What is a plasmid?

A plasmid is correctly identified as a small, circular DNA molecule that exists independently of chromosomal DNA within bacterial cells. Plasmids can carry genes that confer advantageous traits to the bacteria, such as antibiotic resistance, and they play a crucial role in molecular biology and biotechnology for cloning and gene expression purposes.

Their unique structure allows for easy manipulation in genetic engineering, as scientists can insert or modify genes within plasmids to study their functions or produce proteins. This capability has made plasmids invaluable tools in research and the development of genetically modified organisms.

In contrast, the other options describe different molecular entities. Linear fragments of chromosomal DNA do not retain the circular nature that defines plasmids, viral DNA refers to the DNA incorporated during a viral infection, and modified amino acids pertain to protein synthesis rather than DNA structure. Thus, the characteristics of plasmids distinctly set them apart from these other biological components.