Chapter 14: Gene Expression – From Gene to Protein (Study Guide)

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Chapter 14: Gene Expression – From Gene to Protein

Concept 14.1 – Genes Specify Proteins

This section explores the foundational concept that genes encode proteins, the molecular workhorses of the cell. It covers the experimental evidence, the central dogma, and the flow of genetic information.

Beadle and Tatum’s Experiment: Demonstrated that genes act by regulating distinct steps in metabolic pathways, leading to the "one gene–one enzyme" hypothesis.
Central Dogma of Molecular Biology: Information flows from DNA → RNA → Protein.
Gene Expression: The process by which DNA directs the synthesis of proteins through transcription and translation.
Template Strand: The DNA strand that is transcribed into mRNA.
Codons: Triplets of nucleotides in mRNA that specify amino acids.
Genetic Code: Universal, redundant, and unambiguous; each codon specifies one amino acid.
Start and Stop Codons: AUG (start), UAA, UAG, UGA (stop).
Reading Frame: The correct grouping of nucleotides to produce the intended polypeptide.
Example: The gene for hemoglobin encodes the protein responsible for oxygen transport in blood.

Concept 14.2 – Transcription: DNA-Directed mRNA Synthesis

Transcription is the process by which a gene’s DNA sequence is copied to make an mRNA molecule. This section details the steps and regulatory mechanisms involved.

Initiation: RNA polymerase binds to the promoter region with the help of transcription factors in eukaryotes.
Elongation: RNA polymerase moves along the DNA, synthesizing mRNA in the 5' to 3' direction.
Termination: Transcription ends when RNA polymerase reaches a terminator sequence.
Promoter: DNA sequence where RNA polymerase attaches and initiates transcription.
Transcription Factors: Proteins that assist the binding of RNA polymerase to the promoter.
Example: The TATA box is a common promoter element in eukaryotic genes.

Concept 14.3 – Eukaryotes Modify Primary mRNA Transcripts

In eukaryotes, the primary mRNA transcript (pre-mRNA) undergoes several modifications before becoming mature mRNA that can be translated.

5' Capping: Addition of a modified guanine nucleotide to the 5' end for stability and ribosome recognition.
3' Polyadenylation: Addition of a poly(A) tail to the 3' end for stability and export from the nucleus.
Splicing: Removal of non-coding introns and joining of coding exons by the spliceosome.
Alternative Splicing: Allows a single gene to code for multiple proteins by varying exon combinations.
Example: The human dystrophin gene undergoes alternative splicing to produce different protein isoforms.

Concept 14.4 – Translation: mRNA-Directed Protein Synthesis

Translation is the process by which the sequence of an mRNA molecule directs the incorporation of amino acids into a polypeptide chain, forming a protein.

Ribosome Structure: Composed of large and small subunits, with binding sites for mRNA and tRNAs (A, P, and E sites).
tRNA: Transfers specific amino acids to the growing polypeptide chain; contains an anticodon complementary to mRNA codons.
Initiation: Assembly of the translation machinery at the start codon (AUG).
Elongation: Sequential addition of amino acids as the ribosome moves along the mRNA.
Termination: Occurs when a stop codon is reached; the completed polypeptide is released.
Polyribosomes: Multiple ribosomes translating a single mRNA simultaneously, increasing efficiency.
Equation:
Example: Sickle cell anemia results from a single amino acid substitution in the β-globin protein.

Concept 14.5 – Mutations Can Affect Proteins

Mutations are changes in the DNA sequence that can alter the structure and function of proteins. This section discusses types of mutations and their effects.

Point Mutations: Changes in a single nucleotide pair (substitutions, insertions, deletions).
Missense Mutation: Substitution results in a different amino acid.
Nonsense Mutation: Substitution creates a stop codon, leading to a truncated protein.
Frameshift Mutation: Insertions or deletions that alter the reading frame, often resulting in nonfunctional proteins.
Silent Mutation: Substitution does not change the amino acid due to redundancy in the genetic code.
Example: Cystic fibrosis is often caused by a deletion of three nucleotides, resulting in the loss of a single amino acid in the CFTR protein.