Question

Write a note on the packaging of DNA in prokaryotes.

Answer 1

1. In prokaryotes like E. coli, cell size is almost 2-3 µm long.
2. They lack a well-organized nucleus i.e. nucleus is without nuclear membrane and nucleolus.
3. The nucleoid is a small, circular, highly folded, naked ring of DNA which is 1100 µm (approximately 1.1 mm) long in perimeter, containing about 4.6 million base pairs.
4. The nucleoid is to be packaged into a cell that is approximately 2-3 µm long.
5. Hence, the negatively charged DNA becomes circular, reducing the size to 350 µm in diameter. This is further reduced to 30 µm in diameter because of folding/looping. 40-50 domains (loops) are formed.
6. The formation of loops is assisted by RNA connectors.
7. Each domain is further coiled and supercoiled, thereby reducing the size down to 2 µm in diameter.
8. This coiling (packaging) is assisted by positively charged HU (Histone like DNA binding proteins) proteins and enzymes like DNA gyrase and DNA topoisomerase I, for maintaining super coiled state.