Circular Vs. Linear Chromosomes

It is indisputable that DNA has taken over Biology, and its properties are essential in the teaching of biological sciences at all levels. Students learn a great number of facts about this important biopolymer and sometimes there is not enough time to ask some critical questions behind some DNA facts. Students in the biological sciences learn that bacterial DNA occurs naturally in a circular form, while higher eukaryotes generally have linear genetic material. But why is there this difference? What architecture of DNA is favored and what advantages or disadvantages are in play?

The key towards what kind of DNA is favored lies in the existence of telomeres, the ends of linear genetic material. Telomeres are the reason molecular biologists attribute that linear fragments of DNA are less stable than circular ones. These ends of a linear fragment are more prone to degradation by intracellular enzymes, due to having an unprotected OH group. This drives chemical degradation to happen more frequently in linear DNA. This is clearly not a problem in circular DNA because it lacks these vulnerable, exposed ends.

The presence of telomeres also has great implications in replication. Higher eukaryotes have telomeres at the end of their chromosomes, which are shortened every replication cycle. This shortening is related to DNA damage and is one of the reasons for aging in higher eukaryotes. Organisms with circular chromosomes do not present this problem because the amount of DNA is maintained the same in every replication cycle. This is another clear advantage of circular DNA, which might be related to the higher duplication rate of organisms with circular genetic material.

We can see that telomeres entail a clear advantage of circular chromosomes over linearized ones. But why then did higher organisms abandoned this circular architecture as their complexity evolved? The quest will continue…

REFERENCES

1. Vologodskii, V. V. 1999. Circular DNA. Online Biophysical Chemistry Textbook. Ed. 5. Bloomfield.

2. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., Walter, P. 2008. Chapter 5: DNA Replication, Repair, and Recombination. Molecular Biology of the Cell. S. Masson, E. Jeffcock, M, Anderson, S. Granum, editors. Garland Science/New York, USA. 281–293

3. Aubert, G. Landsdorp, P. M. 2008. Telomeres and Aging. Physiol Rev. 88(2):557–79.