Gene Study Guide

📖 Core Concepts Gene (Mendelian vs molecular) – Heritable unit of trait vs DNA segment transcribed to RNA. Genotype vs phenotype – Complete DNA makeup vs observable traits (environment‑influenced). Protein‑coding vs non‑coding genes – Produce mRNA → protein vs functional RNAs (tRNA, rRNA, miRNA, ribozymes). Transcription unit – DNA region (including promoter) that yields a messenger or non‑coding RNA. Operon – Bacterial poly‑cistronic transcription unit regulated by a repressor/operator. Semiconservative replication – Each daughter DNA retains one parental strand + one newly synthesized strand. Alleles, dominance & recessiveness – Different gene versions; dominant expressed with any partner, recessive only when homozygous. Polygenes – Multiple genes jointly influencing a single trait. --- 📌 Must Remember DNA directionality: synthesis 5′→3′; template read 3′→5′. Genetic code: 64 codons → 20 amino acids; redundancy (multiple codons per amino acid). Human genome: 19 000 protein‑coding genes; 26 000 non‑coding genes. Promoter strength: strong → high transcription rate; weak → low rate. RNA processing steps: 5′ cap → splicing (introns removed) → poly(A) tail (200 A’s). Mendelian segregation: two alleles per locus, one passed to each gamete. Independent assortment: alleles at different loci segregate independently (unless linked). Gene linkage: close genes on same chromosome tend to be inherited together. --- 🔄 Key Processes Transcription RNA polymerase binds promoter (e.g., TATA box). Unwinds DNA; reads template 3′→5′. Synthesizes RNA 5′→3′, adding complementary ribonucleotides. RNA Processing (eukaryotes) 5′ capping (7‑methylguanosine). Spliceosome removes introns, joins exons. Cleavage & polyadenylation → poly(A) tail (200 nt). Translation 40S ribosomal subunit binds 5′ cap, scans to start codon (AUG). tRNA anticodon pairs with codon; peptide bond forms, elongating chain N‑terminus → C‑terminus. Termination at stop codon; release factors dissociate ribosome. DNA Replication (Semiconservative) Helicase unwinds helix → replication forks. Leading strand synthesized continuously 5′→3′. Lagging strand synthesized as Okazaki fragments (5′→3′), later joined. Operon Regulation (prokaryotes) Repressor binds operator → blocks RNA polymerase. Inducer (or absence of corepressor) releases repressor → transcription proceeds. --- 🔍 Key Comparisons Mendelian gene vs molecular gene Mendelian: abstract hereditary unit defined by phenotype. Molecular: concrete DNA sequence that is transcribed. Protein‑coding vs non‑coding gene Coding: produces mRNA → protein. Non‑coding: produces functional RNA (tRNA, rRNA, miRNA, ribozyme). Prokaryotic genome vs eukaryotic genome Prokaryotes: single circular chromosome (+ plasmids). Eukaryotes: multiple linear chromosomes packaged with histones. Strong promoter vs weak promoter Strong: high transcription initiation frequency. Weak: low initiation frequency. Operon (poly‑cistronic) vs eukaryotic monocistronic transcription Operon: several genes on one mRNA; coordinated regulation. Eukaryote: one gene → one mRNA (usually). --- ⚠️ Common Misunderstandings “All genes code for proteins.” – False; many genes encode functional RNAs. “Promoters are part of the gene.” – Regulatory regions are gene‑associated, not the transcribed unit itself. “RNA polymerase reads DNA 5′→3′.” – It reads the template strand 3′→5′ while synthesizing RNA 5′→3′. “All introns are removed in every cell.” – Alternative splicing can retain certain introns, creating isoforms. “Linkage means no recombination.” – Low probability, not zero; crossing‑over can still occur. --- 🧠 Mental Models / Intuition “DNA as a two‑track highway.” – Each strand is a lane; during replication each lane keeps one original track and builds a new partner lane. “Gene as a recipe.” – Promoter = “open the kitchen”; exons = “ingredients”; introns = “pages you cut out”; poly(A) tail = “preservative” extending shelf‑life. “Operon as a light switch.” – Repressor = switch off; inducer = flip on, allowing the whole circuit (multiple genes) to run. --- 🚩 Exceptions & Edge Cases Alternative promoters – One gene may have multiple promoters, yielding different transcription start sites. Non‑canonical start codons – Rarely, translation can begin at codons other than AUG (e.g., GUG, UUG). RNA editing – Post‑transcriptional modifications can change nucleotide sequence (e.g., A→I editing). Polycistronic eukaryotic mRNAs – Some viral and mitochondrial genomes produce poly‑cistronic transcripts. --- 📍 When to Use Which | Situation | Choose | Why | |-----------|--------|-----| | Predicting protein product | Protein‑coding gene model (exons → mRNA → translation) | Direct coding information. | | Explaining phenotype without protein change | Non‑coding RNA (miRNA, ribozyme) | Function arises from RNA itself. | | Analyzing bacterial gene clusters | Operon model | Genes co‑transcribed & co‑regulated. | | Determining inheritance pattern | Mendelian vs linkage analysis | Independent assortment vs linked loci. | | Designing gene knockout | CRISPR‑Cas9 or engineered nuclease | Precise double‑strand break → repair. | | Interpreting expression differences | Promoter strength / enhancer activity | Transcription initiation level drives mRNA abundance. | --- 👀 Patterns to Recognize “TATA box → transcription start” – Presence of TATA often marks a strong promoter. “Poly(A) tail + 5′ cap → mature mRNA – Both modifications signal export‑competent transcript. “Multiple exons → possibility of alternative splicing – Look for exon skipping or inclusion patterns. “Genes clustered on same chromosome → potential linkage – Close proximity reduces recombination frequency. “Operon genes share functional pathway – Co‑regulation usually reflects related metabolic roles. --- 🗂️ Exam Traps Distractor: “Promoters are transcribed” – Promoters are regulatory DNA, not part of the RNA product. Trap: “All introns are always removed” – Alternative splicing can retain introns in specific isoforms. Misleading choice: “RNA polymerase synthesizes DNA” – It synthesizes RNA; DNA polymerase does DNA synthesis. Near‑miss: “A gene’s locus includes its promoter” – Locus refers to the gene region; promoters are adjacent, gene‑associated but not part of the transcribed unit. Confusing option: “Linkage eliminates recombination” – Linkage reduces, not abolishes, recombination; crossing‑over still possible. ---