Gene transfer and homologous recombination in bacteria

Chap 6

gene flow promotes --- and ---

**genetic evolution and increases the microbial diversity.**

**A foreign DNA entry in bacteria must be followed by a specific genetic event to be acquired.**

What are the 2 types of genetic events ?

Substitution (homologous recombination )

Addition (plasmid integration)

If DNA is not auto replicative or integrated, it will be diluted and lost among the bacterial population.

True or FALSE

TRUE

Homologous recombination is when a ==__**------**__== event occurs

==**double cross over →**== **Substituted or allelic exchange → DNA transmitted to the bacterial daughters**

A pair number of crossing-overs produces a ---- and ---- recombinant

circular and viable

What are the different types of acquisition through a genetic element in the absence of homologous recombination ?

Site-specific recombination (Integron or transposable)

Autonomous replication

What is a transposon ?

DNA sequences able to __**change their location in the genome.**__

Not replicative

Have a transposase responsible for recombination .

Transposon may bring some new genes (resistance and virulence) and modify gene expression

True or False

True

Transposition implies the presence of --- and --- sequences named IR (repeated inversed sequences) Wha

Inverted and repeated

What do transposons carry usually?

RESISTANCE genes

Describe the conservative and replicative transpositions

On the figure

Co-integrate → resolution by crossover of the co-integrate

What are the 3 different types of transposable elements?

* Composite
* Unitary
* Conjugative → able to transfer from b to another by conjugation between several species (BIG)

What is an integron?

Gene capture and expression system __**under cassettes form.**__

Integrons are __**not mobiles**__ by themselves

__**Found on transposons or plasmids**__

What is a cassettes ?

__**Small mobile elements**__

__**Non-replicatives**__, but able to be __**integrated or excised**__ by a specific recombination mechanism made by an __**integrase**__.

Do cassets contain functional gene?

Cassettes do not encode a functional gene of the integron.

A functional platform of integron is constituted by :

▪ *intI* (5’region) encode an __**integrase**__

▪ *attI, a* __**recombination specific site**__

▪ Pc, a __**strong promotor**__

What is a plasmid?

**is a DNA molecule**:

\- Distinct from the chromosomal DNA

\- Non-essential to the cell survive

\- Able to replicate in an autonomous manner

Circular (most common) or Linear

What is a natural transformation?

Process by which a bacteria can actively take up and integrate an exogenous DNA thereby providing a source of genetic diversity.

Explain the mechanism of natural trasnformation

dsDNA is taken up by a bacteria through a pore. It is tehn converted to a ssDNA which can either be __**degraded**__ (most cases) ==**or**== be integrated to __**chromosomal DNA**__ through __**homologous recombination.**__

What is conjugation?

Process by which a bacteria can __**transfer genetic material**__ to each other by ==__**direct contact using a pilus**__==

What form of DNA is transformed during conjugation?

A ==__**plasmid**__== is received during conjugation

WHat is an episome?

a genetic element inside some bacterial cells, especially the DNA , that can ==__**replicate independently**__== of the host and also in association with a chromosome with which it becomes __**integrated**__.

What is difference between episome and plasmid?

\

==__**Plasmids**__== do not integrate into the genome, while the ==__**episome**__== can integrate into the genome.

What are the 2 different groups of plasmids ?

Conjugative →transfer by conjugation then replication

Non conjugative → No transfer, only replication

What is transduction?

**Introduction of DNA to a recipient bacterium which is carried by a bacteriophage.**

Explain the. mechanism of transduction.

When bacteriophage attach to the bacteria via a specific receptor, then there is perforation of the cell wall by the phage and injects DNA into the bacteria

What are the 2 type of mechanism for transduction?

**Generalized transduction or non specific →** able to transfer several bacterial genes.



**Specialized transduction →**able to transfer some metabolism properties.

Compare the three mechanism of DNA transfer

Natural transformation

Conjugation

Transduction

Transposition

Natural transformation: bacteria take up DNA from envirnment

Conjugation: transfer of plasmid through pili

Transduction: transfer of DNA by bacteriophages

Transposition: Integration of DNA with trasnposons containing IR domains