Final answer:
Ribosomal and Histone genes are called repetitive sequences due to the presence of many copies in the human genome. These multiple copies are essential for their roles in protein synthesis and DNA packaging, respectively, and are also known as constitutive or housekeeping genes.
Step-by-step explanation:
Ribosomal and Histone genes have many copies in the human genome and are therefore called repetitive sequences. The presence of multiple copies of these genes is essential because they serve fundamental roles in cellular processes. For instance, ribosomal RNAs (rRNAs) are crucial for protein synthesis in the ribosome, where they contribute to the ribosome's structure and functional activity. The 45S pre-rRNA, for instance, is processed by cleavage, and there can be as many as 200-400 copies of the 45S gene in eukaryotic cells. These genes are distributed among five acrocentric chromosomes in humans.
Histone genes, on the other hand, encode proteins that are important for the packaging and organization of DNA into chromatin. This packaging allows the long strands of DNA to fit into the cell nucleus and plays a role in gene regulation. Similar to rRNAs, histones are consistently required by cells, and thus, having multiple copies ensures that the demand for these protein products can be met at all times. These genes are typically referred to as constitutive genes or housekeeping genes because they are always needed and expressed by the cell.