Horizontal Gene Transfer Practice Questions

Correct answer: Transformation

Transformation is the process by which a competent bacterium takes up free DNA fragments released by dead or lysed cells in its environment. This was famously demonstrated by Frederick Griffith in his experiments with Streptococcus pneumoniae.

Correct answer: A state of physiological competence

Competence is a specific physiological state that allows bacteria to alter their cell wall and membrane to permit the entry of high-molecular-weight DNA. Some bacteria are naturally competent, while others can be induced into competence in a laboratory setting.

Correct answer: Sex pilus

The sex pilus (or F pilus) is a proteinaceous appendage produced by the donor cell (F+) that attaches to the recipient cell (F-). It then retracts to bring the two cells into close proximity for the formation of a conjugation bridge.

Correct answer: A cell with the F plasmid integrated into its chromosome

An Hfr strain occurs when the F plasmid integrates into the host's circular chromosome via site-specific recombination. During conjugation, Hfr cells attempt to transfer the entire chromosome, leading to high rates of genetic exchange.

Correct answer: Random packaging of bacterial host DNA into a phage head

Generalized transduction occurs during the lytic cycle when a bacteriophage accidentally packages random fragments of the host bacterial DNA instead of the viral genome. This 'transducing particle' then delivers the bacterial DNA to a new recipient cell.

Correct answer: Lysogenic cycle

Specialized transduction occurs when a temperate phage (prophage) exits the lysogenic cycle incorrectly, taking adjacent chromosomal genes with it. Because the phage integrates at a specific site (e.g., attB), only genes neighboring that site are transferred.

Correct answer: It becomes F+ and can act as a donor

In F+ x F- conjugation, a single strand of the F plasmid is transferred and then replicated in the recipient. This converts the F- recipient into a fertile F+ donor cell capable of further conjugation.

Correct answer: Transposase

Transposase is the enzyme encoded by the transposon that recognizes inverted repeat sequences at the ends of the element. It catalyzes the excision and integration of the transposon into a new location.

Correct answer: IS elements only carry the gene for transposase

Insertion Sequences are the simplest type of transposable element, containing only the genetic information required for movement (transposase). Complex (or composite) transposons carry additional genes, such as those for antibiotic resistance, flanked by IS elements.

Correct answer: F' plasmids carry part of the host chromosome

An F' plasmid is formed when an integrated F plasmid is excised from the chromosome imprecisely, bringing some chromosomal DNA with it. When this F' plasmid is transferred, the recipient receives those specific chromosomal genes along with the plasmid.

Correct answer: One strand is degraded and the other enters the cell

In most natural transformation systems, an endonuclease degrades one strand of the DNA at the cell surface. The remaining single strand is then protected by DNA-binding proteins and transported across the membrane into the cytoplasm.

Correct answer: Transformation

Transformation relies on 'naked' DNA being present in the environment. Because this DNA is not protected by a viral capsid or a conjugation bridge, the addition of DNase to the medium will degrade the DNA and prevent transformation.

Correct answer: Conjugation

R-plasmids (Resistance plasmids) often carry genes for multiple antibiotic resistances and the genes necessary for their own transfer. Conjugation is the primary and most efficient method for the rapid spread of these plasmids through bacterial populations.

Correct answer: A replication method that transfers a single DNA strand

During conjugation, a nick is made in the plasmid at the origin of transfer (oriT). Rolling circle replication then occurs, where the nicked strand is 'unrolled' and transferred to the recipient, while the remaining strand serves as a template for synthesis.

Correct answer: The R-strain took up capsule DNA from the dead S-strain

This was the first demonstration of transformation. The live, non-pathogenic R-strain bacteria took up the DNA encoding the capsule from the heat-killed S-strain, transforming them into virulent, capsule-producing S-strain bacteria.

Correct answer: Outer membrane lipopolysaccharide

Bacteriophages are highly specific and must bind to particular receptors on the host cell surface. In Gram-negative bacteria, these receptors are often components of the outer membrane, such as LPS, porins, or transport proteins.

Correct answer: To bind to the oriT and nick the DNA to initiate transfer

The relaxosome is a group of proteins that binds to the origin of transfer (oriT) on the plasmid. An enzyme called relaxase within this complex nicks the DNA, allowing the single strand to be displaced and moved into the recipient cell.

Correct answer: Is the passage of genes from parent to offspring

Vertical gene transfer is the standard inheritance of genetic material through reproduction (binary fission in bacteria). Horizontal gene transfer involves the movement of genetic material between non-related individuals, often across species lines.

Correct answer: F' conjugation

In F' conjugation, the recipient receives an F plasmid that already contains some chromosomal genes from the donor. Since the recipient still has its own complete chromosome, it ends up with two copies of those specific genes, making it a merodiploid.

Correct answer: Incubation with calcium chloride followed by heat shock

Chemical transformation using calcium chloride (CaCl2) helps neutralize the negative charges on the DNA and the cell membrane. A subsequent brief heat shock creates temporary pores in the membrane, allowing the DNA to enter the cell.