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DNA Packaging

DNA Packaging

Chromatin Packaging

Chromatin Packaging

Each human somatic cell contains 6 billion base-pairs of DNA. Each base-pair is 0.34 nm long, which means that each diploid cell contains a staggering 2 meters of DNA. How is such a long DNA strand packed inside a nucleus measuring only 10 - 20 microns in diameter?
The chromatin
In combination with specialized DNA binding protein called Histones, the DNA double helix forms a compact DNA: protein complex called chromatin. The chromatin itself is further compacted into higher-order...

DNA Topoisomerases

DNA Topoisomerases

Topoisomerases are enzymes that relax overwound DNA molecules during various cell processes, including DNA replication and transcription. These enzymes regulate positive and negative DNA supercoiling without changing the nucleotide sequence. DNA overwinding in a clockwise direction results in positively supercoiled DNA, whereas underwinding in a counterclockwise direction produces negatively supercoiled DNA.
Types and Mechanism of action
Topoisomerases are divided into two main types. ...

Fixing Double-strand Breaks

Fixing Double-strand Breaks

The double-stranded structure of DNA has two major advantages. First, it serves as a safe repository of genetic information where one strand serves as the back-up in case the other strand is damaged. Second, the double-helical structure can be wrapped around proteins called histones to form nucleosomes, which can then be tightly wound to form chromosomes. This way, DNA chains up to 2 inches long can be contained within microscopic structures in a cell. A double-stranded break not only damages...

Single-Strand DNA Binding Proteins

Single-Strand DNA Binding Proteins

For successful DNA replication, the unwinding of double-stranded DNA must be accompanied by stabilization and protection of the separated single strands of the DNA. This crucial task is performed by single-strand DNA-binding (SSB) proteins. They bind to the DNA in a sequence-independent manner, which means that the nitrogenous bases of the DNA need not be present in a specific order for binding of SSB proteins to it. The binding of SSB proteins straightens single-stranded DNA (ssDNA) and makes...

Chromatin Packaging

Chromatin Packaging

Each human somatic cell contains 6 billion base pairs of DNA. Each base pair is 0.34 nm long, meaning each diploid cell contains a staggering 2 meters of DNA. This long DNA strand is packed inside a nucleus measuring only 10-20 microns in diameter with the help of specialized DNA-binding proteins called histones. Together they form a compact DNA-protein complex called chromatin. The chromatin is further compacted into higher-order structures. The highest level of compaction is achieved during...

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Related Experiment Video

Updated: May 2, 2026

Stretching Short Sequences of DNA with Constant Force Axial Optical Tweezers

Stretching Short Sequences of DNA with Constant Force Axial Optical Tweezers

Published on: October 13, 2011

DNA overwinds when stretched.

Jeff Gore¹, Zev Bryant, Marcelo Nöllmann

¹Department of Physics, University of California, Berkeley, California 94720, USA.

|July 25, 2006

Summary

This summary is machine-generated.

Contrary to intuition, DNA overwinds under tension, reaching maximum twist around 30 pN before unwinding. This DNA mechanical property, twist-stretch coupling, has implications for DNA-binding proteins.

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Area of Science:

Biophysics
Molecular Biology
Structural Biology

Background:

DNA is typically modeled as an isotropic rod, neglecting its chiral structure.
Anisotropic mechanical properties, like twist-stretch coupling, may be crucial for DNA function.

Purpose of the Study:

To directly measure twist-stretch coupling in single DNA molecules.
To investigate DNA's mechanical response to varying tension.

Main Methods:

Rotor bead tracking was employed to precisely measure twist-stretch coupling.
Single DNA molecules were subjected to controlled tension.

Main Results:

DNA overwinds under tension up to approximately 30 pN, contrary to simple physical intuition.
Above 30 pN, DNA begins to unwind as tension increases.
Observed twist-stretch coupling predicts and confirms DNA lengthening when overwound under constant tension.

Conclusions:

The study reveals counterintuitive mechanical properties of DNA, including overwinding under tension.
A model explaining these properties suggests an origin for DNA's high torsional rigidity.
Findings impact understanding of DNA-binding proteins that manipulate DNA structure.