Question

Which of the following statements is not correct about a plasmid?
A. It has a circular DNA
B. It has an antibiotic-resistant gene
C. It has the ability of autonomous replication
D. Its DNA is as long as chromosomal DNA

Answer 1

Hint: A plasmid is an extracellular DNA material present besides the chromosomal DNA. It is generally found in bacterial cells. It carries some special traits for bacteria like antibiotic resistance, the origin of replication, f-factors or fertility factors, etc. these features make it a compatible tool as a vector in rDNA technology.

Complete answer: The plasmid is an extra piece of DNA present in a bacterial cell along with the bacterial chromosomal DNA. Almost all bacterial cells carry plasmids, but some of the species lack plasmid. The plasmid contains its own set of information and also it shows some special characteristic functions in the bacterial cell. Cells carrying plasmids show special survival characteristics. Usually, a plasmid contains only extra genes that are required in certain conditions only.

Let us understand each given statement in context to plasmids to know whether they are correct or incorrect.
-Option A says that plasmid has circular DNA. It is true. All of the plasmids are usually made up of double-stranded circular DNA molecules. Unlike the chromosomal DNA, they are present in a confined region in a circular defined shape.
-Option B says that plasmid has an antibiotic-resistant gene. As stated earlier that the presence of plasmids increases the survival rates for bacterial cells, the plasmid also carries antibiotic-resistant genes. These genes help bacteria to survive in antibiotic conditions by secreting various protective enzymes or proteins.
-Option C says that plasmids have the ability to replicate autonomously. This characteristic is due to the presence of the origin of the replication sequence on the plasmids. This sequence can initiate the replication of plasmids independently.
-Option D says that the plasmid’s DNA is as long as chromosomal DNA. As the plasmid DNA contains only extra genes that are required in certain conditions only, the DNA of the plasmid carries shorter sequences than the chromosomal DNA. Thus, this statement is incorrect.
Therefore, the right answer is option D.

Note: Plasmid is a special characteristic feature of bacterial cells, it is also present in some eukaryotic single-celled organisms. It serves as an important vector tool in recombinant DNA technology or rDNA technology. Genes of interest can be carried by plasmids into host cells where they can replicate independently and express them.