What is the Difference Between Constitutive and Facultative Heterochromatin

The main difference between constitutive and facultative heterochromatin is that constitutive heterochromatin is a permanent factor in a particular cell type, whereas facultative heterochromatin is not a permanent character of each cell of particular cell type. Furthermore, constitutive heterochromatin includes repetitive and structural genes in the telomeres and centromeres, while the formation of facultative heterochromatin often depends on morphogenesis or differentiation. 

Constitutive and facultative heterochromatin are the two types of heterochromatin which can occur in the nucleus.  

Key Areas Covered 

1. What is Constitutive Heterochromatin
     – Definition, Structure, Importance
2. What is Facultative Heterochromatin
     – Definition, Structure, Importance
3. What are the Similarities Between Constitutive and Facultative Heterochromatin
     – Outline of Common Features
4. What is the Difference Between Constitutive and Facultative Heterochromatin
     – Comparison of Key Differences

Key Terms 

Constitutive Heterochromatin, Facultative Heterochromatin, Heterochromatin, LINE-Sequences, Satellite DNA

What is Constitutive Heterochromatin 

Constitutive heterochromatin is a type of heterochromatin the cell, remaining in the condensed state throughout the cell cycle and during the development of the cell. Therefore, it is a permanent factor for a particular type of cells. Generally, this type of heterochromatin is made up of highly repetitive DNA. Thus, constitutive heterochromatin does not get transcribed, but it plays a role in the chromosome structure. Moreover, centromeric and telomeric regions present throughout the cell cycle are examples of constitutive heterochromatin.

Figure 1: C-banding

Furthermore, the type of repetitive sequences in constitutive heterochromatin is satellite DNA, containing a large number of short and tandemly-repeated sequences including Alpha-satellite DNA, DNA satellite I, II and III. They fold themselves to form highly compact structures. Also, due to the instability of satellite DNA, constitutive heterochromatin is highly polymorphic. Besides, this type of heterochromatin can be strongly stained by the C-banding technique.  

What is Facultative Heterochromatin 

Facultative heterochromatin is a type of reversible heterochromatin found in a cell. It is not a conserved factor to a particular set of cells. Additionally, the genes in facultative heterochromatin have the potential to undergo expression at a particular level of development. Therefore, this type of heterochromatin is easier to condense and decondense depending on the cell type. Moreover, the inactivation of one of the two X chromosomes in the female somatic cells occurs by the formation of facultative heterochromatin.

Figure 2: Gene Expression Model in Heterochromatin

Moreover, facultative heterochromatin contains a type of repetitive sequences known as LINE-type repeated sequences. Further, this type of repeated sequences in the genome is present throughout the genome, promoting the condense of chromatin. However, this type of heterochromatin is not rich in satellite DNA. Therefore, they are not polymorphic as well. Furthermore, facultative heterochromatin does not produce C banding patterns. 

Similarities Between Constitutive and Facultative heterochromatin 

• Constitutive and facultative heterochromatin are the two types of heterochromatin found in the eukaryotic nucleus. 
• Both are tightly-packaged or condensed DNA inaccessible to DNA polymerases. 
• They play a key role in the regulation of gene expression. 

Difference Between Constitutive and Facultative Heterochromatin 

Definition 

The constitutive heterochromatin refers to the regions of chromosomes that are invariably heterochromatic, containing highly repetitive sequences of DNA, which are genetically-inactive and serves as a structural element of the chromosome. On the other hand, the facultative heterochromatin refers to the regions of the chromosomes, which become heterochromatic in certain cells and tissues; as an example, it makes up the inactive X chromosome in female somatic cells. 

Occurrence 

Moreover, constitutive heterochromatin is a permanent factor to a particular cell type while facultative heterochromatin is not a permanent character of each cell of particular cell types. Thus, this is one main difference between constitutive and facultative heterochromatin.

Stability 

While constitutive heterochromatin is stable, facultative heterochromatin is reversible. 

Made up of  

Further, constitutive heterochromatin is made up of satellite DNA while facultative heterochromatin is made up of LINE- sequences. 

Polymorphism 

Also, another difference between constitutive and facultative heterochromatin is that constitutive heterochromatin contains polymorphism while facultative heterochromatin does not contain polymorphism. 

C Bands 

Besides, constitutive heterochromatin contains C bands while facultative heterochromatin does not contain C Bands. 

Function 

While constitutive heterochromatin has a structural function, facultative heterochromatin has genes with potential for expression at a particular point of development. 

Examples 

For example, constitutive heterochromatin occurs in the centromeres and telomeres while facultative heterochromatin inactivates X chromosome in female somatic cells. 

Conclusion 

Constitutive heterochromatin is a type of heterochromatin, which is a permanent factor of a particular type of cells. Therefore, it occurs in the same condensed form throughout the cell cycle. The type of repetitive DNA that occurs in the constitutive heterochromatin is satellite DNA, which occurs in centromeres and telomeres. On the other hand, facultative heterochromatin is a type of reversible heterochromatin, which can be condensed or decondensed based on the type of cells. Moreover, it is made up of LINE-sequences dispersed throughout the genome. Additionally, the inactivation of the X chromosome in female somatic cells is an example of facultative heterochromatin. Therefore, the main difference between constitutive and facultative heterochromatin is their structure and importance. 

 References:

1. Mattei, Marie-Genevièvee, and Judith Luciani. “Heterochromatin, from Chromosome to Protein.” Atlas of Genetics and Cytogenetics in Oncology and Haematology, Available Here.

Image Courtesy:

1. “C-banding” By Rcann3 – Own work (CC BY-SA 4.0) via Commons Wikimedia   
2. “Gene expression heterochromatin” By Rcann3 – Own work (CC BY-SA 4.0) via Commons Wikimedia