Question

Extra-chromosomal DNA occurs in
a. Mitochondria
b. Ribosomes
c. Nucleus
d. Chromosomes

Answer 1

Mostly the DNA is present in chromosomes inside the nucleus of a cell. But if DNA is not present on the chromosome rather at any other site outside the nucleus of the cells or in the cytoplasm it is called extra-chromosomal DNA. As you all know in eukaryotic cells there are many organelles like endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, and plant cells contain chloroplast. Among these mitochondria and chloroplasts are thought to be derived from more primitive aerobic bacteria and photosynthetic bacteria respectively. They were engulfed. This theory is called the endosymbiotic theory.

Complete answer:
Structures containing DNA are as follows -
• Chromosome
• Chromatin
• Nucleus
• Nucleolus
• Mitochondria
• Chloroplast
Among these mitochondria and chloroplasts containing bacterial DNA is present in cytoplasm so DNA present in these structures is considered as extrachromosomal DNA.
Ribosomes contain RNA and do not contain DNA.
Therefore nucleus and chromosome contain chromosomal DNA.

Hence, the correct answer is option (A).

Additional information:
• Mitochondria is a sausage-shaped cell organelle and is the main site for aerobic respiration. It produces cellular energy in the form of ATP, hence they are called “power-houses” of the cell.
• Mitochondria divide by fission independent of the cell cycle.
• Mitochondria besides containing a single circular DNA molecule like a bacterial chromosome it also contains ribosomes.

Note: Structures containing RNA are cytoplasm, nucleoplasm, chromosome, chromatin, ribosomes.Plasmids also extra-chromosomal DNA that is used as vectors to transfer the piece of DNA attached to it. Thea is able to make many copies of certain genes. A vector is one that can transfer foreign DNA.