Unit 6: Gene Expression and Regulation

📚Study Guide: Gene Expression and Regulation

Unit 6: Gene Expression and Regulation

This unit bridges the gap between genotype and phenotype by exploring how genetic information stored in DNA is converted into functional proteins. The central dogma of molecular biology--DNA -> RNA -> protein--governs this flow of information. Transcription produces messenger RNA (mRNA) from a DNA template, while translation synthesizes polypeptide chains at ribosomes according to the mRNA codon sequence. Understanding the genetic code, including start and stop codons, and the roles of tRNA and rRNA is fundamental. Beyond the basic mechanics, this unit delves into gene regulation in prokaryotes and eukaryotes. The lac operon in E. coli exemplifies prokaryotic regulation, where an inducer (allolactose) removes a repressor to activate transcription. Eukaryotic regulation is far more complex, involving transcription factors, enhancers, silencers, chromatin remodeling, and epigenetic modifications like DNA methylation and histone acetylation. Mutations, the ultimate source of genetic variation, are also covered extensively. Students must distinguish between point mutations (silent, missense, nonsense) and frameshift mutations (insertions, deletions), and understand how each affects protein structure. Biotechnology techniques, including gel electrophoresis, PCR, and CRISPR, are increasingly tested as students must apply molecular biology concepts to real-world scenarios. On the AP exam, this unit appears in almost every free-response section, often requiring students to predict the effect of a mutation on protein structure or to explain regulatory mechanisms.

Key Concepts

• DNA Structure and Replication: DNA is a double helix of antiparallel strands (5' to 3' orientation) held by hydrogen bonds between complementary bases (A-T, G-C). Replication is semiconservative; each new DNA molecule contains one old and one new strand. DNA polymerase synthesizes new strands in the 5' to 3' direction, requiring an RNA primer.
• Transcription: RNA polymerase binds to a promoter region and synthesizes pre-mRNA from the template strand. In eukaryotes, pre-mRNA undergoes processing: 5' cap, poly-A tail addition, and splicing (introns removed, exons joined). Alternative splicing allows one gene to produce multiple proteins.
• Translation: Occurs at ribosomes. mRNA codons are read in the 5' to 3' direction. tRNA anticodons bring specific amino acids. The A site accepts aminoacyl-tRNA, the P site holds the growing peptide, and the E site releases empty tRNA. Translation ends at a stop codon (UAA, UAG, UGA).
• Gene Regulation in Prokaryotes: The lac operon contains a promoter, operator, and structural genes (lacZ, lacY, lacA). When lactose is absent, the lac repressor binds the operator, blocking transcription. When lactose is present, allolactose binds the repressor, allowing RNA polymerase to transcribe the genes.
• Gene Regulation in Eukaryotes: Transcription factors bind to promoters and enhancers to recruit RNA polymerase. Chromatin structure affects accessibility; DNA methylation and histone deacetylation typically repress transcription, while histone acetylation promotes it. MicroRNAs (miRNAs) can block translation or degrade mRNA.
• Mutations: Point mutations substitute one base (silent: no amino acid change; missense: different amino acid; nonsense: premature stop codon). Frameshift mutations (insertions or deletions not in multiples of 3) shift the reading frame, altering all downstream amino acids.

Vocabulary

• Promoter: A DNA sequence upstream of a gene where RNA polymerase and transcription factors bind to initiate transcription.
• Codon: A sequence of three mRNA nucleotides that specifies a particular amino acid or a stop signal during protein synthesis.
• Anticodon: A sequence of three nucleotides on a tRNA molecule that is complementary to an mRNA codon.
• Operon: A cluster of genes in prokaryotes transcribed as a single mRNA and regulated by a single promoter and operator.
• Epigenetics: Heritable changes in gene expression that do not involve changes to the underlying DNA sequence, such as DNA methylation and histone modification.
• RNA Splicing: The removal of introns (non-coding sequences) and joining of exons (coding sequences) from pre-mRNA to produce mature mRNA.

Processes and Diagrams to Know

• DNA Replication Fork: Label leading strand (continuous synthesis), lagging strand (Okazaki fragments), DNA polymerase, helicase, primase, ligase, and single-strand binding proteins.
• lac Operon: Be able to draw the operon in active (lactose present) and inactive (lactose absent) states, labeling the repressor, operator, promoter, and structural genes.
• Transcription and Translation Diagram: Show RNA polymerase at DNA, ribosome with A/P/E sites, and tRNA delivering amino acids.

Experimental Designs

• Gel Electrophoresis: Separates DNA fragments by size. Smaller fragments travel farther. Used for DNA fingerprinting, paternity testing, and analyzing PCR products.
• Transformation: Inserting recombinant DNA (plasmid with gene of interest) into bacteria, which then express the foreign gene.

Common Mistakes

• Confusing Transcription and Replication Enzymes: DNA polymerase replicates DNA; RNA polymerase transcribes RNA. They are NOT interchangeable.
• Using the Coding Strand for mRNA: mRNA is complementary to the template strand and identical to the coding strand (except T->U). If asked to write mRNA, use the template strand as the guide.
• Thinking All Mutations Are Harmful: Silent mutations have no effect on the protein. Some mutations are neutral or even beneficial, providing raw material for evolution.
• Forgetting that Prokaryotes Lack Splicing: Prokaryotic mRNA does not undergo the extensive post-transcriptional modification seen in eukaryotes. Translation can begin while transcription is still occurring.

AP Exam Strategies

• Write Out Sequences: When given a DNA strand and asked for mRNA or amino acids, write out the complementary sequence explicitly to avoid errors.
• Use the Codon Table Efficiently: Locate the first base on the left, second on the top, third on the right. Circle or highlight the codon to avoid misreading.
• Explain the 'Why': In regulation questions, don't just describe what happens. Explain WHY: e.g., "The repressor changes shape when bound to allolactose, preventing it from binding the operator, which allows RNA polymerase access to the promoter."
• Connect Mutations to Phenotypes: If a nonsense mutation creates a premature stop codon, predict that the resulting truncated protein will likely be nonfunctional, leading to a loss-of-function phenotype.

Real-World Applications

• CRISPR-Cas9 Gene Editing: This revolutionary technology uses guide RNA to direct the Cas9 nuclease to specific DNA sequences, allowing precise gene editing for treating genetic diseases like sickle cell anemia.
• Cancer Epigenetics: Aberrant DNA methylation patterns can silence tumor suppressor genes or activate oncogenes, driving cancer progression and informing therapeutic strategies.
• Insulin Production: Recombinant DNA technology allows bacteria to produce human insulin by inserting the human insulin gene into bacterial plasmids, demonstrating transformation and gene expression.