Bacterial Genetics & DNA Transfer Microbiology and Immunology Levinson ليفنسون وراثة بكتريا
Bacterial Genetics & DNA Transfer
I. Introduction to Bacterial Genetics
Definition: Bacterial genetics studies how genetic material changes and transfers between cells, affecting traits like growth and antibiotic resistance.
Genetic Material: In bacteria (e.g., E. coli), it consists of a single circular DNA molecule.
Haploid Nature: Bacteria are haploid (one chromosome/one gene copy), meaning any mutation is immediately expressed. In contrast, human cells are diploid.
Genome Size: Mycoplasma has the smallest genome among free-living bacteria.
II. Mutations
A mutation is a change in the DNA base sequence that may produce a new phenotype.
Types of Mutations:
Base Substitution: One base is replaced by another.
Missense: Changes one amino acid.
Nonsense: Creates a stop codon, producing a nonfunctional protein.
Frameshift Mutation: Insertion or deletion of base pairs, shifting the reading frame.
Transposon Insertion: Mobile DNA elements disrupt gene function.
Conditional Lethal: e.g., Temperature-sensitive mutations used in vaccines (influenza).
Causes (Mutagens):
Chemical: Nitrous acid, base analogues (5-bromouracil), and intercalating agents (benzpyrene).
Radiation: X-rays (DNA breaks) and UV light (Pyrimidine/Thymine dimers).
III. DNA Transfer
Within Bacterial Cells:
Transposons: "Jumping genes" that spread antibiotic resistance.
Programmed Rearrangements: Movement of silent genes to expression sites for antigenic variation (e.g., N. gonorrhoeae).
Between Bacterial Cells (Horizontal Gene Transfer):
Conjugation: Direct cell-to-cell contact via a sex pilus. F⁺ (donor) transfers plasmid to F⁻ (recipient).
Hfr Cells: High-frequency recombination where the F plasmid integrates into the chromosome.
Transduction: DNA transfer via a bacteriophage (virus).
Generalized: Any gene transferred.
Specialized: Specific genes near the phage insertion site.
Medical Importance: Pathogenicity (Toxins like Cholera, Botulinum).
Transformation: Uptake of free DNA from the environment.
IV. Recombination
Integration of transferred DNA into the recipient's chromosome.
Homologous: Requires sequence similarity.
Non-homologous: Little or no similarity required.
Biology : Ali Yahia
Thi Qar University
1. Multiple Choice Questions (MCQ)
Q1: Which mechanism of DNA transfer requires a bacteriophage?
a) Transformation b) Conjugation c) Transduction d) Transposition
Q2: Bacteria are haploid organisms, which means:
a) They have two copies of each gene. b) Mutations are immediately expressed. c) They lack DNA.
Q3: Ultraviolet (UV) light causes DNA damage by forming:
a) Free radicals b) Pyrimidine dimers c) Base substitutions
2. True or False
Q1: Missense mutation creates a stop codon. (False - That is Nonsense mutation) .
Q2: Transposons can move within the same cell from the chromosome to a plasmid. (True) .
Q3: Transformation involves cell-to-cell contact through a sex pilus. (False - That is Conjugation) .
3. Fill in the Blanks
Q1: The process of taking up free DNA from the environment is called _________ (Transformation).
Q2: _________ mutations are used in the production of live attenuated vaccines like influenza. (Temperature-sensitive / Conditional lethal) .
Q3: Integration of DNA that requires extensive sequence similarity is known as _________ recombination. (Homologous) .
4. Comparison (Conjugation vs. Transduction vs. Transformation)
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