[Music] hi everyone this is andy from med school eu and we're going to continue on the topic of this cell and in today's video we're specifically going to talk about the cell cycle and we're going to begin today's lecture with the topic of the cell cycle in prokaryotic organisms because they differ from prokaryotic to eukaryotic organisms so let's begin with that and typically the process of prokaryotic cell cycle is called binary fission that's the entire process and i have it outlined right here now we're going to go over this cycle and outline the three different periods that exist now first we have the first period which is the b period this one is outlined by the cell growth so it would be growth so once the cell is made from so let's say the two previous cells we're going to just take one let's just say we're going to take this one it enters into cell growth phase and that's the b period cell growth and it continues on and it can grow it can enlarge it can synthesize things or whatever the procreative cell really does however this period is outlined mostly by the cell growth and it is it stops so the b period is goes up until dna replication now as soon as the dna replication begins as you can see this would be the origin of replication so as soon as the replication begins the b period ends and the c period begins and it's outlined by the dna replication so we're going to highlight that rapidly location and what happens is that the dna is replicated it it is doubled so there are two chromosomes now now this part right here is still unreplicated part and this this would be what marks once once it reaches this state right here where pretty much the entire chromosome is replicated and separate two opposite sides of the cell then that marks the end of the c period now after the c period we have the d period and the d period typically begins with um the end of dna replication and it is outlined by something called cytoplasm division or the pinching of the cell membrane that would as you can see here i outlined that the cell membrane would pinch inward to divide the two cells that have separated their two chromosomes into different parts of the cell so at this point replication is complete and cell division begins as the plasma membrane grows inward and a new cell wall is synthesized and what it does is it finishes off the cycle with the full separation of the cell producing two daughter cells that would be shown right here and then each daughter cell would enter back into the same cycle going to the b period and the c period and the d period so the d period would be mostly outlined with cell division so that is typically the the nature of cell cycle in prokaryotic cells so a couple of things to note is uh is that when nutrients are abundant so there's there's lots of food there's lots of things to supply energy for the prokaryotic cell what happens is that prokaryotic cells have no need for the b period they kind of they kind of skip over the b period not entirely however they go through the b period a lot faster once if nutrients are available and they can go grow quickly enough to divide their cytoplasm as soon as dna replication is complete and chromosomes are separated so this the d period is very short as well because the nutrients are available and it doesn't even hesitate to divide the cytoplasm as soon as this stage is reached and under such optimal conditions populations of bacteria like e coli that exists in in our bodies these bacterias these bacteria cells can double every 20 minutes and 20 minutes they can double their population isn't that isn't that fascinating now we're going to take a look at the cell cycle in eukaryotic cells so what uh what we have is the eukaryotic cell typically undergoes three primary stages and i have outlined them right here in three different colors so the one going through in this big long cycle is called interface interface stage and that's the first one that we're going to discuss now inter interphase stage comprises of three phases of the cell cycle so the first phase is called g1 phase in which the cell carries out its function and in some cases it grows so how we can label this here is it goes through its function and growth that's what outlines g1 phase as you can see it goes through here so just to clarify this here we have two different daughter cells after each cell cycle after each round of cytokinesis and mitosis we get two daughter cells and now we just taken one of those daughter cells through another cycle just to depict what happens to that daughter cell now what we have is again it begins right away as soon as the daughter cell is born it begins with the g1 phase of the interface stage and interface would be all of all of this part right here so interface is the longest part of the three stages and it starts off with the g1 phase called going through a function and it's growth and it's outlined that it ends or it goes up until the dna begins to replicate so the dna hasn't replicated yet nothing has happened within the chromosomes what happens is the cell simply grows and does its function the cell makes various rnas proteins and other types of cellular molecules but do not make dna they do not synthesize or replicate dna another important thing to know is that in the g1 phase a lot of the cells stop their cell cycle they stop dividing so the cells are not some cells are not destined to divide immediately as soon as g1 phase is over so what they do is they go through this g0 phase that prevents them from division so for example if in some cases the cell in g0 may may start dividing again so as you can see it goes back and re-enters the interface cycle and it goes through the vision however some some cells do not and they are kind of in this g0 state forever so so for example most cells in the human nervous system they stop the dividing once they're fully mature so once the cell is mature it's done its growth has done a function it stops dividing it stops at this g0 which makes it hang in the g1 phase or for example if um the