- Chromatin is found in two varieties: euchromatin and heterochromatin.
- In 1928, Heitz defined heterochromatin as, -those regions of the chromosome that remain condensed during interphase and early prophase are called chromocentre.
- The rest of the chromosome which is in a non condensed state was called euchromatin.
- Originally, the two forms were distinguished cytologically by- how intensely they stained – the euchromatin is less intense, while heterochromatin stains intensely, indicating tighter packing.
- Heterochromatin is usually localized to the periphery of the nucleus.
Heterochromatin
- Heterochromatin mainly consists of genetically inactive satellite sequences, and many genes are repressed to various extents, although some cannot be expressed in euchromatin at all. Both centromeres and telomeres are heterochromatic, as is the Barr body of the second inactivated X chromosome in a female Heterochromatic regions are considered into three structures namely chromomeres, chromocentres and knobs.
- Chromomeres are regular feature of all prophase chromosomes, large enough to reveal them, but their number, size; distribution and arrangement are specific for a particular species at a particular stage of development.
- Chromocentres are heterochromatic regions of varying size which occur near the centromeres in proximal regions of chromosome arms.
- At mid-prophase, many chromocentres can be resolved into strings of chromomeres, which are larger than chromomere found in distal regions. In some dipteran salivary glands, the chromocentres of different chromosomes fuse to form a large chromocentre.
- The relative distributions of chromocentres are sometimes considered to be of considerable evolutionary value.
- Knobs are spherical heterochromatin bodies which may have a diameter equal to the chromosome width but may reach a size having a diameter which is several times the width of the chromosome.
- Very distinct chromosome knobs can be observed in maize at pachytene stage.
- Knobs are valuable chromosome markers for distinguishing chromosomes of related species and races.
Function
- Heterochromatin is believed to serve several functions, from gene regulation to the protection of the integrity of chromosomes; all of these roles can be attributed to the dense packing of DNA, which makes it less accessible to protein factors that bind DNA or its associated factors.
- For example, naked double-stranded DNA ends would usually be interpreted by the cell as damaged DNA, triggering cell cycle arrest and DNA repair.
- Some regions of chromatin are very densely packed with fibres displaying a condition comparable to that of the chromosome at mitosis.
- Heterochromatin is generally clonally inherited; when a cell divides the two daughter cells will typically contain heterochromatin within the same regions of DNA, resulting in epigenetic inheritance.
- Variations cause heterochromatin to encroach on adjacent genes or recede from genes at the extremes of domains.
- Transcribable material may be repressed by being positioned (in cis) at these boundary domains.
- This gives rise to different levels of expression from cell to cell, which may be demonstrated by position-effect variegation. Insulator sequen-ces may act as a barrier in rare cases where constitutive heterochromatin and highly active genes are juxtaposed (e.g. the 5’HS4 insulator upstream of the chicken β-globin locus, and loci in two Saccharomyces spp).
Types of Heterochromatin
Constitutive heterochromatin
- All cells of a given species will package the same regions of DNA in constitutive heterochromatin, and thus in all cells any genes contained within the constitutive heterochromatin will be poorly expressed.
- For example, all human chromosomes 1, 9, 16, and the Y chromosome contain large regions of constitutive heterochromatin. In most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres.
Facultative heterochromatin
- The regions of DNA packaged in facultative heterochromatin will not be consistent between the cell types within a species, and thus a sequence in one cell that is packaged in facultative heterochromatin (and the genes within poorly expressed) may be packaged in euchromatin in another cell (and the genes within no longer silenced). However, the formation of facultative heterochromatin is regulated, and is often associated with morphogenesis or differentiation.
- An example of facultative heterochromatin is X-chromosome inactivation in female mammals: one X chromosome is packaged as facultative heterochromatin and silenced, while the other X chromosome is packaged as euchromatin and expressed.
- Among the molecular components that appear to regulate the spreading of heterochromatin include the Polycomb-group proteins and non-coding genes such as Xist. The mechanism for such spreading is still a matter of contrive.
Euchromatin
- Euchromatin is a lightly packed form of chromatin that is rich in gene concentration, and is often (but not always) under active transcription.
- Unlike heterochromatin, it is found in both eukaryotes and prokaryotes.
- Euchromatin comprises the most active portion of the genome within the cell nucleus.
Structure
- The structure of euchromatin is reminiscient of an unfolded set of beads along a string, where those beads represent nucleosomes.
- Nucleosomes consist of eight proteins known as histones, with approximately 147 base pairs of DNA wound around them; in euchromatin this wrapping is loose so that the raw DNA may be accessed.
