Question

Circular DNA molecule occurs in
A. Viruses
B. Bacteria, Chloroplasts, and Mitochondria
C. Bacteria and Chloroplasts only
D. Bacteria only

Answer 1

Hint: DNA or Deoxyribonucleic acid is the control unit of the cell. It guides the cell for all metabolic activities and is the primary source for the production of proteins and enzymes required by the cell. Generally, it is found inside the nucleus in eukaryotic cells. But in prokaryotes or semi-autonomous organelles, it is presently free in the cytoplasm. Also, in eukaryotes, it is found in linear shape but in prokaryotes, it may be circular or single-stranded.

Complete step by step answer: DNA is a complex molecule. It is made up of nucleotides. These nucleotide bases have encoded information on them for the production of proteins and enzymes that are required by the cell. The DNA is like the master unit of the cell. It guides, assists, and commands it. Generally in eukaryotic cells or complex organisms, the DNA is present as a double helical structure condensed to form chromosomes enclosed in the nucleus of the cell. It is present as linear strands for the process of replication, transcription, and translation.
In the viruses, the DNA can be single-stranded or double-stranded. Furthermore, most of the viruses have RNA as their genetic material. If DNA is present in a virus as their genetic material, it is present as a very short linear strand inside the capsid of the virus. As the virus is a very minute organism it cannot carry complex or circular DNA strands.
In bacteria, the DNA is present freely in the cytoplasm. But the region where the DNA is present is called the nucleoid. The bacterial genome ranges from 130 kbp to 14 Mbp. As bacteria is a prokaryotic organism the DNA is circular and has no nucleus around it.

The semi-autonomous organelle chloroplast contains its own DNA that assists it to make required proteins required majorly for the process of photosynthesis in the plants. The DNA molecule is circular and is called ct-DNA. It is usually employed in the biotechnology field as a plasmid vector.
The other semi-autonomous organelle Mitochondria also contains its own DNA molecule that is used to synthesize proteins. It is also circular and is known as mt-DNA. It is also used as a potent unit in genetic manipulation experiments.
Therefore, the bacteria, chloroplast, and mitochondria consist of circular DNA molecules. so, the right answer is option B.

Note: In the circular type of DNA there are no endpoints. The DNA attains conformational changes to form a closed circular structure. Almost all the prokaryotes carry circular DNA. The prokaryotes do not have any true nucleus, thus the DNA floats free in the cytoplasm. Also, in prokaryotes like bacteria, extranuclear material is presently called the plasmid. It is the extra piece of circular DNA that bacteria carry. It exhibits some special characteristics.