Heterochromatin vs Euchromatin: Structure and Function

Euchromatin and Heterochromatin

Heterochromatin is abnormally condensed chromatin. It appears similar to fungal chromatin but is transcriptionally inactive. During mitosis, chromatin is divided into 46 chromosomes. For cell division, DNA must be doubled during the S phase of interphase. Heterochromatin, being highly condensed, replicates last. There are two types of heterochromatin: constitutive and facultative.

Constitutive Heterochromatin

Constitutive heterochromatin is discernible from mitotic chromatin. It is located around the centromere of chromosomes and forms alternating bands in mammalian chromosome arms.

Facultative Heterochromatin

Facultative heterochromatin is condensed only at certain physiological times in the cell, and its amount depends on the degree of cellular development. Embryonic cells have very little because most genes are still expressed. However, mature or differentiated cells have a significant amount.

Chemical Composition and Organization in the Genome

Chromatin contains DNA and basic proteins (histones) as permanent components throughout the cell's life.

The fundamental composition of chromatin includes:

• DNA: The most important component, serving as the repository of genetic information transcribed from mother to daughter cell without data loss.
• RNA: Transient components present during genesis, acting as vectors of genetic information within the cell, from the nucleus to the cytoplasm.
• Proteins: Attached to DNA temporarily or permanently, fundamental to chromatin's origin, its evolution to the chromosome, and the operation and control of genetic activity.

Histones

Histones are basic proteins with a low molecular weight (11,000-21,000 Daltons), rich in positively charged amino acids.

Nonhistone Chromosomal Proteins (NHCP)

NHCPs are specific regulators of genetic information transcription, forming a heterogeneous group difficult to classify. We can distinguish:

1. Proteins involved in the metaphysical structure of the chromosome.
2. Enzymes involved in chromosome restoration and replication.
3. Proteins involved in the transcription of DNA into different types of RNA.
4. Enzymes that control gene expression or DNA reading.
5. High mobility group proteins responsible for selecting nucleosomes associated with active genes.