- Each core histone possesses a `tail’ structure which can vary in several ways; it is thought that these variations act as “master control switches” which determine the overall arrangement of the chromatin.
- In particular, it is believed that the presence of methylated lysine 4 on the histone tails acts as a general marker for euchromatin.
Appearance
- Euchromatin generally appears as light-colored bands when stained in GTG banding and observed under an optical microscope; in contrast to heterochromatin, which stains darkly.
- This lighter staining is due to the less compact structure of euchromatin.
- The basic structure of euchromatin is an elongated, open, 10nm microfibril, as noted by electron microscopy.
- It should be noted that in prokaryotes, euchromatin is the only form of chromatin present; this indicates that the heterochromatin structure evolved later along with the nucleus, possibly as a mechanism to handle increasing genome size.
Function
- Euchromatin participates in the active transcription of DNA to mRNA products.
- The unfolded structure allows gene regulatory proteins and RNA polymerase complexes to bind to the DNA sequence, which can then initiate the transcription process.
- Not all euchromatin is necessarily transcribed, but in general that which is not is transformed into heterochromatin to protect the genes while they are not in use.
- There is therefore a direct link to how actively productive a cell is and the amount of euchromatin that can be found in its nucleus.
- It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling gene expression and replication, since such processes behave differently on densely compacted chromatin- this is known as the `accessibility hypothesis’.
- One example of constitutive euchromatin that is ‘always turned on’ is housekeeping genes, which codes for the proteins needed for basic functions of cell survival
🔹 1. Who first defined heterochromatin in 1928?
A. Watson
B. Heitz ✅
C. Crick
D. Morgan
🧠 Explanation: Heitz first described heterochromatin based on its staining properties.
🔹 2. Euchromatin is characterized by:
A. Dense staining
B. Peripheral localization
C. Loose DNA packing ✅
D. Inactive genes
🧠 Explanation: Euchromatin is loosely packed and lightly stained, allowing transcription.
🔹 3. Which chromatin is transcriptionally active? (NEET PYQ)
A. Constitutive heterochromatin
B. Facultative heterochromatin
C. Euchromatin ✅
D. All of the above
🧠 Explanation: Euchromatin allows access to RNA polymerase and regulatory factors.
🔹 4. What is a key feature of heterochromatin?
A. Open structure
B. High gene activity
C. Tightly packed DNA ✅
D. Abundant in mitochondria
🧠 Explanation: Heterochromatin is highly condensed and mostly gene-silent.
🔹 5. Chromocentres are typically located:
A. Near telomeres
B. Near centromeres ✅
C. In nucleolus
D. In cytoplasm
🧠 Explanation: Chromocentres are heterochromatin-rich regions near centromeres.
🔹 6. Knobs are prominently found in:
A. Drosophila
B. Humans
C. Maize ✅
D. Yeast
🧠 Explanation: Chromosome knobs in maize serve as cytogenetic markers.
🔹 7. Facultative heterochromatin is best represented by:
A. Satellite DNA
B. Housekeeping genes
C. X-inactivation in females ✅
D. Telomeric DNA
🧠 Explanation: One X chromosome in females becomes transcriptionally inactive.
🔹 8. Which type of chromatin shows position-effect variegation?
A. Euchromatin
B. Constitutive heterochromatin
C. Facultative heterochromatin ✅
D. Nucleolar chromatin
🧠 Explanation: Variable gene silencing due to chromatin position.
🔹 9. Assertion (A): Constitutive heterochromatin is always inactive.
Reason (R): It contains genes for cell survival.
A. A and R are true; R explains A
B. A and R are true; R does not explain A
C. A is true, R is false ✅
D. A is false, R is true
🧠 Explanation: Constitutive heterochromatin is inactive; cell survival genes are in euchromatin.
🔹 10. DNA in nucleosomes of euchromatin is wrapped around:
A. RNA molecules
B. Eight histones ✅
C. Five histones
D. Chromocentres
🧠 Explanation: Each nucleosome consists of a histone octamer.
🔹 11. GTG banding stains which chromatin darker?
A. Euchromatin
B. Heterochromatin ✅
C. Both equally
D. Neither
🧠 Explanation: Heterochromatin stains dark due to compactness.
🔹 12. What structure is absent in euchromatin but present in heterochromatin?
A. Histones
B. DNA
C. Chromomeres ✅
D. Nucleosomes
🧠 Explanation: Chromomeres are more characteristic of heterochromatin.
🔹 13. In which form is chromatin most accessible to transcription machinery?
