Genetic Transfer in Bacteria: Transduction Mechanism
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Questions and Answers

What is the primary mechanism of genetic transfer in bacterial conjugation?

  • Transformation
  • Horizontal gene transfer (correct)
  • Transduction
  • Vertical gene transfer

    • What is the term for bacterial cells that require a specific nutrient for growth?

  • Auxotrophs (correct)
  • Mutants
  • Wild-type
  • Prototrophs

    • What is the term for the process by which a bacteriophage incorporates a fragment of bacterial chromosomal DNA into a newly made phage particle?

  • Transformation
  • Mutation
  • Transduction (correct)
  • Conjugation

    • What is the result of the incorporation of a fragment of DNA into a recipient cell's chromosome through recombination?

    <p>Genetic variation</p> Signup and view all the answers

    What is the term for the process by which a bacterial cell takes up a fragment of DNA from its environment?

    <p>Transformation</p> Signup and view all the answers

    Who discovered genetic transfer in bacteria in 1946?

    <p>Joshua Lederberg and Edward Tatum</p> Signup and view all the answers

    What is the term for bacterial cells that can synthesize all their nutrients from components in their growth medium?

    <p>Prototrophs</p> Signup and view all the answers

    What is the term for the circular segment of DNA transferred to a recipient cell during genetic transfer?

    <p>Plasmid</p> Signup and view all the answers

    What was the primary reason behind Lederberg and Tatum's conclusion that some genetic material was transferred between the two bacterial strains?

    <p>The growth of bacterial cells on the plates with a different genotype</p> Signup and view all the answers

    What was the purpose of the filter at the bottom of the U-tube apparatus used by Bernard Davis?

    <p>To allow the passage of the genetic material but prevent the passage of bacterial cells</p> Signup and view all the answers

    What was the outcome of Lederberg and Tatum's experiment that led to the conclusion that genetic material was transferred between the two bacterial strains?

    <p>The growth of bacterial cells on the plates with a different genotype</p> Signup and view all the answers

    What was the primary conclusion drawn from the result of Lederberg and Tatum's experiment?

    <p>The bacterial cells must make physical contact for transfer of genetic material to occur</p> Signup and view all the answers

    What was the significance of the U-tube apparatus used by Bernard Davis?

    <p>It demonstrated the requirement of physical contact for transfer of genetic material</p> Signup and view all the answers

    What was the primary role of the growth medium in Lederberg and Tatum's experiment?

    <p>It provided the necessary nutrients for the growth of bacterial cells</p> Signup and view all the answers

    What was the outcome of Bernard Davis's experiment using the U-tube apparatus?

    <p>The transfer of genetic material was prevented</p> Signup and view all the answers

    What was the limitation of Lederberg and Tatum's experiment?

    <p>The inability to distinguish between the two possibilities of genetic material transfer</p> Signup and view all the answers

    What was the purpose of applying pressure or suction in Davis's experiment?

    <p>To facilitate the movement of liquid through the filter</p> Signup and view all the answers

    What is the term for the transfer of DNA from one bacterium to another following direct cell-to-cell contact?

    <p>Conjugation</p> Signup and view all the answers

    What is the designation for bacteria that lack an F factor?

    <p>F-</p> Signup and view all the answers

    What is the purpose of the genes named tra or trb followed by a capital letter?

    <p>To play a role in the transfer of DNA</p> Signup and view all the answers

    What was the result of Davis's experiment when he placed samples of bacteria on growth media?

    <p>Neither strain of bacteria grew</p> Signup and view all the answers

    What is the term for the small circular piece of DNA that plays a role in conjugation?

    <p>F factor</p> Signup and view all the answers

    Which of the following is a characteristic of conjugation?

    <p>It requires direct physical contact between bacteria</p> Signup and view all the answers

    What is the function of the F factor in certain donor strains of E. coli?

    <p>To play a role in the transfer of DNA</p> Signup and view all the answers

    What is the primary function of sex pili in conjugation?

    <p>To act as attachment sites for F- bacteria</p> Signup and view all the answers

    What is the consequence of successful contact between donor and recipient cells during conjugation?

    <p>The donor cell begins the transfer process</p> Signup and view all the answers

    What is the role of relaxase in the mechanism of transfer?

    <p>To remain bound to the end of the T-DNA</p> Signup and view all the answers

    What is the function of the exporter complex in conjugation?

    <p>To pump DNA into the recipient cell</p> Signup and view all the answers

    What is the result of the formation of a conjugation bridge between donor and recipient cells?

    <p>The donor and recipient cells are drawn closer together</p> Signup and view all the answers

    What is the role of the relaxosome in the mechanism of transfer?

    <p>To recognize the origin of transfer in the DNA sequence</p> Signup and view all the answers

    What is the consequence of the shortening of sex pili during conjugation?

    <p>The donor and recipient cells are drawn closer together</p> Signup and view all the answers

    What is the characteristic of the exporter complex that enables it to pump DNA into the recipient cell?

