6.24
Plasmidi
I plasmidi sono molecole di DNA extracromosomico presenti nei batteri, negli archei e in alcuni microrganismi eucariotici come i lieviti. Queste picco…
Batteri, archaea e alcuni microbi eucariotici possiedono molecole di DNA extracromosomiche chiamate plasmidi.
I plasmidi sono piccole molecole di DNA circolari, di solito con meno di 30 geni, anche se alcuni possono essere lineari.
Il numero di copie si riferisce al numero di copie di plasmidi in una cellula, che varia da una nei plasmidi a copia singola a oltre 100 nei plasmidi a più copie.
I plasmidi di solito si replicano in modo indipendente, mentre alcuni, come gli episomi, si integrano nel DNA cromosomico per replicarsi insieme ad esso.
Il fattore F dell'episoma media il trasferimento del DNA attraverso la coniugazione codificando i geni per il trasferimento dei pili sessuali e dei plasmidi.
I plasmidi, sebbene non essenziali, aiutano la sopravvivenza. Alcune specie di Pseudomonas metabolizzano gli inquinanti ambientali attraverso enzimi codificati da plasmidi.
I plasmidi conferiscono benefici ecologici, come consentire al rizobio di formare noduli che fissano l'azoto.
Possono anche codificare batteriocine per eliminare i batteri concorrenti.
Alcuni plasmidi aumentano la patogenicità, come quelli nei ceppi di E. coli che codificano tossine e fattori di attaccamento, causando diarrea.
I plasmidi di resistenza, come l'R100, conferiscono resistenza a più antibiotici.
View the full transcript and gain access to JoVE Core videos
Q1: What are plasmids and where are they found in cells?
Plasmids are small, circular extrachromosomal DNA molecules found in bacteria, archaea, and some eukaryotic microbes. They typically contain fewer than 30 genes, though some may be linear. Unlike chromosomal DNA, plasmids replicate independently using the host's DNA replication machinery, providing cells with additional genetic material beyond their main chromosome.
Q2: What is copy number and how does it vary among plasmids?
Copy number refers to the number of plasmid copies present in a single cell. Single-copy plasmids exist as one copy per cell, while multicopy plasmids can reach over 100 copies per cell. This variation allows bacteria to control gene dosage and expression levels, with higher copy numbers generally increasing the availability of plasmid-encoded genes and their products.
Q3: How do episomes differ from typical plasmids?
Episomes are plasmids that can integrate into the host chromosome and replicate along with it, unlike most plasmids that replicate independently. The F factor is a well-known episome that encodes genes for sex pili formation, facilitating direct DNA transfer between bacterial cells during conjugation and promoting genetic exchange among populations.
Q4: What ecological and survival advantages do plasmids provide bacteria?
Plasmids confer significant survival benefits despite being nonessential. Some Pseudomonas species use plasmid-encoded enzymes to metabolize environmental pollutants, aiding bioremediation. Rhizobium bacteria carry plasmids enabling nitrogen-fixing nodule formation in legumes, enhancing soil fertility. Other plasmids encode bacteriocins that eliminate competing bacteria, providing advantages in resource-limited environments.
Q5: How do plasmids contribute to bacterial pathogenicity?
Certain plasmids enhance pathogenicity by encoding virulence factors. Pathogenic E. coli strains harbor plasmids that encode toxins and attachment factors, causing severe diarrheal diseases in humans. These plasmids allow bacteria to produce harmful substances and adhere to host tissues, increasing their ability to cause infection and disease.
Q6: What are resistance plasmids and why are they medically significant?
Resistance plasmids, such as R100, confer resistance to multiple antibiotics, enabling bacteria to survive antibiotic exposure. These plasmids pose a significant challenge in medical treatment and contribute to the growing issue of antibiotic resistance. Their spread among bacterial populations complicates infection control and reduces the effectiveness of antibiotics as therapeutic agents.
Q7: How does the F factor plasmid facilitate genetic exchange between bacteria?
The F factor episome encodes genes responsible for forming sex pili, hair-like structures that enable direct contact between bacterial cells. During conjugation, the F factor mediates plasmid DNA transfer from donor to recipient cells, promoting horizontal genetic exchange. This mechanism allows bacteria to share beneficial genes, including those for antibiotic resistance and metabolic capabilities.