A. Heterochromatin
B. Euchromatin ✅
C. Knobs
D. Chromocentres
🧠 Explanation: Euchromatin is loosely packed, facilitating transcription.
🔹 14. What differentiates euchromatin from heterochromatin at molecular level?
A. DNA polymerase activity
B. Histone tail modifications ✅
C. ATP production
D. RNA types
🧠 Explanation: Euchromatin often has methylated lysine 4 on histone tails.
🔹 15. What causes epigenetic inheritance in heterochromatin?
A. DNA sequence
B. DNA methylation
C. Chromatin state inheritance ✅
D. RNA silencing
🧠 Explanation: Heterochromatin is clonally inherited during cell division.
🔹 16. Which chromosome feature is most associated with chromomeres?
A. Metaphase chromosomes
B. Pachytene chromosomes
C. Prophase chromosomes ✅
D. Anaphase chromosomes
🧠 Explanation: Chromomeres are visible as granules during prophase.
🔹 17. Chromatin with consistently silent gene expression in all cells is:
A. Facultative heterochromatin
B. Euchromatin
C. Constitutive heterochromatin ✅
D. Satellite chromatin
🧠 Explanation: Constitutive heterochromatin is stable and never transcriptionally active.
🔹 18. Knobs can be useful in:
A. Transcription control
B. Cytoplasmic inheritance
C. Chromosome identification ✅
D. Ribosome formation
🧠 Explanation: Knobs act as chromosomal markers in maize.
🔹 19. Barr body is formed by:
A. Euchromatin
B. Y chromosome
C. Constitutive heterochromatin
D. Facultative heterochromatin ✅
🧠 Explanation: Inactivated X chromosome forms a Barr body in females.
🔹 20. Which histone modification is a marker of euchromatin?
A. Acetylation of H1
B. Methylation of lysine 4 on H3 ✅
C. Ubiquitination of H2A
D. Phosphorylation of H3
🧠 Explanation: H3K4me is a marker for euchromatin and gene activation.
🔹 21. Euchromatin appears under the microscope as:
A. Granular dark spots
B. Light-stained areas ✅
C. Condensed dots
D. Black bodies
🧠 Explanation: Less dense euchromatin stains lightly in GTG banding.
🔹 22. Which chromatin is found in prokaryotes?
A. Euchromatin ✅
B. Heterochromatin
C. Both
D. Nucleolar chromatin
🧠 Explanation: Prokaryotes contain only euchromatin; heterochromatin evolved later.
🔹 23. What triggers position-effect variegation?
A. Centromeric fusion
B. DNA duplication
C. Gene relocation near heterochromatin ✅
D. Chromosome deletion
🧠 Explanation: Gene silencing occurs when euchromatic genes are relocated close to heterochromatin.
🔹 24. Which chromatin type undergoes morphogenesis-related silencing?
A. Euchromatin
B. Constitutive heterochromatin
C. Facultative heterochromatin ✅
D. Mitochondrial chromatin
🧠 Explanation: Facultative heterochromatin is dynamic and developmentally regulated.
🔹 25. Polycomb-group proteins are associated with:
A. Histone production
B. Spreading of facultative heterochromatin ✅
C. Telomere elongation
D. Ribosome assembly
🧠 Explanation: Polycomb-group proteins silence genes by chromatin remodeling.
🔹 26. Which chromatin structure has higher nucleosome accessibility?
A. Knobs
B. Chromocentres
C. Euchromatin ✅
D. Centromeres
🧠 Explanation: Loose packaging allows easier access in euchromatin.
🔹 27. Constitutive heterochromatin is enriched in:
A. Highly repetitive DNA ✅
B. Introns
C. Promoter sequences
D. tRNA genes
🧠 Explanation: It consists of repetitive satellite DNA, mainly near centromeres.
🔹 28. Which component is NOT found in heterochromatin?
A. Nucleosomes
B. Active genes ✅
C. DNA
D. Histones
🧠 Explanation: Heterochromatin is generally gene-silent.
🔹 29. Histone tails act as:
A. DNA methylators
B. Nucleolar regulators
C. Master control switches ✅
D. Spindle fiber anchors
🧠 Explanation: Post-translational modifications of histone tails regulate chromatin structure.
🔹 30. The term “chromocentre” refers to:
A. Central nuclear chromatin
B. Centromere DNA only
C. Fused heterochromatic regions near centromeres ✅
D. Knob fusion
🧠 Explanation: Chromocentres are pericentromeric heterochromatin aggregations.
🔹 31. Which chromatin type is generally clonally inherited?
A. Euchromatin
B. Facultative heterochromatin
C. Heterochromatin ✅
D. Ribosomal chromatin
🧠 Explanation: Heterochromatin state is typically preserved across cell divisions.
