The genetic material of an organism is composed of billions of base pairs of DNA. It is arranged into chromosomes for the tight packaging inside the nucleus. Chromosomes are composed of DNA associated with proteins, forming a complex structure known as chromatin. 40% of the chromosomes are DNA, and the remaining 60% is proteins. Histones are the proteins associated with DNA. DNA is wrapped around a core formed by histones, forming a structure known as a nucleosome. The nucleosome is the basic unit of a chromosome or chromatin fiber. A nucleosome can be considered as a DNA coil. Hence, a chromosome is made up of DNA supercoils.
Key Areas Covered
1. What are Histones
– Definition, Types, Role
2. How Do Histone Proteins Help in the Coiling of DNA
– Formation of Nucleosomes
Key Terms: Chromatin, Chromatosome, Coils, DNA, Histone Core, Linker DNA, Nucleosome, Supercoils
What are Histones
Histones are a type of positively-charged proteins that serve as the basic type of proteins found in the chromosomes. The five types of histones are H1, H2A, H2B, H3, and H4. The main function of histone proteins is to help in the condensed packaging of DNA inside the nucleus. The interaction between histones and DNA is shown in figure 1.
Figure 1: Histone-DNA Interaction
The four types of histones involved in the formation of the histone core are H2A, H2B, H3, and H4. DNA wraps around the histone core to form coils of DNA. Histones play a major role in the regulation of gene expression by forming two types of chromatin known as euchromatin and heterochromatin. Euchromatin contains loosely-packaged DNA and shows high expression rates. However, heterochromatin contains tightly-packaged DNA, rarely-expressing the genes in the region.
How Do Histone Proteins Help in the Coiling of DNA
The genome of an organism is made up of a large number of nucleotides, encoding the whole genetic information for the development and functioning of the organism. All these nucleotides should be contained within a tiny space in the microscopic structure known as nucleus. Therefore, a mechanism is required for the tight-packaging of DNA into the nucleus. Histones are involved in the formation of a highly-condensed structure of DNA coils by wrapping DNA around a core of histones. This coiled structure is known as a nucleosome. The wrapped DNA around a histone core is shown in figure 2.
Figure 2: Nucleosome
The histone core is made up of a histone octamer, which is made up of a combination two of the four types of histones, H2A, H2B, H3, and H4. A DNA stretch that is 146 base pairs long is wrapped around the histone core in the nucleosome. This wrapping forms approximately 1.7 turns on the histone octamer. Then, a fifth type of histone known as H1 binds to the histone core, allowing the wrapping of another 20 base pairs of DNA around the histone core. The resultant structure is known as a chromatosome. Hence, a 166 base pairs long DNA stretch is wrapped around a chromatosome. Thousands of nucleosomes are joined together by DNA stretches known as linker DNA. Linker DNA consists of 20 base pairs. This forms long chains of nucleosomes that gives the beads on a string structure under the microscope. The packaging of DNA into nucleosomes shortens the length of the DNA strand in sevenfold. The diameter of the formed chromatin fiber is 20 nm. However, chromatin is further coiled into a 30 nm fiber, forming higher-order structure.
Nucleosome represents a coil of DNA. It serves as the basic structural and repeating unit of the chromosome that generates the beads on a string appearance. This means that a chromosome is made up of DNA supercoils.
Conclusion
The genome of most organisms is made up of long chains of nucleotides, which should be packaged into the nucleus. Histones are the associated proteins that allow the tight-packaging of DNA into the nucleus. A piece of DNA is wrapped around a core of histones, producing a DNA coil known as a nucleosome. As chromosomes are made up of a series of nucleosomes, it is considered to have a supercoiled nature in the structure of a chromosome.
Reference:
1. Iftikhar, Jannat. “Role of Histone in DNA packaging.” LinkedIn SlideShare, 14 Dec. 2013, Available here.
Image Courtesy:
1. “Figure 04 03 05a” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Nucleosome” By Spellcheck assumed – Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia
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