Cell cycle (mitosis) M G2 (gap2) (growth) G1 (gap1) (growth) S (DNA synthesis) variations in cell cycles: early in development, cells divide quickly Frog eggs: · These eggs are very large (0.5mm) M · The cells divide very quickly because they are trying to go from huge to normal sized · So, all cell signalling works better when cells get down to 10- S 20 microns . So the cells go from mitosis to S phase leaving out G1 and G2 (don't need to grow because they are already huge but they do need to replicate their DNA each time) . This whole process takes about 60 minutes M / Variations in cell cycles: Neurons exit G2 G1 · Neurons are on the other (exit) Go extreme S · Neurons go into G0 and stop dividing . Neurons have very intricate cells that make the axons and dendrites which make it hard for the neuronal cells to replicate. Variations in cell cycles: stem cells are quiescent . Most of the time, stem cells are quiescent (quiet)/ non dividing/ they have gone into G0 · Want to protect stem cells from environmental damage (which is the biggest threat to cells during cell division) . So we want the stem cells to rarely divide but let the progenitors (daughter cells) divide a lot. · Stem cells are kept throughout our lifetime, so we want to keep them safe · Stem cells can come out of GO. o They can self-renew or Cell cycle entry Self-renewal Differentiation GO G1/S G2/M Cell cycle exit IT- HSC ST- HSC Quiescent HSC Active HSC Multi-lineage blood cells LT-HSC differentiate Variations in cell cycles: senescent cells stop dividing and permanently exit the cell cycle . Normal cells can be exposed to lots of different stimuli and will become senescent (permanently stop dividing) Measuring proliferation directly . To determine if cells in a tissue are proliferating: o Measure the incorporation of "H- Thymidine or bromodeoxyuracil (BrdU) into the DNA o (BrdU is much quicker/ more widely used)
This can be detected by an antibody o The cells that pick up the BrdU and place it into the DNA will be in S phase. Therefore, cells in this phase will be labelled (fluoresce) (There are so many cells dividing in our cells dividing in our guts because they are constantly being damaged by the acidity and pathogens, etc. so the safest thing to do is kill those cells. They will undergo apoptosis at a very regular rate) DNA content through the cell cycle DNA Content 4n 2n G1 S G2 & M 2n- representing each chromosome from our mother and father 4n- representing the replicated cells Applying fluorescence activated cell sorter (FACS) analysis to the cell cycle 1) Cells are stained with a dye that becomes fluorescent when it binds to DNA 2) The amount of fluorescence is directly proportional to the amount of DNA ff) The FACS measures the level of cells in G1 phase fluorescence of individual