Bacterial Genetics and Horizontal Gene Transfer

Vocabulary flashcards covering vertical vs. horizontal transfer, Griffith's 1928 experiment, transformation, conjugation, and generalized transduction.

Vertical transfer

The transfer of genetic material from a parent organism to its offspring, typically through standard reproduction.

Horizontal transfer

The transfer of genetic material from one cell to another unrelated cell through mechanisms other than reproduction.

Griffith

The British bacteriologist who in 1928 observed horizontal gene transfer while studying streptococcus pyogenes (causes strep throat)

Streptococcus pyogenes

The pathogen species that causes strep throat and was used by Griffith in his 1928 experiments on bacterial transformation.

Smooth streptococcus cells

Wild-type (normal) Streptococcus pyogenes cells that produce a thick sugary capsule, forming wet, smooth colonies and causing fatal infections in mice by concealing themselves from the host immune system.

Rough streptococcus cells

Mutated Streptococcus pyogenes cells that lack a capsule, forming drier, rough-looking colonies, and are non-lethal to host mice because the immune system can clear them.

Transforming principle

Term coined by Griffith in 1928 for the unknown genetic material acquired by living rough cells from heat-killed smooth cells that transformed them into capsule-producing cells

Transformation

A mechanism of horizontal gene transfer where a competent bacterial cell absorbs DNA fragments from its environment and integrates them into its chromosome.

Competent cell

Bacterial cell capable of absorbing extracellular DNA fragments from its environment during transformation

Homologous recombination

Combines two similar DNA molecules, allowing absorbed environmental DNA fragments to be inserted into the bacterial chromosome

F plasmid

A circular piece of non-chromosomal DNA, also known as the F factor or fertility factor, that carries genes for forming an F pilus and performing conjugation.

F plus cell

A bacterial cell that possesses the F plasmid and can form an F pilus to initiate conjugation.

F minus cell

Bacterial cell that lacks the F plasmid and acts as the recipient during bacterial conjugation

F pilus

A hollow tubular structure made of pilin protein that extends from an F plus cell to attach to an F minus cell for DNA transfer during conjugation.

Pilin

The specific protein that forms the structural makeup of a bacterial pilus.

Conjugation

A mechanism of horizontal gene transfer in which an F plus cell attaches to an F minus cell via an F pilus and passes a copy of plasmid DNA across the connection.

High frequency recombinant

A bacterial cell in which the F plasmid has integrated directly into the main circular bacterial chromosome through homologous recombination.

Transduction

A mechanism of horizontal gene transfer in bacteria where host DNA is transferred from one bacterial cell to another by a virus.

1. Attachment & Injection→ viruses attach to bacterium & inject its DNA

2. Host DNA Destruction→ Viral DNA causes the bacterial chromosome to break down

3. Viral production→ Host resources are used to make new viral DNA & proteins

4. Assembly & Lysis→ New viruses assemble, & the bacterial cell bursts releasing them

Generalized transduction

An error during viral packaging where a viral protein shell mistakenly encapsulates a fragment of host bacterial chromosome instead of viral nucleic acid and transfers it to a new host cell. Bacterial DNA integrates it into the recipient chromosome (homologous recombination) recipient gains new traits but does not become infected, no viral DNA was transferred

Virus accidentally carries bacterial DNA → new traits, but no viral infection.

Inoculation 1: Smooth strain of streptococcus injected into mice→

Mice die, autopsy=living, capsule producing smooth streptococcus cells

Inoculation 2: Rough strain of streptococcus injected into mice→

Mice live, Autopsy=living rough streptococcus cells(Few) without a capsule

Capsule

Thick polysaccharide capsule conceals the smooth pathogen from the host immune system → cannot give an effective response. Lacking a capsule, rough cells are readily recognized & cleared by the host immune response

Inoculation: Heat killed smooth strain streptococcus injected→

Mice survived→ Autopsy=no living bacteria

Inoculation 4: Heat killed smooth & live rough mixture streptococcus injected

mice die→ autopsy=living capsule producing smooth streptococcus

Transforming prinicple

Live rough cells aquired a “transforming prinicple” from the dead smooth cells, which genetically transformed them into smooth, capsule producing cells

Mechanism of Hfr Chromosomal transfer

Hfr cells have F factor integrated into their chromosomes, during conjugation, chromosomal DNA is transferred through the conjugation bridge but bc the chromosome is so big and the bridge is fragile it usually breaks b4 the chromosome can transfer. The recipient remains F-, but recieives a large piece of donor DNA, which can recombine with its own DNA (homologous recombination), potentially leading to new genetic traits