🔹 32. Which insulator prevents spread of heterochromatin in chicken β-globin locus?
A. Xist
B. 5’HS4 ✅
C. XIST
D. LacZ
🧠 Explanation: 5’HS4 acts as a barrier to heterochromatin spread.
🔹 33. Euchromatin is actively involved in:
A. RNA degradation
B. DNA repair
C. Transcription ✅
D. Protein translation
🧠 Explanation: Its open structure supports active transcription.
🔹 34. Housekeeping genes are located in:
A. Facultative heterochromatin
B. Constitutive heterochromatin
C. Euchromatin ✅
D. Telomeres
🧠 Explanation: These essential genes are always active and found in euchromatin.
🔹 35. Which chromatin type shows reversible silencing?
A. Euchromatin
B. Constitutive heterochromatin
C. Facultative heterochromatin ✅
D. Ribosomal chromatin
🧠 Explanation: Gene silencing in facultative heterochromatin is reversible and tissue-specific.
🔹 36. Which sequence is essential for facultative heterochromatin formation?
A. Xist ✅
B. Lac operon
C. Satellite DNA
D. LINEs
🧠 Explanation: Xist is a non-coding RNA responsible for X-inactivation.
🔹 37. Euchromatin’s fiber diameter is generally:
A. 5 nm
B. 10 nm ✅
C. 20 nm
D. 30 nm
🧠 Explanation: Euchromatin forms 10 nm fibers (“beads on a string”).
🔹 38. What distinguishes facultative from constitutive heterochromatin?
A. DNA sequence
B. Gene expression status ✅
C. GC content
D. Nucleosome count
🧠 Explanation: Facultative heterochromatin varies between cell types in gene activity.
🔹 39. The chromatin that undergoes condensation during mitosis is:
A. Euchromatin
B. Heterochromatin ✅
C. Ribosomal DNA
D. Satellite chromatin
🧠 Explanation: Heterochromatin remains condensed even outside mitosis.
🔹 40. What term describes variable gene expression based on chromatin position?
A. Genetic drift
B. Chromosomal crossover
C. Position-effect variegation ✅
D. X-inactivation
🧠 Explanation: Gene expression varies when moved near heterochromatin.
🔹 41. What color does euchromatin appear in GTG banding?
A. Dark
B. Light ✅
C. Black
D. No stain
🧠 Explanation: Less condensed euchromatin stains lightly.
🔹 42. Which heterochromatin type may change with cell type?
A. Constitutive
B. Facultative ✅
C. Structural
D. Satellite
🧠 Explanation: Facultative heterochromatin is dynamic and cell-specific.
🔹 43. Which of the following is an evolutionary implication of heterochromatin?
A. Chromatin remodeling
B. Increased recombination
C. Species-specific chromocentre patterns ✅
D. Gene duplication
🧠 Explanation: Distribution of chromocentres is species-specific.
🔹 44. Which of the following is NOT true about euchromatin?
A. Loose packaging
B. Active transcription
C. Constitutes Barr body ✅
D. Contains housekeeping genes
🧠 Explanation: Barr body is heterochromatic.
🔹 45. Heterochromatin formation helps prevent:
A. mRNA degradation
B. Misidentification of telomeres as DNA breaks ✅
C. Protein folding
D. Histone production
🧠 Explanation: Compacted chromatin prevents DNA damage response activation.
🔹 46. The presence of nucleosomes confirms:
A. DNA polymerase activity
B. Prokaryotic DNA
C. Chromatin packaging ✅
D. Protein denaturation
🧠 Explanation: Nucleosomes are the basic chromatin packaging unit.
🔹 47. Which is not a heterochromatin-associated structure?
A. Chromomeres
B. Chromocentres
C. Knobs
D. Nucleolus ✅
🧠 Explanation: Nucleolus is for ribosome biogenesis, not heterochromatin.
🔹 48. In maize, knobs are best seen at:
A. Interphase
B. Prophase
C. Metaphase
D. Pachytene ✅
🧠 Explanation: Knobs become prominent in pachytene of meiosis.
🔹 49. Which domain prevents spread of heterochromatin into active genes?
A. Enhancer
B. Silencer
C. Insulator ✅
D. Promoter
🧠 Explanation: Insulators block the propagation of chromatin states.
🔹 50. The accessibility hypothesis explains:
A. DNA replication
B. Gene editing
C. Regulation via chromatin structure ✅
D. Chromosome movement
🧠 Explanation: Open chromatin = active genes; closed chromatin = repression.