What is the role of plasmids in bacteria, including copy number and typical content?

• A. Plasmids are essential chromosomal DNA required for survival
• B. Plasmids are linear chromosomes that integrate into the host genome
• C. Plasmids are identical copies of chromosomal DNA
• D. Plasmids are extra-chromosomal DNA that carry genes such as antibiotic resistance and virulence factors; copy number ranges from low to high

Answer: (D)

Plasmids are extrachromosomal DNA elements that replicate independently of the bacterial chromosome. They are usually circular and nonessential for basic survival, but they carry genes that can give the cell advantages in certain environments. The content commonly includes antibiotic resistance genes, virulence factors, or additional metabolic functions. Copy number refers to how many copies of the plasmid exist per cell, and this can range from low (a single copy) to high (tens or hundreds of copies), influencing how much of the plasmid-encoded trait is expressed. Plasmids are not essential chromosomal DNA, not linear chromosomes that integrate into the genome, and not identical copies of chromosomal DNA.