in this video we're going to take a look at how cells divide and in particular at the role of chromosomes All multisellular ukarotic organisms whether they're humans plants or walruses require a continuous supply of new cells which can be used for growth development and repair This is particularly true for young organisms who need to grow in size but it's also true for older organisms who must constantly replace lost cells For instance the skin cells which continuously fall off our bodies The process through which these cells are generated is known as the cell cycle and it can be thought of as the life cycle of the cell From the time it was made to the time it divides into two new cells Within this cell cycle there are three main stages you need to be aware of growth DNA replication and mitosis and division also known as cytochinesis In the first step the cell grows in size and at the same time increases the number of subsolar structures it contains such as mitochondria and ribosomes Next the DNA is duplicated so that the two new cells will each have a full set of DNA In order to understand the next stage it's worth taking a closer look at the form that DNA takes within a cell Usually when a cell is not dividing and is just going about its normal functions the DNA is spread out in long strings However once a cell starts to prepare for division its DNA condenses into chromosomes which are basically coiled up packets of DNA The DNA of each chromosome contains a large number of genes and these control the development of different characteristics such as eye color Ukarotic cells such as animals have two copies of each chromosome which we call a pair One of these copies is from their mother and the other copy is from their father Humans have 23 pairs of chromosomes So in total they have 46 chromosomes Other species though have different numbers of chromosomes and this is one of the reasons that animals of different species can't reproduce to have fertile offspring As the cell continues to prepare for division it duplicates each of the 46 chromosomes but the duplicate stays attached to the original chromosome that it was copied from so that together they form an X shape Notice that each chromosome is colored half green This is to highlight the fact that the right half of each chromosome is a duplicate of the left half that we started with You might sometimes hear each half referred to as an arm The result is that the DNA on the right arm of each X-shaped chromosome is the same as the DNA on the left arm of that chromosome Once the cell is ready to divide all 46 of the cell's chromosomes line up along the center of the cell At this point fibers from either side of the cell attached to their respective half of each chromosome These are called cell fibers and they pull the arms of the chromosomes to opposite sides of the cell We call these two sides poles As you can see this breaks the chromosomes in half into two separate arms This happens to all 46 of the chromosomes even though we've only demonstrated it with four here Now that all of the genetic material has been divided among the two sides of the cell the third and final stage takes place which is division or more properly called cytochinesis For this stage the cell membrane and cytoplasm pull apart forming two daughter cells Notice that half of the chromosomes have ended up in the left hand cell and the other half in the right hand one Therefore each cell has the same DNA and the two cells are identical to each other and to the parent cell we started off with These resulting daughter cells can then contribute to growth development or repair and undergo the cell cycle all over again If you haven't heard yet you can find all of our videos on our website cognito.org You'll also find questions flashcards exam style questions and past papers And we track all of your progress so that you always know what to study next So sign up for free by clicking here or browse our playlist here on YouTube