Mitosis


Cells can divide, and in unicellular organisms, this makes more organisms. In multicellular organisms, cell division is used for growth, development, and repair of the organism. Cell division is controlled by DNA, but exact copies of the DNA must be given to the daughter cells (note use of “mother” and “daughter”). Bacteria reproduce by a simple process called binary fission. They have one chromosome which is attached to the cell membrane. This chromosome replicates, then the two copies are pulled apart as the cell grows. Eventually the cell pinches in two to make two cells. Eukaryotes do mitosis. In mitosis, each daughter cell gets about half of the cytoplasm from the mother cell and one set or copy of the DNA.


Before cell division occurs, the cell first has to replicate the chromosomes so each daughter cell can have a set. When the chromosomes are replicated and getting ready to divide, they consist of two, identical halves called sister chromatids which are joined by a central region, the centromere. Each chromosome is one long molecule of DNA and special proteins. DNA makes up the genes, and we say that genes are “on” chromosomes, or chromosomes “contain” or are made of genes. Some of the proteins in the chromosomes “turn off” the genes that are not needed in that cell. For example, while every cell in your body contains exactly the same genes, you don’t need your eye-color gene operational in cells in your big toe, nor toenail-shape genes active in cells in your stomach.


Two basic types of cells occur in the bodies of eukaryotes. Somatic cells are general body cells. These have the same number of chromosomes as each other within the body of an organism. The number of chromosomes in somatic cells is consistent among organisms of the same species, but varies from species to species. These chromosomes come in pairs, where one chromosome in each pair is from the mother and one is from the father. Actually, since most organisms have more than one pair of chromosomes, it would also be correct to say that the organism received one set of chromosomes from its mother and one matching set from its father, and that these sets match in pairs. The other type of cells found in eukaryotes is gametes or sex cells, consisting of eggs in females and sperm in males. These special reproductive cells have only one set (half as many) of chromosomes consisting of one chromosome from each pair. In humans ONLY, the somatic cells have 46 chromosomes arranged in 23 pairs (= two sets of 23 each), while gametes have 23 individual chromosomes (= one set). In fruit flies, somatic cells have 8 chromosomes (= 4 pairs or 2 sets) and gametes have 4 chromosomes (= 1 set). Geneticists use the term “-ploid” to refer to one set of chromosomes in an organism, and that term is typically combined with another wordstem that describes the number of sets of chromosomes present. For example, a cell with one set of chromosomes is called haploid, a cell with two sets of chromosomes is diploid, and a cell with four sets of chromosomes (not usually a “normal” condition, but sometimes possible) is tetraploid.


Technically, mitosis is specifically the process of division of the chromosomes, while cytokinesis is officially the process of division of the cytoplasm to form two cells. In most cells, cytokinesis follows or occurs along with the last part of mitosis.


Remember centrioles? They consist of nine sets of three microtubules, occur in animal cells only, and are involved in division of the chromosomes. Each animal cell has a pair of centrioles located just outside the nucleus. The two centrioles in the pair are oriented at right angles to each other. Just before mitosis, the centrioles replicate, so the cell now has four (two sets of two) as it starts mitosis.






The stages in mitosis include (interphase), metaphase, anaphase, and telophase. Remembering “IPMAT” or Intelligent People Meet At Three (or is that Twelve?) can help you remember the stages in order. Strictly speaking, interphase is the stage in which a cell spends most of its life and is not part of the process of mitosis, per se, but is usually discussed along with the other stages.


Interphase may