Final answer:
DNA methylation is the process where a methyl group is added to DNA, generally leading to gene silencing. It is an epigenetic modification that can be inherited and is influenced by environmental factors, affecting gene expression.
Step-by-step explanation:
DNA methylation is epigenetic modification of the DNA molecule. This involves the addition of a methyl group (CH₃) to the cytosine base of DNA, usually at CpG islands, which are regions with a high density of CG pairs. The addition of a methyl group to DNA, generally at the 5' position of the cytosine ring, can lead to gene silencing. Methylation patterns can be heritable, meaning that they can be passed down from one generation to the next. Changes in the methylation status of DNA are influenced by environmental factors and can affect gene expression. This methylation typically results in a tighter, more condensed DNA structure, making the DNA less accessible for gene transcription activities, thereby decreasing gene expression.
DNA methylation is a key player in regulating gene activity without altering the DNA sequence itself. This process is significant for normal development, as well as playing a role in various diseases when methylation patterns are disrupted. Therefore, the answer to the student's question is 1) The addition of a chemical modification (CH₃) to DNA.