Final answer:
Histone proteins are primarily responsible for the tight packing of DNA, forming nucleosomes which organize into higher order structures. Cohesin proteins are key in holding replicated DNA together at the centromere during cell division.
Step-by-step explanation:
The tight packing of DNA in eukaryotic cells is largely due to structures called nucleosomes, which consist of DNA wrapped around a core of eight histone proteins. This forms the 'beads on a string' structure, with DNA-histone complexes linked by linker DNA. Through additional levels of compaction, such as the formation of a 30-nm chromatin fiber and association with scaffolding proteins, DNA is further condensed. During mitosis, cohesin proteins play a crucial role by holding sister chromatids together, especially in the centromere region, which becomes highly condensed and appears as a constricted area under a microscope.
In prophase I of meiosis, homologous chromosomes are held together by cohesin proteins, and their alignment is maintained by the synaptonemal complex. After DNA replication during the S phase of interphase, sister chromatids are bound together by cohesin proteins at the centromere and become visible under a light microscope when fully compact.