As we're all humans, I thought I'd start with an example of human genetics and we're going to talk a little bit about ploidy (how many copies of each chromosome different organisms have) and how that relates to the cell cycle so, humans have 23 different chromosomes numbered, in order, largest to smallest, from 1 to 23 and humans are diploid di meaning two so, each of us, in most of our cells, have two sets of the 23 chromosomes or, a diploid chromosome number of 46 This is often referred to as the 2n number where n is the number of chromosomes and 2 indicates that an individual diploid has 2 sets of those chromosomes so, where do these chromosomes come from? well, we all started when this event happened: when the sperm met the egg at fertilization and inside that sperm there was one copy of each of the chromosomes chromosome 1, chromosome 2, and up to the 23rd chromosome and the same thing was true inside the oocyte or the egg the egg carried a single version of each of those chromosomes, 1, 2, 3, 4, etc., up to 23 that way, each of us has an equal amount (almost, anyway), of contribution of genetic material from our fathers and from our mothers and when that fusion occurs, it results in a fertilized embryo that has 2n it has two copies of 23 chromosomes: one set that came from our father, and one set that came from our mother But, all of our cells do not always have just two copies of chromosomes so, this is what happens during the cell cycle. You may appreciate, from previous classes, that, when cells divide, they have to replicate the DNA that is inside of them So, we can start with a cell that has just gone through mitosis, which is sometimes abbreviated "M" It will have 2n copies of chromosomes This single fertilzed embryo, the one cell embryo, has two copies of every chromosome. A copy of chromosome 1 from dad; a copy of chromosome 1 from mom and so on When the cell wants to divide, or is going to divide, anyway, to become two cells, through the process of cell division, each of those two resulting cells needs also to have 2n copies of the chromosomes So, if we have 46 chromosomes (2 copies of 23) in this cell, Then how are we going to get to twice that number of chromosomes (46 + 46 = 92 copies total) in this organism after the first cell division? After mitosis, we have part of the cell cycle that's called interphase - this is G1 (a growth phase) then, after the growth phase, where nothing is changing to the number of chromosomes inside the cell, we have synthesis, or S phase - and this is where DNA is replicated so these 46 (every one of those 46) chromosomes is copied So, now this cell, temporarily, has 92 chromosomes After that part of the cell cycle, there's another stage of interphase (growth 2, or G2) and then the cell enters mitosis and divides, and that's when - this is the critical part - that's when these 92 chromosomes (4 copies of each of chromosomes 1, 2, 3, 4, and so on) divide in half - they segregate - between the two daughter cells, so that one receives two copies of every chromosome, again, one from dad (so, one copy of chromosome - remember that a chromosome is represented, double-strand, double-helix, by a single line, in this case…) One copy of chromosome 1 from dad, one copy of chromosome 1 from mom, and there are those in each of those two daughter cells and then we could move on to the second chromosome There's a second copy of chromosome 2 from dad in each of those cells and a second copy of chromosome 2 from mom in each of those cells So, finally, if we were going to plot, over the cell cycle, the amount of DNA (n, numbers of copies of each of the chromosomes) we could start with G1, interphase this is where ploidy is determined this is when geneticists define how many chromosomes every cell is going to have That's how we know, for example, that humans are diploid organisms We start with two copies of every chromosome in the G1 phase then, the cell undergoes mitosis - not mitosis, but rather what comes next? Synthesis phase and in synthesis, there is a doubling in the amount of genetic material in the cell, so we go from 2n to 4n. So, temporarily, this is the tricky part… after mitosis, when you look in G2 phase, this organism's cell is temporarily 4n, which is also called tetraploid So, 4 copies (temporarily) of every chromosome Then, during mitosis, that number is halved through the process of cell division, back to 2n Two copies of every chromosome, as was shown here. If this was meiosis, then there would be two rounds of cell division, resulting in every resulting daughter cell having one copy of every chromosome So, in a nutshell, then, to summarize: this is how the DNA content of a cell changes across the cell cycle In G1 phase is when ploidy is determined and then the number of chromosomes doubles during synthesis phase, it stays the same during G2, and then at mitosis that level of ploidy (the number of copies of DNA) returns to 2 for a diploid organism What I'd like you to do for next class is to produce the same chart, but what would happen if you were defined by geneticists to be a tetraploid organism? Draw its ploidy chart according to its phase in the cell cycle And bring that to class with you next time