A section of a chromosome is called the gene, which is dna, which codes for a protein which well look at in a later key area. In terms of the number of chromosomes in living organisms, every species of living organism has a characteristic number of chromosomes and we call this its chromosome conflict. So, for example, humans have 46 chromosomes or 23 pairs. Dogs have 78 potato plants, have 24, every species of living organism has their own characteristic number of chromosomes, and we call this a chromosome complement in terms of the number of chromosomes in cells. A cell that contains a single set of chromosomes is called haploid, whereas a cell that contains two matching sets of chromosomes are called diploid dye, meaning two all normal body cells, for example, skin cells, muscle cells, liver cells, are all diploid thats the standard number of chromosomes. We find in our normal body cells, however, sex cells or gametes such as sperm and egg are haploid. They only contain a single set of chromosomes and thats, because a haploid egg and a haploid sperm are going to combine through fertilization to produce a diploid zygote, but well come to this in a different key area. The way that we make new cells or cell division is a process called mitosis, and it is required to produce new cells for both growth and for the repair of damaged cells. This is why we need mitosis. The other thing that happens during this is while new cells are produced from the original cell, which we call the molar cell.
They produce two genetically identical daughter cells, and these maintain the diploid chromosome complement. What this means is the number of chromosomes in the molar cell will be the same as the number of chromosomes in each daughter cell. They are not half each time. They maintain that same number of chromosomes, so they maintain the diploid. Chromosome complement were now going to take a quick look at the stages of mitosis, so how cell division takes place at the start, chromosomes become more visible as they double up to form two identical chromatids, so these become visible when cell division is about to take place. The second stage, the chromosomes, will shorten and thicken, and we can now see this distinctive chromosome shape. The chromosome is made up of the two chromatids which are joined in the middle by a centromere in stage three. The chromosomes line up at the equator and the membrane surrounding the nucleus disintegrates, and that structure called spindle fibers starts to fill the cell. These spindle fibers then attach to the centromere of each chromosome, which is lined up at the equator in the next stage. Spindle fibers pull the chromatids apart and they pull these chromatids to the opposite ends or what we call poles of the cell. So, if you imagine the cell as a globe, the chromosomes line after the equator and the chromatids are pulled to the opposite, poles of the cell. Finally, a nuclear membrane will now form around each set of chromosomes at the top and at the bottom of the cell.
At the opposite, poles, the cell then pinches in the middle and its divided into those two genetically identical daughter cells. So for one molar cell, we now have two genetically identical daughter cells, which has maintained the chromosome complement in terms of different types of cells. We have specialized cells which are able to perform a certain function so, for example, skin cells, sperm cells, brain cells, these have one job and they can only replicate to become that same form of cell, so skin cells can only replicate to form new skin cells. For example, they cannot become a different type of cell and this is what we call specialized. However, there are unspecialized forms of cells and we call these stem cells in humans and animals. Stem cells are unspecialized cells that are able to divide to produce more stem cells. A process that we say is self reviewing, so a single stem cell can make more stem cells or they can produce cells that then go on to become specialized cells. So a stem cell could produce a stem cell that then differentiates into a skin cell or a red blood cell, or a muscle cell, for example, and this is why these are very important in medicine and research. Finally, theres something that we call a hierarchy of cells and our cells can be specialized in order to work together to form tissues again like skin or muscle. These tissues can then form organs, and these groups of organs can work together in order to form systems.