Explanation:
Genetics owes a great debt to good drawing. Handmade graphic depictions of mitotic chromosomes by Walther Flemming (Figure 1) and meiotic chromosomes by Walter Sutton provided an early record of the physical path of chromosomes during cell division. The physical movement of chromosomes could then be correlated with cells' patterns of genetic inheritance. (The idea that genes were carried on cytological structures is now known as the chromosome theory.) Using such methods, researchers determined that although mitosis and meiosis are both forms of cell division, the results of these processes are actually quite different.
Gene Transmission in Mitosis
View Full-Size ImageFigure 1
Mitosis occurs in somatic cells; this means that it takes place in all types of cells that are not involved in the production of gametes. Prior to each mitotic division, a copy of every chromosome is created; thus, following division, a complete set of chromosomes is found in the nucleus of each new cell. Indeed, apart from random mutations, each successive duplicate cell will have the same genetic composition as its parent, due to the inheritance of the same chromosome set and similar biological environment. This works well for replacing damaged tissue or for growth and expansion from an embryonic state.
Because the genes contained in the duplicate chromosomes are transferred to each successive cellular generation, all mitotic progeny are genetically similar. However, there are exceptions. For example, there are genetic variations that arise in clonal species, such as bacteria, due to spontaneous mutations during mitotic division. Furthermore, chromosomes are sometimes replicated multiple times without any accompanying cell division. This occurs in the cells of Drosophila larvae salivary glands, for example, where there is a high metabolic demand. The chromosomes there are called polytene chromosomes, and they are extremely large compared to chromosomes in other Drosophila cells. These chromosomes replicate by undergoing the initial phases of mitosis without any cytokinesis (Figure 2). Therefore, the same cell contains thick arrangements of duplicate chromosomes side by side, which look like strands of very thick rope. Scientists believe that these chromosomes are hyper-replicated to allow for the rapid and copious production of certain proteins that help larval growth and metamorphosis (Gilbert, 2008).
Gene Transmission in Meiosis
Three photomicrographs show polytene chromosomes. The chromosomes look like horizontal tubes composed of white, grey, and black bands against a black background. They look like thick, striated lengths of rope.
Figure 2: Examples of polytene chromosomes
Pairing of homologous chromatids results in hundreds to thousands of individual chromatid copies aligned tightly in parallel to produce giant, "polytene" chromosomes.
© 2007 Nature Publishing Group Novikov, D. et al. High-pressure treatment of polytene chromosomes improves structural resolution. Nature Methods 4, 483 (2007). All rights reserved. View Terms of Use
Although he did not know it, Walther Flemming actually observed spermatozoa undergoing meiosis in 1882, but he mistook this process for mitosis. Nonetheless, Flemming did notice that, unlike during regular cell division, chromosomes occurred in pairs during spermatozoan development. This observation, followed in 1902 by Sutton's meticulous measurement of chromosomes in grasshopper sperm cell development, provided definitive clues that cell division in gametes was not just regular mitosis. Sutton demonstrated that the number of chromosomes was reduced in spermatozoan cell division, a process referred to as reductive division. As a result of this process, each gamete that Sutton observed had one-half the genetic information of the original cell. A few years later, researchers J. B. Farmer and J. E. S. Moore reported that this process—otherwise known as meiosis—is the fundamental means by which animals and plants produce gametes (Farmer & Moore, 1905).
