If we follow a typical chromosome in a typical human cell, it alternates between unreplicated and replicated states, and between relatively uncondensed and condensed. The replication is easy to explain. If a cell has made the commitment to divide, it first needs to replicate its DNA. This occurs during S phase. Before S phase, chromosomes consist of a single piece of double-stranded DNA and after they consist of two identical double-stranded DNA molecules.
The condensation is a more complex story because eukaryotic DNA is always wrapped around some proteins. Figure 3.5.1 shows the different levels commonly found in cells. During interphase, a chromosome exists mostly as a 30 nm fibre. This allows it to fit inside the nucleus and still have the DNA be accessible for enzymes performing RNA synthesis, DNA replication, and DNA repair. At the start of mitosis, these processes halt and the chromosome becomes even more condensed. This is necessary so that the chromosomes are compact enough to move to the opposite ends within the cell. When mitosis is complete the chromosome returns to its 30 nm fibre structure. Recall that each of our cells has a maternal and a paternal chromosome 1. Figure 3.5.2 shows what these chromosomes look like during the cell cycle.
- Figure 3.5.1 Chromatin Structures by Richard Wheeler at en.wikipedia, CC BY-SA 3.0, via Wikipedia
- Figure 3.5.2 Original by M. Harrington (2017), CC BY-NC 3.0, MRU Open Genetics Lectures
Harringon, M. (2017). Figure 8. Appearance of maternal and paternal chromosome 1 …[diagram]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 15, p. 6). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf