acid.

• Ribosome Structure and Function: Ribosomes are composed of rRNA and proteins, with large and small subunits. They facilitate the coupling of tRNA anticodons with mRNA codons during protein synthesis.

• Steps of Translation:

  1. Initiation: Ribosome assembles around the start codon (AUG) on mRNA.

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

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

• Free vs. Bound Ribosomes: Ribosomes are free in the cytoplasm unless the polypeptide being synthesized has a signal sequence that directs the ribosome to the rough endoplasmic reticulum (ER).

• Protein Synthesis in Prokaryotes vs. Eukaryotes:

  • Prokaryotes: Transcription and translation are coupled; both occur in the cytoplasm.

  • Eukaryotes: Transcription occurs in the nucleus; translation occurs in the cytoplasm. mRNA is processed before translation.

Concept 17.5: Mutations of One or a Few Nucleotides Can Affect Protein Structure and Function

Mutations are changes in the genetic material that can impact gene products in various ways.

• Point Mutations: Changes in a single nucleotide pair of a gene.

• Types of Mutations (Ranked by Impact):

  1. Frameshift Mutation: Insertion or deletion of nucleotides that alters the reading frame, usually resulting in a nonfunctional protein.

  2. Nonsense Mutation: Changes a codon to a stop codon, leading to premature termination of translation.

  3. Missense Mutation: Changes one amino acid to another; effect depends on the role of the altered amino acid.

  4. Silent Mutation: Alters a codon but does not change the amino acid due to redundancy in the genetic code.

  Reasoning: Frameshift mutations generally have the greatest impact because they change the entire downstream amino acid sequence. Nonsense mutations truncate the protein, often making it nonfunctional. Missense mutations may or may not affect function, while silent mutations have no effect on the protein sequence.

Example Table: Types of Point Mutations and Their Effects

Key Equations

• Central Dogma:

Additional info: Some explanations and the table were expanded for clarity and completeness based on standard biology textbooks.