A DNA region called a CpG island, found in gene promoters, has a high frequency of cytosine and guanine pairs. What happens when the cytosine in these pairs is heavily methylated together with deacetylated histones?
AThe DNA region becomes tightly coiled and transcriptionally inactive
BThe DNA region becomes loosely coiled and transcriptionally hyperactive
CThe DNA sequence itself is permanently rewritten at that site
DRNA polymerase binds more efficiently to that promoter region
Answer & Solution
Correct answer: A. The DNA region becomes tightly coiled and transcriptionally inactive
1. CpG islands are stretches with a high frequency of cytosine and guanine dinucleotide pairs found in gene promoter regions.
2. Within a CpG island, the cytosine member of the pair can be methylated by adding a methyl group.
3. Highly methylated, or hypermethylated, DNA regions combined with deacetylated histones become tightly coiled.
4. This tightly coiled state is transcriptionally inactive, meaning RNA polymerase cannot access and bind that promoter.
5. The option describing loose coiling and hyperactivity is the opposite of what hypermethylation and deacetylation produce.
6. These modifications change how DNA interacts with proteins, not the underlying nucleotide sequence, so the option about rewriting the sequence is incorrect.
7. Improved RNA polymerase binding is inconsistent with a tightly coiled, transcriptionally inactive region.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 16 "Gene Expression", section Epigenetic Control_
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