Final answer:
Variants have been identified for histones H1, H2A, H2B, and H3. Histone modifications can lead to epigenetic changes, affecting gene expression without altering the DNA sequence.
Step-by-step explanation:
Identifying Histone Variants
Variants have been identified in different types of histone proteins, which are crucial for DNA packaging. Among histones, variants are known to occur in H2A and H2B, as well as in H3 and H1. Each of these histones plays a specific role in the structure and function of chromatin within the cell nucleus. For instance, Histone H2B.1 and Histone H2B.4 are variants of the H2B family, which together with H2A, H3, and H4, form the nucleosome core around which DNA is wound.
Epigenetic changes can be facilitated by modifications to these histones, such as methylation by enzymes like Histone-lysine N-methyltransferase EZ1 and Histone-lysine N-methyltransferase EZ2. These changes can affect chromatin structure and, consequently, gene expression without altering the DNA sequence itself. For example, histone acetylation typically results in the loose packing of nucleosomes, which can allow DNA to be more accessible for transcription.
Hence, the correct answer to the student's question is a) H1 and H3, as variants have been identified in both these histones.