Provide a brief description of two different types of histone modification and how they impact transcription.

Question: Explain the roles of histone modifications in transcription, and provide two examples of such modifications with their respective influences on transcription. Answer: Histone modifications, such as acetylation and methylation, play a crucial role in regulating gene expression by altering the chromatin structure and accessibility, ultimately affecting transcription. Two examples of histone modifications include: 1. Histone Acetylation: This involves the addition of an acetyl group to specific lysine residues in histone proteins. Acetylation is generally associated with transcriptional activation, as it neutralizes the positive charge on the lysines and leads to a more relaxed chromatin structure. This allows the DNA to be more accessible to transcription factors and RNA polymerase, promoting transcription. 2. Histone Methylation: This involves the addition of a methyl group to specific lysine or arginine residues in histone proteins. Methylation can either promote or repress transcription depending on the specific residue being methylated and the presence of other modifications. For example, methylation of lysine residues on histone H3, such as H3K4 and H3K36, is associated with transcriptional activation, while methylation of other residues, such as H3K9 and H3K27, leads to transcriptional repression.