Practice 4 structured short- and long-answer medical questions on Gene Transfer and Transcription in Bacteria. Review model answers for exam revision.
Answer: Genetic variation in bacteria is essential for survival, adaptation, and evolution. Bacteria acquire new genetic traits, such as antibiotic resistance, through gene transfer and structural recombination. Unlike eukaryotes, bacteria utilize three primary horizontal gene transfer mechanisms: transformation, transduction, and conjugation. ## Structural Recombination in Bacteria Structural recombination refers to the genetic exchange between two DNA molecules, leading to the incorporation of new material into the bacterial genome. This is a major driver in the spread of antibiotic resistance. ### 1. Transformation **Definition:** The process by which bacteria take up naked DNA from their surroundings and integrate it into their genome. This DNA usually originates from lysed (dead) bacterial cells. **Mechanism:** - Cell lysis releases DNA fragments into the environment. - Competent recipient bacteria (cells capable of taking up extracellular DNA) bind and internalize the DNA. - The acquired DNA is incorporated into the host chromosome via recombination. ### 2. Transduction **Definition:** Gene transfer mediated by a bacteriophage (bacterial virus) acting as a vector. **Mechanism:** - A bacteriophage infects a bacterium and replicates. - During assembly, fragments of host bacterial DNA may be accidentally packaged into the viral capsid. - Upon infecting a new host, the phage injects the previous host's DNA. - The recipient incorporates this foreign DNA into its genome. **Phage Cycles:** - **Lytic cycle:** Rapid replication leading to host cell lysis. - **Lysogenic cycle:** The phage DNA (prophage) integrates into the bacterial genome and remains dormant until induced to enter the lytic cycle. ### 3. Conjugation **Definition:** The direct transfer of genetic material (usually plasmids) between two bacterial cells through a sex pilus. **Mechanism:** - Controlled by the F-plasmid (fertility factor). - **F+ cells (donors)** possess the plasmid and produce a sex pilus. - **F- cells (recipients)** lack the plasmid. - The pilus draws the cells together, forming a conjugation bridge. - A copy of the F-plasmid is transferred, converting the recipient into an F+ cell. --- # Transcription in Prokaryotes and Eukaryotes Transcription is the synthesis of RNA (mRNA, tRNA, rRNA) from a DNA template. It occurs in the 5' to 3' direction. ### Location - **Prokaryotes:** Occurs in the cytoplasm. - **Eukaryotes:** Occurs in the nucleus. ## RNA Polymerase in Prokaryotes Prokaryotes utilize a single type of RNA polymerase holoenzyme consisting of several subunits: - **Core Enzyme:** - **2 Alpha (α) subunits:** Required for enzyme assembly. - **1 Beta (β) subunit:** Involved in DNA template recognition and catalytic activity. - **1 Beta-prime (β') subunit:** The heaviest subunit; possesses helicase-like activity to unwind the DNA double helix. - **Sigma (σ) factor:** A regulatory subunit that recognizes and binds to the promoter region to initiate transcription. ## RNA Polymerases in Eukaryotes Eukaryotes possess three specialized RNA polymerases:
Answer: Model answer pending
Answer: Model answer pending
Answer: Model answer pending