Difference Between Constitutive and Facultative Heterochromatin

Key Difference – Constitutive vs Facultative Heterochromatin
 

Chromosomes are condensed structures composed of Deoxyribose nucleic acids (DNA). It is a well-organized structure, and the basic unit of the DNA packaging is the nucleosome. The packaging of DNA into the chromosome involves many steps. When chromosomes are observed under a microscope after staining, different regions can be observed such as darkly stained regions and lightly stained regions. Darkly stained regions are known as Heterochromatin, and they are the regions that have densely packed DNA. Lightly stained regions are known as Euchromatin, and they are the regions that have loosely packed DNA. Heterochromatin can be further classified as Constitutive heterochromatin and facultative heterochromatin. Constitutive heterochromatin refers to the regions of DNA in the chromosome found throughout the cell cycle. They are mainly found near the peri-centromeric regions and telomeric regions of the chromosome. Facultative heterochromatin is regions of the DNA in which the genes are silenced by modifications. Therefore, they are only activated under certain conditions and not found throughout the cell. The key difference between constitutive and facultative heterochromatin is the functionality of the two types. Constitutive heterochromatin is present throughout the cell cycle and does not code for proteins, whereas facultative heterochromatin refers to silenced DNA regions of the chromosome that are activated under specific conditions.

CONTENTS

1. Overview and Key Difference
2. What is Constitutive Heterochromatin
3. What is Facultative Heterochromatin
4. Similarities Between Constitutive and Facultative Heterochromatin
5. Side by Side Comparison – Constitutive vs Facultative Heterochromatin in Tabular Form
6. Summary

What is Constitutive Heterochromatin?

Constitutive heterochromatin refers to the darkly stained condensed regions of DNA that are found throughout the chromosome of the eukaryotes. They are found in the peri-centromeric and telomeric regions of the chromosome. Constitutive heterochromatin regions are visualized using the C banding technique. Under the microscope, the constitutive heterochromatin appears to be much darkly stained.

The composition of the constitutive heterochromatin is mainly based on the high copy number of tandem repeats. These tandem repeats can be satellite DNA, minisatellite DNA or microsatellite DNA. These regions are highly repetitive and polymorphic. Therefore, at present, they are used as markers in DNA fingerprinting and paternity testing.

The main function of constitutive heterochromatin is observed during the cell division process, where it is predicted that constitutive heterochromatin is required for the segregation of sister chromatids. It is also useful in the proper functioning and formation of the centromere.

Although both centromeric and telomeric DNA are composed of constitutive heterochromatin, both centromeric and telomeric DNA are not conserved throughout the genome. Centromeric sequences are not conserved in many species, but telomeric sequences are thought to be more conserved across species. Both regions do not contain genes but are important as they play a prominent structural role.

Replication of constitutive heterochromatin takes place during the late S phase. Histone modifications are done to form the constitutive heterochromatin, where the most common modifications include – histone hypoacetylation, histone H3-Lys9 methylation (H3K9), and cytosine methylation. These modifications are heritable therefore, fall under the broad topic of epigenetics. Genetic mutations may lead to defects in the constitutive heterochromatin regions leading to different genetic complications (Robert’s Syndrome)

What is Facultative Heterochromatin?

Facultative heterochromatin regions are the DNA regions that are not found throughout the chromosome, and thus, they are not consistent between different species. This DNA code for genes that are expressed poorly.

The facultative heterochromatins are silenced genes that are expressed under specific conditions. These conditions include;

• Temporal (e.g., developmental states or specific cell-cycle stages)
• Spatial (e.g., nuclear localization changes from the center to the periphery or vice versa due to exogenous factors/signals)
• Parental/heritable (e.g., monoallelic gene expression)

The genes are silenced by chromatin modulation processes. The classic example of facultative heterochromatin modification is X chromosome inactivation in females, where one set of the X chromosome is inactivated so that the genetic composition of X chromosome in males and females are balanced.

Facultative heterochromatin has a high possibility to be converted into euchromatin regions; thus, during C banding staining technique, facultative heterochromatin is not stained dark in comparison to constitutive heterochromatin.

What are the Similarities Between Constitutive and Facultative Heterochromatin?

• Both Constitutive and Facultative Heterochromatin types are composed of DNA regions.
• Both Constitutive and Facultative Heterochromatin types are highly condensed regions of DNA.
• Both Constitutive and Facultative Heterochromatin types can be distinguished by C banding staining.
• Both Constitutive and Facultative Heterochromatin types are regulated by epigenetic factors.

What is the Difference Between Constitutive and Facultative Heterochromatin?

Constitutive vs Facultative Heterochromatin
Constitutive heterochromatin refers to the regions of DNA in the chromosome found throughout the cell cycle.	Facultative heterochromatin is regions of the DNA in which the genes are silenced by modifications. Therefore, they are only activated under certain conditions and not found throughout the cell.
Types of Sequences
Satellite, minisatellite and microsatellite sequences are types of constitutive heterochromatin.	Long interspersed nuclear elements are a type of facultative heterochromatin.
Ability to Express
Constitutive heterochromatin is unable to express the genes.	Facultative heterochromatin can be expressed.
C Banding Staining
Constitutive heterochromatin bands stain in dark colour.	Facultative heterochromatin bands do not stain / stain with a light colour.
Polymorphisms
Present among the constitutive heterochromatin.	Absent in facultative heterochromatin.

Summary – Constitutive vs Facultative Heterochromatin

Heterochromatin and Euchromatin are the two main banding patterns observed under C band staining. Heterochromatin appears darkly stained as they are highly condensed. Constitutive and Facultative heterochromatin regions are the main divisions of heterochromatin. The consistent regions found throughout the cell cycle, which are structurally important, are referred to as constitutive heterochromatin. The silenced DNA regions that are eventually converted to euchromatin regions are referred to as facultative heterochromatin. They are expressed only under certain conditions. This is the difference between constitutive and facultative heterochromatin.

Reference:

1.Patrick Trojer, and Danny Reinberg. “Facultative Heterochromatin: Is There a Distinctive Molecular Signature?” Molecular Cell, Cell Press, 11 Oct. 2007. Available here
2.Saksouk, Nehmé, et al. “Constitutive heterochromatin formation and transcription in mammals.” Epigenetics & Chromatin, BioMed Central, 2015. Available here

Image Courtesy:

1.’C-banding’By Rcann3 – Own work, (CC BY-SA 4.0) via Commons Wikimedia 
2.’Sha-Boyer-Fig1-CCBy3.0′ (CC BY 3.0) via Commons Wikimedia