Advanced Placement (AP) Biology Practice Exam

What is one possible effect of DNA methylation on gene expression?

1. Enhances all gene expressions

2. May serve as a long-term control of gene expression

3. Initiates DNA replication

4. Has no effect on gene expression

The correct answer is: May serve as a long-term control of gene expression

DNA methylation typically involves the addition of a methyl group to the DNA molecule, often occurring at cytosine bases in the context of CpG dinucleotides. This modification can have a significant impact on gene expression, primarily by repressing transcription. When methylation occurs in the promoter region of a gene, it can inhibit the binding of transcription factors and other necessary proteins that promote gene expression, leading to transcriptional silencing. This mechanism is essential for long-term regulation, as it can influence how genes are expressed during development and in response to environmental signals. For instance, genes that are crucial for early embryonic development may be methylated in somatic cells to prevent their expression, effectively “locking” them in a non-active state until they are needed. This long-term control of gene expression via DNA methylation also plays a role in processes such as X-chromosome inactivation in female mammals and genomic imprinting, where only one allele of a gene is expressed based on the parent of origin. Therefore, DNA methylation serves as a vital and reversible mechanism for the long-term control of gene expression, directing cellular function and differentiation without altering the underlying DNA sequence.