Transcriptionally active genes are characterized by:
a. an increase of H1
b. the presence of specialized chromosome remodeling complexes
c. the presence of bound nucleosomes at the promoter regions
d. all of the above is correct
The correct answer and explanation is:
The correct answer is b. the presence of specialized chromosome remodeling complexes.
Explanation:
Transcriptionally active genes undergo a number of changes at the chromatin level to facilitate gene expression. These changes are essential because the DNA must be accessible to the transcription machinery in order for transcription to occur. Here’s why option b is correct:
- Specialized Chromosome Remodeling Complexes (b): These complexes are crucial for the activation of transcription. They work by modifying the chromatin structure, either by sliding, ejecting, or restructuring nucleosomes. This makes the DNA more accessible to the transcription machinery, such as RNA polymerase and transcription factors. These complexes play a key role in the regulation of gene expression.
- Increase of H1 (a): Histone H1 is a linker histone that is involved in the formation of higher-order chromatin structures. In general, an increase in H1 is associated with a more condensed chromatin state, which is typically linked to transcriptionally inactive regions. Therefore, an increase in H1 is not characteristic of transcriptionally active genes.
- Bound Nucleosomes at the Promoter Regions (c): While nucleosomes are present at the promoter regions of many genes, the presence of bound nucleosomes alone does not necessarily indicate transcriptional activity. In fact, transcriptionally active regions tend to have a more open chromatin structure, with nucleosomes displaced or repositioned, particularly at the promoter. This allows transcription factors and RNA polymerase to bind more easily to the DNA.
Overall, transcriptionally active genes are characterized by chromatin remodeling, histone modifications (such as acetylation), and the presence of transcription factors and RNA polymerase, all of which are facilitated by specialized remodeling complexes.