Which of the following are examples of heterochromatin in a mammalian chromosome? Select all that apply:
repetitive DNA
nucleosome
centromeric DNA
Y chromosome
X chromosome
Barr body
telomeric DNA
The correct answer and explanation is :
Correct Answer:
- Repetitive DNA
- Centromeric DNA
- Y Chromosome
- X Chromosome
- Barr Body
- Telomeric DNA
Explanation:
Heterochromatin is a tightly packed form of chromatin that is transcriptionally inactive, meaning it is not actively involved in gene expression. It plays an essential role in maintaining genome stability, regulating gene expression, and protecting chromosomal integrity. In mammalian chromosomes, heterochromatin can be constitutive (always condensed) or facultative (can switch between condensed and relaxed states).
- Repetitive DNA – Heterochromatin is often composed of repetitive DNA sequences that do not encode proteins but contribute to structural functions. These sequences are highly condensed and play a role in chromosomal integrity.
- Centromeric DNA – The centromere is a region of a chromosome that consists of heterochromatin. It is responsible for the attachment of spindle fibers during cell division and is rich in repetitive DNA sequences.
- Y Chromosome – The Y chromosome is largely composed of heterochromatin, especially in non-recombining regions, making it transcriptionally inactive in many parts.
- X Chromosome – In female mammals, one of the two X chromosomes is inactivated to compensate for dosage differences between males and females. This inactivated X chromosome is mostly heterochromatic.
- Barr Body – The Barr body is the condensed, inactivated X chromosome found in female somatic cells. It is an example of facultative heterochromatin, as it can be reactivated in certain conditions.
- Telomeric DNA – Telomeres, which cap the ends of chromosomes, consist of repetitive sequences and are tightly packed into heterochromatin. They protect the ends of chromosomes from degradation and fusion.
Incorrect Option:
- Nucleosome – A nucleosome is a fundamental unit of chromatin consisting of DNA wrapped around histone proteins. It is present in both euchromatin (loosely packed, active) and heterochromatin, making it not exclusive to heterochromatin.