Question

The genomic content of the nucleus is constant for a given species whereas the extra chromosomal DNA is found to be variable among the members of a population. Explain.

Answer 1

Genomic content of the nucleus

The DNA content is constant for each species so as to ensure species viability. Both asexual and sexual reproduction is designed in such a way that it maintains the ploidy of the subsequent generations involving DNA duplications or special cell divisions followed by fusions wherever necessary. Therefore, the genomic content is constant but the extra chromosomal content can vary.


Extra chromosomal content in eukaryotes

In eukaryotes, extrachromosomal DNA is present in mitochondria and plastids.
The abundance of mitochondria can be higher in case of highly active organisms, as compared to less active ones. Besides, there is variation in its quantity when different types of cells are compared. Similarly, the abundance of plastids can be different
among plants of the same species.


Extra chromosomal content in prokaryotes

Plasmids may or may not be present in bacteria. Certain types of plasmids when present in bacteria can transfer its copy to another bacteria by a process termed as conjugation. Therefore, the number of plasmids and hence the amount of extra-chromosomal DNA may vary in a specific population of bacteria.