What is a plasmid?
a. An organelle found in many bacteria and certain eukaryotes
b. A circular DNA molecule that replicates independently of the main chromosome(s)
c. A type of virus that has a DNA genome and infects certain types of human cells, including lung and respiratory tract tissue
d. A type of virus that has an RNA genome, codes for reverse transcriptase, and inserts a cDNA copy of its genome into cells

Verified step by step guidance

1. Understand that a plasmid is a genetic structure in cells, often used in molecular biology.
2. Recognize that plasmids are typically found in bacteria, but can also be present in some eukaryotes.
3. Identify that plasmids are distinct from the main chromosomal DNA, as they are usually circular and can replicate independently.
4. Note that plasmids are not viruses, so options describing viral characteristics (such as options c and d) are not correct.
5. Conclude that the correct description of a plasmid is option b: a circular DNA molecule that replicates independently of the main chromosome(s).

Key Concepts

Plasmid

A plasmid is a small, circular DNA molecule found in bacteria and some eukaryotes that replicates independently of the chromosomal DNA. Plasmids often carry genes that confer advantageous traits, such as antibiotic resistance, and are used in genetic engineering due to their ability to transfer genes between cells.

Independent Replication

Independent replication refers to the ability of plasmids to replicate autonomously within a host cell, separate from the chromosomal DNA. This characteristic allows plasmids to be maintained in multiple copies within a cell, facilitating the spread of genetic traits and their use in molecular cloning.

Genetic Engineering

Genetic engineering involves the manipulation of an organism's genome using biotechnology. Plasmids are crucial tools in this process, as they can be engineered to carry specific genes and introduced into host cells to study gene function, produce proteins, or develop genetically modified organisms.