Difference Between Chromatin and Nucleosome

Main Difference – Chromatin vs Nucleosome

DNA is the genetic material of most organisms. Typically, eukaryotic genomes are much larger than prokaryotic genomes. Many organisms have around 10⁹-10¹⁰ base pairs in their genome. However, this long DNA strands should be packed inside the nucleus. DNA are wrapped with a protein called histone to produce chromatin and then chromosomes. Chromatin and nucleosome are two terms used to describe the tight-packaging of genetic material inside the nucleus. The main difference between chromatin and nucleosome is that chromatin is the general term for the DNA wrapped with histones whereas nucleosome is the basic, repeating structural unit of chromatin.

Key Areas Covered

1. What is Chromatin
     – Definition, Structure, Role
2. What is Nucleosome
     – Definition, Structure, Role
3. What are the Similarities Between Chromatin and Nucleosome
     – Outline of Common Features
4. What is the Difference Between Chromatin and Nucleosome
     – Comparison of Key Differences

Key Terms: Chromatin, Chromatosome, Chromosomes, DNA, Euchromatin, Heterochromatin, Histone Core, Linker DNA, Nucleosome

What is Chromatin

Chromatin is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. The main purpose of chromatin is to tightly pack the DNA inside the cell nucleus. Chromatin regulates the gene expression and allows DNA replication in addition to the packaging. It also prevents DNA damage. Chromatin can be observed under the microscope as thread-like, looped structures during the interphase. Chromatin is shown in figure 1.

Figure 1: Chromatin

Chromatin Types

The interphase chromatin consists of two types: euchromatin and heterochromatin. 

Euchromatin

The loosely-packed form of chromatin is known as euchromatin. It contains the actively expressed genes in the genome. The loose packaging of chromatin allows the transcription of genes in that region. The diameter of the euchromatin fiber is 30 nm. Euchromatin consists of loops with 40-100 kb regions in the genome. Euchromatin is also genetically active as chromosome crossover occurs in this regions.

Heterochromatin

The tightly-packed form of chromatin is known as heterochromatin. Heterochromatin contains both transcriptionally and genetically inactive DNA which provides structural support to the genome during its chromosomal stages. Two types of heterochromatin can be identified: constitutive heterochromatin and facultative heterochromatin. Constitutive heterochromatin consists of no genes. Facultative heterochromatin consists of inactive genes.

What is Nucleosome

Nucleosome refers to the main structural unit of the eukaryotic chromatin that consists of a length of DNA coiled around a core of histones. This means the core particles that form chromatin are the nucleosomes. A nucleosome consists of 146 base pairs long DNA stretches, wrapped around a core of histone. The histone core is made up of eight histone proteins. The histone octamer is formed by combining two of each four histones, H2A, H2B, H3, and H4. The DNA stretch wraps approximately 1.7 turns of DNA. Another 20 base pairs of DNA is wrapped around an H1 protein, completing two turns of DNA around the histone core. The forming structure is known as chromatosome. Ultimately, a nucleosome consists of 166 base pairs of wrapped DNA. The structure of the nucleosome is shown in figure 2.

Figure 2: Nucleosome

Each chromosome consists of thousands of nucleosomes that are interconnected by DNA stretches known as linker DNA. The length of the linker DNA is around 20 base pairs. The nucleosomes plus linker DAN gives the appearance of beads-on-a-string.

Similarities Between Chromatin and Nucleosome

• Chromatin and nucleosome are two terms used to describe the tight packaging of DNA inside the nucleus.
• Both chromatin and nucleosome are found in eukaryotes.
• Both chromatin and nucleosome are structures made up of DNA wrapped around histone proteins.
• Both chromatin and nucleosome are involved in the formation of chromosomes.

Difference Between Chromatin and Nucleosome

Definition

Chromatin: Chromatin is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells.

Nucleosome: Nucleosome is the main structural unit of the eukaryotic chromatin that consists of a length of DNA coiled around a core of histones.

Significance

Chromatin: Chromatin is the general term for DNA wrapped around histones.

Nucleosome: Nucleosome is the basic repeating, structural unit of chromatin.

Correlation

Chromatin: Chromatin forms the chromosomes.

Nucleosome: Nucleosomes form chromatin.

Appearance

Chromatin: Chromatin appears as a thread-like, looped structure.

Nucleosome: Nucleosome appears as beads on a string.

Length

Chromatin: A chromatin loop consists of 40-100 kb DNA.

Nucleosome: A nucleosome consists of 166 base pairs of wrapped DNA.

Diameter

Chromatin: The diameter of a chromatin fiber is 30 nm.

Nucleosome: The diameter of a nucleosome is 11 nm.

Condensity

Chromatin: Chromatin is more condensed than nucleosomes.

Nucleosome: Nucleosomes are the least condensed chromosome structures.

Conclusion

Chromatin and nucleosome are two structures made up of DNA and histones. Both structures are important in the tight packaging of DNA inside the nucleus. Chromatin is the general term for DNA plus histones. Nucleosome is the basic structural unit of chromatin. The main difference between chromatin and nucleosome is the correspondence of two structures.

Reference:

1. Annunziato, Anthony C. “DNA Packaging: Nucleosomes and Chromatin.” Nature News, Nature Publishing Group, Available here.

Image Courtesy:

1. “Sha-Boyer-Fig1-CCBy3.0” (CC BY 3.0) via Commons Wikimedia
2. “Nucleosome organization” By Darekk2 – Own work (CC BY-SA 3.0) via Commons Wikimedia