Heterochromatin is the part of the chromosome in which the DNA does not have coding genes. Euchromatin is the part of the chromosome in which the coding DNA sequences are present.

The heterochromatin region of the chromosome is highly condensed. These regions appear dark on staining due to a compact arrangement of chromatin, the DNA content is more. These regions have silenced or suppressed genes by mechanisms such as DNA methylation which prevents their involvement in the transcription and thus coding for protein synthesis.

An example of heterochromatin is the X chromosome inactivation in female mammalian where the X chromosomes is packed into transcriptionally inactive heterochromatin. This is because female mammals have two X chromosomes so to prevent duplicates one of them is inactivated. Due to this, it is one of the last chromosomes to undergo replication in mitosis.

The euchromatin region of the chromosome is loosely packed and has lesser content of DNA. These are lightly stained. These sequences have active genes. These are involved in the process of transcription. Around 90% of our genome is euchromatin.

Practice Questions

MCAT Official Prep (AAMC)

Practice Exam 4 B/B Section Question 31

Sample Test B/B Section Question 30

Key Points

• Heterochromatin is highly packed parts of the chromosome that have inactive genes and thus are not involved in transcription. These are the dark staining regions due to more quantity of DNA.

•  X chromosome inactivation is an example of heterochromatin where the female mammal has 2 X chromosomes and one of them is inactivated to prevent having duplicates. It is one of the last chromosomes to replicate as it is transcriptionally silent due to tight packing as heterochromatin.

• Euchromatin is loosely packed parts of the chromosome that have active genes and thus are involved in transcription. These are the light staining regions due to the lesser quantity of DNA.

Key Terms

Genome: the cell’s complete genetic information packaged as a double-stranded DNA molecule

Transcription: the synthesis of RNA under the direction of DNA

Gene: unit of heredity made up of DNA

Heterochromatin: part of the chromosomes, which is a firmly packed form and is genetically inactive

Euchromatin: uncoiled (loosely) packed form of chromatin and is genetically active

DNA methylation: the biological process by which methyl groups are added to the DNA molecule making it inactive

Chromatin: a complex of DNA and protein found in eukaryotic cells

Transcription: the copying of a gene’s DNA sequence to make an RNA molecule