Title Cell Cycle Handout.pdf ✅

Unit 1: Introduction to Genetics and Cell Biology 3 in the nucleus during most times. The chromosomes presented in media are tightly packed chromatin fibers which are wound over and over again to condense to its maximum. Chromatin can not be that densely packed all the time because genes have to be expressed at any moment. Packing them into the visible chromosomes we see will make it very difficult for cells to express the genes that they need, when they need to. Figure 3.2. Configuration of chromatin during the cell cycle. (A) Chromosomes in interphase are loosely packed and take up chromosome territories. (Source: Springer Science) (B) Once a cell enters the M phase, they begin to condense and form sister chromatids. The image shows an electron micrograph of a condensed chromosome and a schematic of its parts. Sister chromatids are 2 identical chromosomes that have been attached at their centromere. (Source: Campbell et al., 2009) Outside of the M phase in mitosis, chromosomes are actually loosely packed and take up regions in the nucleus. These chromosome territories are distinct and despite containing thousands upon thousands of base pairs, these chromosomes do not get tangled up. Interphase chromosomes are shown in Figure 3.2.A. It is still possible to specifically label chromosomes based on distinct regions they are found in. Each chromosome has unique sequences and structural features that define them. It is actually possible to stain specific chromosomes and be able to locate them apart from the other chromosomes. Several regions of each chromosome are labelled and the combination of these labels designate which chromosome it is. The computer then colors in the chromosome so they are easier to tell apart. This is what was done for Figure 3.2.A. We can clearly see regions of the cell which contain the loosely packed chromosomes. We have mentioned