10 Difference Between Constitutive and Facultative Heterochromatin (With Table)

Heterochromatin is a type of chromatin that is densely packed. The chromatin occurs in the nucleus of eukaryotic cells. 

The condensed structure of heterochromatin prevents the DNA and RNA polymerase from accessing the DNA. This also prevents DNA replication and transcription. 

Heterochromatin is further classified as constitutive and facultative heterochromatin. The activation and location are what bring the difference. 

So, what is the main difference between constitutive and facultative heterochromatin? The former does not code proteins and is present throughout the cell cycle while the latter refers to a silenced DNA region of a chromosome that is activated under specific conditions. 

Constitutive heterochromatin consists of structural and repetitive genes in the centromeres and telomeres. Facultative heterochromatin formation depends on differentiation and morphogenesis.  

This article provides further differences between constitutive and facultative heterochromatin in a tabular form. Take the time to read and get to know the relationship between these two forms of heterochromatin.

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Comparison Table ( Constitutive vs Facultative Heterochromatin)

Basic Terms Constitutive HeterochromatinFacultative Heterochromatin
Description Refers to DNA regions in the chromosomes found throughout the cell cycle. Refers to silenced DNA regions in the chromosomes that are only activated under certain conditions. 
Types of SequenceSatellite, minisatellite and microsatellite.Long interspersed nuclear elements. 
Ability to ExpressUnable to express the genes.Can be expressed to the genes. 
Occurrence Permanent factor to a particular cell type.Not a permanent factor of particular cell types. 
Stability More stable Reversible 
Content Satellite DNA Line-Sequence 
Polymorphism Present Absent 
C Bands Present Absent 
Function Structural function Genes with the potential of expression at a specific point of development. 
Examples Centromeres and telomeres Inactive X chromosomes in female somatic cells 

What Is Constitutive Heterochromatin?

It is a form of heterochromatin that remains in a condensed state throughout the cell cycle and development stage. It is a permanent factor in a particular cell type. 

This heterochromatin form contains highly repetitive DNA that prevents transcription and replication. It plays a vital role in structural function. 

Examples of constitutive heterochromatin are centromeres and telomeres. The type of sequence in this heterochromatin form is satellite, minisatellite, and microsatellite. 

These sequences usually form highly compact structures. The instability of satellite DNA makes this constitutive heterochromatin highly polymorphic. It later stains heterochromatin during the C-banding technique. 

What Is Facultative Heterochromatin?

It is a form of reversible heterochromatin in a cell. It is also not a considered factor in a particular type of cell. The genes have the opportunity to undergo expression at every stage of development. 

Facultative heterochromatin is easier to condense and decondense depending on the cell type. It also results in the inactivation of one of the two X chromosomes in female somatic cells. 

This form of heterochromatin contains a Line-sequence that is repetitive. It is common throughout a genome to promote condensation of chromatin. 

Facultative heterochromatin is not rich in satellite DNA which makes them non-polymorphic. Besides that, they do not produce C banding patterns. 

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Main Difference between Constitutive and Facultative Heterochromatin

  1. Constitutive heterochromatin is a DNA region found throughout the cell cycle. Facultative heterochromatin is a silenced DNA only activated under a specific condition.
  2. Constitutive heterochromatin has a satellite DNA sequence. The Facultative heterochromatin has a line sequence of DNA.
  3. Facultative heterochromatin can express genes. Constitutive heterochromatin cannot express genes. 
  4. Constitutive heterochromatin is stable. Facultative heterochromatin is reversible. 
  5. Constitutive heterochromatin contains C bands. Facultative heterochromatin lacks C bands.

Similarities between Constitutive and Facultative Heterochromatin

  1. Both are found in the eukaryotic cell nucleus
  2. Both have highly packed DNA to inhibit accessibility by the polymerase.
  3. Both help in the regulation of gene expression. 
  4. Both are forms of heterochromatin.

In Conclusion

Constitutive heterochromatin occurs in a dense form throughout the cell cycle and is a permanent factor in a particular cell type. Satellite DNA occurs in constitutive heterochromatin in centromeres and telomeres. 

Facultative heterochromatin is a reversible form of heterochromatin. It can be condensed and decondensed depending on the type of cell. It consists of Line-sequence that spread out in the genome. It is responsible for the inactivation of one X chromosome in female somatic cells. 

The core difference between constitutive and facultative heterochromatin is based on structure and significance. Learning how heterochromatin works are crucial in the study of genetics. 

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