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Topic: Eukaryotic Chromosome Organization

Supercoiling means that DNA is either under-wound (less than one turn of the helix per 10 base pairs) or over-wound (more than one turn per 10 base pairs) from its normal relaxed state. Supercoiling is essential in several biological processes, such as compacting DNA.

In a “relaxed” double-helical segment of DNA, the two strands twist around the helical axis once every 10.4 to 10.5 base pairs of sequence. Adding or subtracting twists can make the DNA molecule more or less strained.

Extra helical twists are positive and lead to positive supercoiling, while fewer twists of the helix cause negative supercoiling. For example, if the ends of a DNA molecule are joined to form a circular DNA structure if the helix were supercoiled it would contort and form a figure of 8 shape due to the immense strain in the molecule.

Because the length of DNA can be thousands of times that of a cell, packaging this genetic material into the cell or nucleus (in eukaryotes ) is a difficult feat. The supercoiling of DNA reduces the space and allows for much more DNA to be packaged.

Key Points

• As a general rule, the DNA of most organisms is negatively supercoiled.

• The simple figure eight is the simplest supercoil and is the shape a circular DNA assumes to accommodate one too many or one too few helical twists.

• DNA supercoiling is essential for DNA packaging within all cells.

Key Terms

supercoiling: the coiling of the DNA helix upon itself; can disrupt transcription and lead to cell death.

DNA: a biopolymer of deoxyribonucleic acids (a type of nucleic acid) that has four different chemical groups, called bases: adenine, guanine, cytosine, and thymine.

base pair: a unit consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA

nucleus: centre of a cell contains most genetic material

eukaryotes: organisms whose cells have a nucleus enclosed within membranes

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