    <p>It is composed of 10-15 proteins that span both inner and outer membranes</p> Signup and view all the answers

    What is the outcome of conjugation in the recipient cell?

    <p>It acquires an F factor</p> Signup and view all the answers

    What is the general term used to describe extra-chromosomal DNA?

    <p>Plasmid</p> Signup and view all the answers

    What is the range of size of plasmids?

    <p>A few thousand to 500,000 bp</p> Signup and view all the answers

    What is the function of episomes?

    <p>To integrate into a chromosome</p> Signup and view all the answers

    What is the primary function of fertility plasmids?

    <p>To allow conjugation</p> Signup and view all the answers

    What is the term used to describe plasmids that contain genes conferring resistance to antibiotics?

    <p>Resistance plasmids</p> Signup and view all the answers

    What is the function of virulence plasmids?

    <p>To carry genes that turn bacterium into pathogenic strains</p> Signup and view all the answers

    What is the characteristic of plasmids that is not required for survival?

    <p>Ability to provide growth advantages to the bacteria</p> Signup and view all the answers

    Study Notes

    Genetic Transfer in Bacteria

    • There are three mechanisms of genetic transfer in bacteria: transformation, transduction, and conjugation.

    Transduction

    • A bacteriophage infects a donor cell and incorporates a fragment of bacterial chromosomal DNA into a newly made phage particle.
    • The phage then transfers this fragment of DNA to a recipient cell, which incorporates the DNA into its chromosome by recombination.

    Transformation

    • When a bacterial cell dies, it releases a fragment of its DNA into the environment.
    • This DNA fragment is taken up by a recipient cell, which incorporates the DNA into its chromosome by recombination.

    Conjugation

    • Genetic transfer in bacteria was discovered in 1946 by Joshua Lederberg and Edward Tatum.
    • They were studying strains of Escherichia coli that had different nutritional growth requirements.
    • Auxotrophs cannot synthesize a particular nutrient and therefore need it in their growth medium.
    • Prototrophs make all their nutrients from components in their growth medium.

    Lederberg and Tatum's Experiment

    • One strain of E. coli was designated met bio thr leu thi, requiring one vitamin (biotin) and one amino acid (methionine).
    • Another strain was designated met+ bio+ thr- leu- thi-, requiring one vitamin (thiamine) and two amino acids, leucine and threonine.
    • The genotype of the bacterial cells that grew on the plates must be met bio thr leu thi.
    • Lederberg and Tatum reasoned that some genetic material was transferred between the two strains.

    Conjugation Requires Direct Physical Contact

    • Bernard Davis later showed that the bacterial strains must make physical contact for transfer of genetic material to occur.
    • Davis placed the two strains on opposite sides of a filter in a U-tube apparatus.
    • Application of pressure or suction promoted the movement of liquid through the filter.
    • Samples of bacteria from either side were then placed on growth media that selected for the met+ bio+ thr+ leu+ thi+ genotype.
    • Nothing grew! Davis concluded that the bacteria must make direct physical contact to transfer genetic material.

    F Factors and Conjugation

    • In E. coli, certain donor strains contain a small circular piece of DNA termed an F factor (for fertility factor).
    • Strains containing an F factor are designated F+, while those lacking it are F-.
    • Genes that play a role in the transfer of DNA are named tra or trb followed by a capital letter.
    • Contact initiates conjugation, which is mediated by sex pili (or F pili) made only by F+ strains.

    Mechanism of Transfer

    • The first step in conjugation is the contact between donor and recipient cells.
    • This is mediated by sex pili, which act as attachment sites for the F- bacteria.
    • Once contact is made, the pili shorten, and the donor and recipient cells are drawn closer together, forming a conjugation bridge.
    • The successful contact stimulates the donor cell to begin the transfer process.
    • The mechanism of transfer involves the relaxosome, relaxase, and an exporter complex of proteins that spans both inner and outer membranes.

    Plasmids

    • A plasmid is the general term used to describe extra-chromosomal DNA.
    • Most are circular, although some are linear.
    • Plasmids are present in many bacteria and a few eukaryotic species.
    • They range in size from a few thousand to 500,000 bp.
    • Some plasmids, called episomes, can integrate into a chromosome.
    • Plasmids have their own replication origin and can replicate independent of the bacterial chromosome.

    Five Different Categories of Plasmids

    • Fertility plasmids allow conjugation.
    • Resistance plasmids (R factors) contain genes conferring resistance to antibiotics.
    • Degradative plasmids carry genes allowing digestion of unusual substances.
    • Col-plasmids contain genes that kill other bacteria.
    • Virulence plasmids carry genes that turn bacteria into pathogenic strains.

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    Related Documents

    BIO222 Molecular Genetics PDF

    Description

    Learn about the transduction mechanism of genetic transfer in bacteria, where a bacteriophage incorporates bacterial chromosomal DNA into a newly made phage particle, transferring it to the recipient cell.

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