The only requirement is that the number be even. The number of chromosomes in each cell varies from species to species. The important point is that chromosomes are how cells package their DNA. In reality, chromosomes don’t look anything like that, and they’re enclosed within a cell’s nucleus. The cells above are shown in a simplified form: all that’s shown is the cell membrane, the cytoplasm, and two chromosomes. Note: Scroll through the text below while continuing to refer to the image above. To start, let’s focus on the image below, which shows a simplified mother cell (on the far left) developing into two daughter cells (on the far right). How is cell division controlled?įor now, let’s take a big-picture look at how one cell divides into two. Too much cell division and cells grow out of control, which is what cancer is all about. Too little cell division and processes like development and repair won’t unfold correctly. Cell division has to be carefully regulated.How do cells know what types of cells they should become? On top of that, how do cells grow into the patterns involved in muscle tissue, in organs like the stomach, or in a structure like a hand? These questions are addressed in tutorials about eukaryotic gene regulation and animal development. As an organism grows, some cells develop into nerve cells, muscle cells, skin cells, and so on.How is our DNA replicated? That’s covered in our tutorial about DNA replication. Each of our cells has an immense amount of DNA: about 2 meters (over 6 feet) if it were fully uncoiled. Every time a cell divides, all of the cell’s DNA has to be copied.Note that we’re not, at the moment, addressing some related (and amazing) issues: The question that we’ll address below is how does one cell divide into two? We all start from just one cell Just before conceptionĪt the start of your existence - in the first moments after your conception - you consisted of one cell. Illustration showing the five stages of mitosis.Looking for a student learning guide? It’s linked on the main menu for your course. The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus.A membrane forms around each set of chromosomes to create two new nuclei.At each pole of the cell a full set of chromosomes gather together.The sister chromatids are then pulled apart by the mitotic spindle which pulls one chromatid to one pole and the other chromatid to the opposite pole.The mitotic spindle fibres attach to each of the sister chromatids.The centrioles are now at opposite poles of the cell with the mitotic spindle fibres extending from them.The chromosomes line up neatly end-to-end along the centre (equator) of the cell.The mitotic spindle, consisting of the microtubules and other proteins, extends across the cell between the centrioles as they move to opposite poles of the cell.At the end of prophase the membrane around the nucleus in the cell dissolves away releasing the chromosomes.The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on.Each chromosome is composed of two sister chromatids, containing identical genetic information.The chromosomes condense into X-shaped structures that can be easily seen under a microscope.During interphase, microtubules extend from these centrosomes.Outside of the nucleus are two centrosomes, each containing a pair of centrioles, these structures are critical for the process of cell division.The DNA in the cell is copied in preparation for cell division, this results in two identical full sets of chromosomes.If not corrected in time, mistakes made during mitosis can result in changes in the DNA that can potentially lead to genetic disorders.The major purpose of mitosis is for growth and to replace worn out cells.During mitosis one cell divides once to form two identical cells.
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