Gene Mapping In Prokaryotes

 

Introduction

Gene mapping in prokaryotes is a cornerstone of microbial genetics and molecular biology. It helps researchers understand the physical locations of genes on a chromosome and how genetic material is transferred and recombined. Unlike eukaryotes, bacteria reproduce asexually, but they possess unique mechanisms—conjugation, transformation, and transduction—that allow for genetic exchange and recombination.

Mapping bacterial genes has been crucial for understanding gene linkage, recombination frequency, mutation studies, and horizontal gene transfer, all of which contribute to genetic diversity and evolution.

1. Bacterial Growth and Conjugation

Bacteria reproduce by binary fission, producing identical daughter cells. However, genetic variation arises through processes such as conjugation, where two bacterial cells exchange DNA through direct contact.

1. Conjugation was first discovered by Lederberg and Tatum (1946) in Escherichia coli.

2. It involves a donor (F⁺) cell containing the F (fertility) plasmid and a recipient (F⁻) cell lacking it.
3. The F plasmid encodes the genes for pilus formation and DNA transfer machinery.
4. DNA is transferred through a sex pilus, forming a conjugation bridge.

During conjugation, if the F factor integrates into the bacterial chromosome, the cell becomes an Hfr (High-frequency recombination) strain, which transfers chromosomal genes sequentially to the recipient.

2. Conjugation and Sexduction: Gene Mapping in Bacteria

Sexduction refers to the process where an F’ (F-prime) plasmid carries additional chromosomal genes. When this plasmid is transferred, it introduces specific chromosomal markers into the recipient.

Steps in Gene Mapping via Conjugation

1. Mating between Hfr and F⁻ strains is initiated.

2. The transfer of DNA begins from a specific origin (OriT).

3. The interrupted mating technique is used—by blending the cells at different time intervals, gene transfer is stopped at specific stages.

4. The order and timing of gene entry reveal gene sequence and relative distance.

Example:
If gene order is A → B → C, and gene B enters after 10 minutes while C enters after 20 minutes, the distance between A–B and B–C is 10 minutes each.

3. Transformation: Gene Mapping in Bacteria

Transformation is the uptake of free, naked DNA fragments from the environment by a recipient bacterium, leading to genetic change.

Key Features:

1. Discovered by Griffith (1928) in Streptococcus pneumoniae.

2. Later demonstrated by Avery, MacLeod, and McCarty (1944) as DNA-mediated.

3. Requires competent cells capable of binding and internalizing DNA.

Mapping through Transformation

1. When two genes are close together, they are more likely to be co-transformed.

2. The frequency of co-transformation indicates the distance between genes.

Example: If genes A and B are frequently co-transformed, they are likely close together on the chromosome.

4. Transduction and Gene Mapping in Bacteria

Transduction involves the transfer of bacterial genes via bacteriophages (viruses that infect bacteria).

Types of Transduction:

1. Generalized Transduction: Any bacterial gene can be transferred. Transduction occurs during the lytic cycle when host DNA is mistakenly packaged into a phage head. Example: P1 phage in E. coli.

2. Specialized Transduction: Only specific genes near the prophage integration site are transferred. Example: λ (lambda) phage transfers genes near the gal and bio loci.

Mapping Principle:

1. Recombination frequency between co-transduced genes helps estimate their genetic distance.

2. The closer the genes, the higher the co-transduction frequency.

5. Gene Mapping in Phages: Mapping the rII Locus – History, Concepts & Applications

The rII locus of T4 bacteriophage was used by Seymour Benzer (1955) to study fine structure mapping.

Key Concepts:

1. The rII mutants produce distinct plaque morphologies on E. coli strains.

1. By analyzing recombination between different rII mutants, Benzer demonstrated that: a. 

   Genes are linear structures b.  Recombination can occur within genes and c. The genetic code is composed of discrete units.

Applications:

1. Foundation for molecular genetics and genomics.

2. Provided insight into gene structure, mutational hotspots, and DNA repair mechanisms.

3. Set the stage for modern sequencing and CRISPR research.

FAQs

Q1. What is gene mapping?
Gene mapping determines the order and relative distance of genes on a chromosome using recombination data.

Q2. Why is conjugation important in gene mapping?
Because it allows sequential gene transfer, helping determine the gene order and distances in minutes.

Q3. What is the difference between transformation and transduction?
Transformation involves DNA uptake from the environment, whereas transduction uses bacteriophages to transfer DNA.

Q4. Who discovered transformation?
Frederick Griffith in 1928.

Q5. What is co-transduction frequency?
It measures how often two genes are transferred together by a phage, indicating their proximity on the chromosome.

MCQs

1. The F plasmid in E. coli controls: a) Mutation b) Fertility and conjugation ✅c) DNA replication d) Transcription

2. Transformation requires: a) Competent cells ✅ b) Pili formation c) Phage infection d) Ribosomes

3. Generalized transduction occurs during: a) Lysogenic cycle b) Lytic cycle ✅ c) Transformation d) Binary fission

4. Hfr strains are characterized by: a) Free F plasmid b) F plasmid integrated into chromosome ✅ c) Lack of plasmid d) Double-stranded RNA

5. The fine structure of the gene was mapped by: a) Hershey and Chase b) Griffith c) Benzer ✅ d) Lederberg

Worksheet Activities

1. Draw a labeled diagram showing conjugation between Hfr and F⁻ cells.

2. Explain the interrupted mating technique and how it helps in gene mapping.

3. Compare transformation, transduction, and conjugation in a tabular form.

4. Describe the Benzer experiment on the rII locus.

5. Explain the role of plasmids in bacterial genetics.

References

1. Griffith, F. (1928). The significance of pneumococcal types. Journal of Hygiene, 27(2):113-159.

2. Lederberg, J., & Tatum, E.L. (1946). Gene recombination in E. coli. Nature, 158:558.

3. Avery, O.T., MacLeod, C.M., & McCarty, M. (1944). Studies on the chemical nature of transforming principle. J. Exp. Med., 79:137–158.

4. Benzer, S. (1955). Fine structure of a genetic region in bacteriophage. Proc. Natl. Acad. Sci., 41(6):344–354.

5. Watson, J.D., et al. (2014). Molecular Biology of the Gene. Pearson Education.

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