Heterochromatin and Euchromatin

Certain segments of the chromosomes, or the entire chromosomes, are more condensed than the rest of karyotype during various stages of the cell cycle. Chromatin material is two types, heterochromatin and euchromatin.

There are several differences between heterochromatin and euchromatin.

(1) Heterochromatin stains deeply while euchromatin stains less deeply.

(2) Heterochromatin is found in the condensed region of the chromosome, and is associated with tight folding and coiling of the chromosome fiber. Euchromatin consists of the diffused or less tightly coiled regions. It undergoes the typical cycle of condensation during division and decondensation during interphase.

(3) Heterochromatin is late replicating. It replicates at the end of the S phase of the mitotic cycle. Euchromatin replicates during the early stage of the S phase.

(4) Heterochromatin does not become acetylated. Euchromatins take up acetic acid on its histone during interphase.

(5) Heterochromatin is more labile than euchromatin and is affected by temperature, sex, age of parents, proximity to the centromere and presence of an additional Y chromosome.

(6) Heterochromatin is relatively inert metabolically. Heterochromatin segment contain relatively few genes in relation to their length but nevertheless a few genes are present. Euchromatin on the other hand is genetically active. Its DNA synthesizes messenger RNA during interphase.

(7) There are two main types of heterochromatin, constitutive heterochromatin and facultative heterochromatin shows heteropycnosis in all cell types. Facultative heterochromatin on the other hand is heteropycnotic only in some special cell types, or at some particular stages of the life cycle.

Constitutive heterochromatin was originally called satellite DNA (S-DNA). It is mostly inactive during protein synthesis. Constitutive DNA is highly repetitive. It consists of comparatively short identical genes (DNA) which are repeated several hundred thousand times.


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