cell is going through its cell cycle and the cell is um it has a mutation or maybe the cell is was produced with the dysfunction so what happens is it goes into this g0 phase where it does not go through division and it is just metabolized by hydrolysis of of of macrophages so they would just simply delete or destroy the cell or it would go through self-destruction because it is a dysfunctional cell so if there's any sort of um mistake that happens within the cell once it's gone through the division or whatever through mitosis it would go into this g0 stage in order to stop its cell cycle and not produce more of these faulty cells now before we talk about the s phase that would be the next one from g1 phase there's this really important point that happens right here in between the g1 and the s and that would be the g1 s checkpoint and the importance of this g1s checkpoint is the cell basically checks for cell size nutrients growth factors and dna damage so if everything's okay and the cell functions normally it has normal size proper dna and everything's okay it goes into the s phase continuing on with interphase and basically the cell is completely committed for cell division that occurs right here however if the cell does not have nutrients if the cell is not proper size or does not have growth factors and it has dna damage then the cell simply will not go through into the s phase it will stop at this g1 s check checkpoint so that's why this one is very very very important g1s checkpoint it will stop at this point and it will not commit to division until either either things are fixed within the cell or the cell is provided nutrients so in the s phase once the cell decides that yes it's going to divide everything's okay we have nutrients we're going to divide the dna replication begins so the s phase is typically associated with dna replication and during the s phase the cell duplicates each chromosome including both the dna and the chromosomal proteins and a continuous synthesis of other molecules so it still continues its function it continues to possibly even grow however what this s phase is really associated with is dna replication it doubles the number of chromosomes or chromosomal pairs it would have the third and the final phase of interphase would be the g2 stage and the the g2 phase is uh refers to the second gap so g what what g stands for is gap because there's a gap in here where the cell does not replicate and there's a gap here where the cell also does not replicate because in the s phase it's already gone through replication so in g2 so if you are asked on the exam which phases of inter interface stage there is no dna replication you would say the gap phase is g1 and g2 because they are depicted by the fact that there's no dna replication within these two different phases of interphase so during g2 the cell continues to synthesize rna and proteins including those for mitosis and it continues to grow so the next stage here that comes is going to be mitosis and what it does is it basically prepares the cell for the stage of mitosis aligning the dna molecules producing different proteins that are needed for the cell to divide and it continues to to do its function so the end of g2 marks the end of interface and what what it has is another major checkpoint right here and that's called the g2 checkpoint and the significance of the g2 checkpoint is that it checks for dna damage because the cell replicated right the another cell the dna replicated within the cell so what it really does is it made replication and therefore mistakes could have happened so the g2 checkpoint does uh does a confirmation that the dna was replicated properly and it's all complete and ready to continue on with the cell division now if there is some sort of errors within the dna the cell detects errors within a dna replication that occurred in the s phase what it will do is it will pause it will stop the cell in the g2 checkpoint and it will repair the damage that is associated with dna replication before it continues on with mitosis however if the damage is irreparable something major went wrong and the damage is cannot be repaired by the cell the cell may undergo the process of apoptosis ptosis and that's basically just self cell destruction and the significance of this is that it prevents the cell to divide with a mutation so if if you've got a mistake in the dna that has replicated it's a mutation and obviously the cell does not want to produce more cells with that mutation therefore it's going to posit this g2 checkpoint and provide and hydrolyze itself it will destroy itself through the process called apoptosis so again let's label the g2 phase uh it continues to do its function growth and prepare prep for division so that's the major functions of the g2 phase now what comes next is going to be mitosis stage so the m stage mitosis the one outline here that is the stage of of that is the second stage of cell cycle which undergoes division cell division now it also contains another checkpoint here and this one's going to be called spindle spindle checkpoint and we're going to talk further about spindle checkpoint and mitosis in our next video where we talk about the chromosomes that are associated with the cell division now the final thing that occurs with the cell cycle the third stage is called cytokinesis and cytokinesis occurs at the end of mitosis sometimes people associate cytokinesis with mitosis as it's a part of it however i'm going to keep things separate just to make it very simple and we're going to talk about mitosis stage and cytokinesis separately however the two processes are aligned together now cytokinesis is typically the process of actually dividing the cytoplasm and splitting the cell into two cells and producing the two daughter cells that would re-enter the cell cycle as we have depicted here all along now with that said is going to conclude our lecture for today and in the next video we're going to talk specifically about cell division and the process of mitosis [Music] you