A diploid human genome consists of around 6 billion base pairs of tightly packaged into 23 chromosomes inside the nucleus. Histones are the proteins involved in the compact packaging of DNA into a microscopic space. Chromatin is the resultant DNA-protein complex. The basic repeating unit of chromatin is the nucleosome. The nucleosome consists of a length of DNA coiled around a core of histone. Histones are positively-charged proteins, and DNA is negatively-charged. The histone core is formed by a protein octamer, combining two of each four histones, H2A, H2B, H3, and H4. Each chromosome consists of thousands of nucleosomes that are interconnected by DNA stretches. The structure of nucleosomes gives a beads-on-string appearance to chromatin.
Key Areas Covered
1. What are Histones
– Definition, Types, Role
2. How are Histones Related to Nucleosomes
– Formation of Nucleosomes
Key Terms: Chromatin, Chromatosome, DNA, Histone Core, Linker DNA, Nucleosome
What are Histones
Histones are basic types of proteins found in chromatin. They are small, positively-charged proteins. H1, H2A, H2B, H3, and H4 are types of histones. In the nucleus, they are associated with DNA, helping to condense it into chromatin. All types of histones are involved in the formation of histone core around which the DNA wraps. The formation of a histone octamer is shown in figure 1.
Figure 1: Histone Core Formation
Histones play a major role in gene regulation by forming two types of chromatin known as euchromatin and heterochromatin. Euchromatin contains loosely-packaged DNA; hence, it shows high expression rates. However, heterochromatin contains tightly-packaged DNA. Therefore, the genes in the heterochromatin are rarely-expressed.
How are Histones Related to Nucleosomes
DNA are negatively-charged molecules due to the presence of a phosphate group in the sugar-phosphate backbone. Hence, positively-charged histones can bind with DNA in a very tight manner. The highly-condensed structure of DNA is formed by the wrapping of DNA around histone protein cores. The wrapped DNA around a histone core is shown in figure 2.
Figure 2: Nucleosome
Nucleosome serves as the basic structural and repeating unit of chromatin, generating the beads on a string appearance. It is made up of a histone core and a stretch of DNA wrapped around it. A histone octamer or the histone core is formed by combining two of the four types of histones, H2A, H2B, H3, and H4. A 146-base pairs-long DNA stretch is wrapped around the histone to form a nucleosome. This DNA stretch wraps approximately 1.7 turns on the histone octamer. Then, another type of histone known as H1 binds to the histone core, allowing another 20 base pairs of DNA. This results in the wrapping of a DNA stretch in two complete rounds around the histone core. This structure is known as a chromatosome. Hence, a 166-base-pairs-long DNA stretch is wrapped around a chromatosome. The structure of the chromatosome is shown in figure 3.
Figure 3: Chromatosome
Every chromosome is made up of thousands of nucleosomes that are joint 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 sting 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.
Conclusion
Chromatin is the condensed structure of DNA made up of a sequence of nucleosomes that are interconnected to each other. Nucleosomes are the structural unit of chromatin. It consists of a stretch of DNA wrapped around a histone core. The histone core is made up of eight histone proteins. Therefore, histones are the core structure of nucleosomes.
Reference:
1. “DNA Packaging: Nucleosomes and Chromatin.” Nature News, Nature Publishing Group, Available here.
Image Courtesy:
1.”Nucleosome structure” By Richard Wheeler (Zephyris) – English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Nucleosome 1KX5 colour coded” By Zephyris at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
3. “Nucleosome organization” By Darekk2 – Own work (CC BY-SA 3.0) via Commons Wikimedia
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