Kaplan Nursing Entrance Practice Exam

What is a plasmid?

1. A small circular piece of RNA

2. An extra piece of independently replicating DNA

3. A linear piece of DNA found in the nucleus

4. A protein structure in bacteria

The correct answer is: An extra piece of independently replicating DNA

A plasmid is best described as an extra piece of independently replicating DNA. Plasmids are small, circular, double-stranded DNA molecules that are found in bacteria, as well as in some archaea and eukaryotic cells. They are distinct from the chromosomal DNA and can replicate independently within the bacterial cell. Plasmids carry genes that can provide bacteria with advantageous traits, such as antibiotic resistance or the ability to metabolize unusual substances. Because they can be transferred between bacteria through mechanisms like conjugation, plasmids play an essential role in genetic diversity and adaptation among bacterial populations. In contrast to the other choices, a plasmid is not a piece of RNA, nor is it linear or found in the nucleus, and it is not a protein structure. The characteristics of plasmids make them important tools in genetic engineering and molecular biology, where they are commonly used to clone, transfer, and manipulate genes.