Gene Expression & Recombinant DNA — Study Guide Summary & Study Notes

🧬 Control of Gene Expression

Gene expression is regulated at many levels so cells can respond to developmental cues and the environment. Key mechanisms include DNA modifications, transcriptional control, post-transcriptional processing, post-translational regulation, and RNA interference.

🧪 DNA Modifications

DNA methylation and histone acetylation are common epigenetic changes that alter chromatin structure and gene accessibility without changing DNA sequence. Genetic imprinting is an epigenetic phenomenon where expression depends on the parent of origin; these marks are heritable through cell divisions.

📝 Regulation of Transcription

Transcriptional control relies on promoters, transcription factors, enhancers, and activators/repressors. In bacteria, operons allow coordinated expression of functionally related genes under a single promoter controlled by regulatory proteins.

✂️ Post-Transcriptional Regulation

After transcription, RNA molecules can be processed by splicing events, alternative splicing, RNA editing, and RNA stability control. These processes determine which mRNA isoforms are produced and how long transcripts persist.

🔧 Post-Translational Regulation

Proteins are regulated after synthesis by mechanisms such as phosphorylation, targeted ubiquitination for proteasomal degradation, inhibition of enzyme activity, and control of ER export. Cytoplasmic determinants partition during cell division to influence cell fate.

🧬 RNA Interference (RNAi)

RNA interference uses double-stranded RNA as a trigger for sequence-specific degradation of matching RNAs. Enzymes like Dicer process dsRNA into micro-RNAs, and the RISC complex uses these small RNAs to bind and cleave target transcripts, effectively silencing genes.

🔬 Recombinant DNA Techniques

Key tools are restriction enzymes (which cut DNA at specific sequences) and ligases (which join DNA fragments). Recombinant techniques allow insertion of genes into vectors and production of cDNA when cloning eukaryotic genes into bacterial systems.

🧫 Bacterial Genetics

Bacteria exchange genetic material by transformation (uptake of free DNA), conjugation (transfer through a sex pilus using plasmids), and transduction (phage-mediated transfer). These processes enable rapid genetic variation and spread of traits.

🦠 Viral Life Cycles and Resistance

Bacteriophages follow lytic or lysogenic cycles; the lysogenic cycle integrates viral DNA into the host genome. Antibiotic resistance can spread quickly through conjugation when resistance genes reside on transferable plasmids.

🧩 Discovery & Cloning of Genes

Scientists use restriction enzymes and ligase to insert human genes into plasmids and transform bacteria to express those genes. cDNA libraries derived from mRNA are used to clone expressed genes that produce proteins in heterologous hosts.

🔁 PCR — Polymerase Chain Reaction

PCR is used to amplify specific DNA sequences rapidly in vitro. It relies on repeated cycles of denaturation, primer annealing, and extension by a thermostable DNA polymerase to generate millions of copies of a target.

✅ Key Takeaways

Multiple, layered controls (epigenetic, transcriptional, post-transcriptional, and post-translational) determine gene expression. Recombinant DNA tools and bacterial gene transfer mechanisms are central to biotechnology and the spread of genetic traits like antibiotic resistance.