 Now we go ahead towards the cell division, cells need to grow, organisms they need to grow, they need to increase in their size sometimes, sometimes they are wounded and they need healing, they have to increase in number. We call this cell division, there are two types of cell, cells if we are considering the division. We will talk about, there are some cells called somatic cells, there are some other cells called germline cells. Somatic cells are most of the cells of the body other than the gametes. The germline cells are the cells which are called gametes which are present in the reproductive organs, testisone ovary of the males and females and these are the cells which are responsible for making the next generation. We are talking about the division in somatic cells first. The somatic cells need to divide for growth, for wound healing, for regeneration or other mechanisms. These cells divide by a process called mitosis, while the germline cells they divide by another process called meiosis. In this, in today's lecture we are going to talk about mitosis. In the diagram above, you can see a dividing cell, this is actually, I must not say a diagram, this is a picture that is taken through microscope and these are the onion cells, onions epidermal cells. You can see when you cut down an onion, you can see a small membrane. These are the cells of that membrane which are dividing. Before talking about division of the cell, the cell division, we have to talk about that what is the whole process of dividing. When cells are produced, after that for some time they normally perform their function. When they need to divide, they have to enter a cyclic process called cell cycle. What is cell cycle? Cell cycle is the sequence of events by which a cell divides and then go back to the non-dividing state. The cell cycle consists of two major phases, one is called an interface which is actually very long phase and it consists of three sub phases called G1 or gap 1, S phase or the synthesis phase and G2 for the gap 2 phase. After the interface, next phase is the M phase or mitosis phase or the division phase. Interphase is actually a preparatory phase for the M phase because cell need to prepare itself for division before it enters into the division phase. The G0 and the gap 0 is also called a resting stage. This is the stage when cells do not divide. Here you can see a diagram which shows the interface and the mitotic phase, the M phase. As you can see that interface makes about 80 to 90 percent of the cell cycle and the remaining small phase is called the mitotic phase M phase. Interphase as you can see is again divided into these three phases as I have described the G1 or gap 1, the S or the DNA synthesis phase, the G2 phase then comes the M phase. After M phase just at the start of G1 cell can enter into the G0 phase or the resting phase and stop dividing. If cell enters into the interface then it means it is going to divide. If it is not going to divide, it is genetically programmed to not to do so then it is staying in the G0 phase. For example, our nerve cells they do not divide, they stay throughout their lifetime, throughout our lifetimes in the G0 phase. This is the reason that if there is an injury to the nerve cells they are never replaced because these cells do not divide for wound healing or for replacing themselves. Now we look at the details of these phases. The gap 1 or G1 in this phase, this is the first phase of the first part of the interface in G1 cells increase in size and as we know that after G1 phase the next phase which is coming is the S or the synthesis phase, synthesis is the DNA synthesis phase in which DNA have to be synthesized. In the gap 1 or G1 phase cell prepare itself for the DNA synthesis we call it replication or duplication. The DNA of the cell have to be duplicated so that the cell produces those materials which are required to make the new DNA, to duplicate or replicate the DNA. There is a G1 checkpoint, there is a control mechanism. This control mechanism checks or G1 checkpoint checks that whether all the materials which are required for entering into the S phase in the synthesis phase are they ready or they are not. If all these materials are ready then cell enters into the S phase. If they are not cell have to complete making of those elements for example if an enzyme required for DNA synthesis is not formed or is formed in lesser quantity then G1 checkpoint stops cell from entering into the S phase and it will make the enzyme and then enters into the S phase. The S phase is the phase in which the DNA is synthesized and the cell actually duplicates its DNA. As we know that DNA is present in the form of chromosomes and chromosomes are present in pairs. For example human beings have 46 chromosomes. These 46 chromosomes are present in 23 pairs. All of these pairs because we know that mitosis is a division which will make two daughter cells and these daughter cells should have an equal number of chromosomes. This is the reason that in the S phase before division cell have to duplicate all of these chromosomes so that both the daughter cells should have an equal number of those chromosomes. So in synthesis phase DNA is synthesized and all of the chromosomes are duplicated they are actually double to their initial number. There is an S check when the DNA is duplicated. This checkpoint at the control mechanism checks that whether the DNA is duplicated and it is is it correctly duplicated if this is not cell have to redo these things. If cell is not able to do so it have it is stopped when it completes its things which are which are required which are required for the duplication and duplication is complete and correct then cells enter into the gap 2 or G2 phase. Now comes the G2 and the gap 2. In this phase cell again continue increasing its size that is it grows and as we know that after gap 2 phase the phase which is coming is the mitosis the division phase. So in gap 2 phase cell collects all those materials which are required for division. The most important element the most important thing which is made or produced during this phase are the microtubules because microtubules are required to move chromosomes to attach with the chromosomes and then move them to correctly towards the poles of the cell. There is a G2 checkpoint which checks whether all the materials required for the M phase complete or not. If they are complete checkpoint allows the cell to go ahead towards the mitosis or the division or if they are not complete then the control mechanism stop the cell over there and cell have to remake all of its materials. When it completes the cell enters into the M or mitosis phase. Now mitosis M phase the division in mitosis basically mitosis in the mitosis cell have to be duplicated cell have to divide into two daughter cells at this division cell growth is totally stopped as we know that in G1 and G2 cell was growing but in in mitosis phase cell stops its growth because for growth cell needs to apply energy and for a division the cell division is a process which require lot of energy. So cell put all of its energy for division only and it do not grow during this phase. There is a check when the cell is dividing there is a checkpoint almost at the end of the mitosis which checks that whether the mitosis is correctly carried out or not. If this is not cell is stopped from entering next phase from dividing if it is ok cell go towards dividing into two.