Final answer:
In a nucleosome, histones H2A and H2B form a dimer, while two copies each of H3 and H4 histones form a tetramer. Chromatin, comprising DNA and protein, becomes tightly packed into visible chromosomes during cell division. DNA replication involves continuous synthesizing of the leading strand and discontinuous synthesis of the lagging strand.
Step-by-step explanation:
The part of the nucleosome that is a dimer is each of the pairs of histones H2A and H2B. Dimers are formed by two identical molecules connected by weak bonds. A tetramer in the nucleosome consists of the four histone molecules H3 and H4 (two copies of H3 and two copies of H4), which join together to form a larger complex.
DNA wraps around these histone proteins to form the nucleosome, which compacts the DNA to fit within the cell nucleus. This complex of DNA and protein makes up chromatin. Chromosomes, which comprise chromatin, are most visible during the metaphase stage of cell division when they are highly compacted. Chromatin exists in two forms within the cell: heterochromatin, which is tightly packed and generally contains inactive genes, and euchromatin, which is less compact and contains active genes available for transcription.
During DNA replication, both strands of the DNA molecule are not copied continuously. One strand, known as the leading strand, is synthesized continuously, while the other strand, known as the lagging strand, is synthesized in small pieces known as Okazaki fragments.