Bacterial and Viral Genetics

Flashcards covering bacterial genetics, plasmid biology, episomes, conjugation, transformation, transduction, and viral reproductive cycles.

Why are bacteria particularly advantageous model organisms for genetic research and biotechnology?

They reproduce rapidly with large numbers of progeny, have haploid genomes that express mutations directly, undergo asexual reproduction that simplifies pure strain isolation, are easy and space-efficient to culture, have compact genomes, and possess methods for gene manipulation and commercial substance engineering.

What is the key functional difference between prototrophic and auxotrophic bacterial strains?

Prototrophic bacteria can synthesize all essential growth compounds on minimal medium (sugar plus inorganic salts), whereas auxotrophic bacteria have mutations preventing them from synthesizing specific essential nutrients, requiring supplementation in their growth medium.

What laboratory technique depicted in this diagram is used to isolate and identify bacterial mutants with specific growth requirements?

Replica plating, which uses a sterile velvet surface to press and transfer colonies from a master plate onto new petri plates with different selective media conditions.

How do the size and coding density of the $E.\text{ coli}$ genome compare to the human genome?

The $E.\text{ coli}$ genome contains approximately $4.6\text{ million}$ base pairs with $90\text{\%}$ coding for proteins, whereas the human genome contains approximately $3.1\text{ billion}$ base pairs with only $1\text{--}1.5\text{\%}$ coding for proteins.

What is an episome, and what is a prominent example found in $E.\text{ coli}$?

An episome is a plasmid capable of replicating independently as well as integrating directly into the host bacterial chromosome; a key example is the Fertility factor ($F\text{ factor}$).

What key functional elements are located on the $F\text{ factor}$ episome mapped below?

It contains an origin of replication ($ori$), genes controlling plasmid replication ($rep$, $inc$), insertion sequences ($IS2$, $IS3$) for chromosomal insertion, genes regulating plasmid transfer between cells, and an origin of transfer ($oriT$) site.

How did Lederberg and Tatum demonstrate genetic exchange in $E.\text{ coli}$ in 1946?

They mixed two distinct auxotrophic strains ($Y10$: $thr^- leu^- thi^-$ and $Y24$: $phe^- cys^- bio^-$) and observed that prototrophic colonies capable of growing on minimal medium were produced via genetic recombination.

What conclusion was established by the Davis U-tube experiment shown in this diagram?

Bacterial conjugation requires direct physical contact between cells, because no recombinant prototrophic bacteria were produced when the two auxotrophic strains were separated by a filter that allowed medium mixing but blocked cell movement.

What step-by-step mechanism occurs during conjugation between an $F^+$ donor and an $F^-$ recipient?

A cytoplasmic bridge forms, one DNA strand of the $F\text{ factor}$ is nicked at $oriT$, the $5'$ end enters the recipient cell, and rolling-circle DNA replication replaces the single strand in both cells, converting the recipient into an $F^+$ cell.

What is an Hfr cell, and how does it participate in chromosomal gene transfer?

An High-Frequency Recombination (Hfr) cell is formed when the $F\text{ factor}$ integrates into the bacterial chromosome. During conjugation, the nicked $oriT$ initiates transfer of the integrated chromosome, allowing donated bacterial genes to recombine with the recipient's genome.

What is an $F'$ cell, and what is the outcome when it conjugates with an $F^-$ cell?

An $F'$ cell forms when an integrated $F\text{ factor}$ excises imprecisely from an Hfr chromosome, carrying bacterial genes with it. Conjugation transfers this plasmid to an $F^-$ recipient, producing two $F'$ cells and creating a partial diploid for those genes.

What are the three major mechanisms of horizontal gene transfer in bacteria?

Conjugation (direct physical contact and plasmid/chromosomal gene transfer), Transformation (uptake of free extracellular DNA from the environment), and Transduction (bacteriophage-mediated gene transfer).

What are the distinct stages of the lytic and lysogenic viral cycles shown in this diagram?

In the lytic cycle, phage DNA enters, degrades host DNA, replicates, synthesizes capsids, and lyses the host cell. In the lysogenic cycle, phage DNA integrates into the bacterial chromosome as a prophage and replicates passively alongside host cell divisions.

How did the 1952 Davis U-tube experiment using auxotrophic strains of $Salmonella\text{ typhimurium}$ demonstrate transduction?

Prototrophic colonies were recovered despite a filter separating the strains, proving that genetic exchange did not require cell-to-cell contact and was instead mediated by a filterable bacteriophage vector.

What cellular interaction is depicted in this electron micrograph?

A bacteriophage attaching to the outer membrane surface of a bacterial host cell prior to injecting its viral genetic material.

What physical structure connecting the two bacterial cells is shown in this scanning electron micrograph?

A conjugation bridge (sex pilus/cytoplasmic connection) formed between donor and recipient bacterial cells to mediate DNA transfer.