By Robert Hazen, George Mason University
The Czechoslovakian monk, Gregor Mendel’s, four laws of classical genetics provide a framework for all subsequent discoveries in molecular and cellular genetics. It raised new questions in genetics, such as, what is the physical and chemical nature of Mendel’s ‘atoms of inheritance’? How is the biological information stored? How is it interpreted? How does it pass from one generation to the next?
Gregor Mendel’s Four Laws
Mendel’s four laws state that, first of all, there exist genes, and these genes carry traits. Secondly, each individual carries two genes associated with each trait; one from the mother and one from the father. Third, genes come in different forms, or alleles. There are dominant alleles, and they are expressed in favor of the recessive allele. Finally, Mendel said that different alleles are sorted and distributed randomly, so that all combinations of alleles are equally likely.
It is interesting to note that Mendel’s work, like much important scientific research, also raised fundamental questions in genetics. As we know, every biological system must carry information in these ‘atoms of inheritance’; they have to be physical structures. So then, what are they? Also, how do alleles differ? Why do they give different traits? Why are some dominant, some recessive? These questions also brought with them new insights by examining cells and the very cellular structures, called chromosomes.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Classical and Cellular Genetics
As opposed to classical genetics, cellular genetics is the study of the transfer of biological information at the level of cells. This research requires microscopes, of course, because cells are so small. They are microscopic structures, and that’s why the discovery and the invention of the microscope, and then subsequent improvements in the microscope, were critical parts in this history of cellular genetics.
This, of course, contrasts with the work of Gregor Mendel, who worked in classical genetics: the study of organisms, which one can deal with in a garden. One can just look and see whether a plant is tall or short, whether it has yellow or green peas, and so forth.
There were two key microscopic discoveries that set the stage for this work. First, of course, is that every living thing is made of cells; the cell is the fundamental building block of life. The second is that all cells arise from previous cells. If one is going to look for the biological information, they have to look for something inside the cell that is passed from one generation to the next. That’s where the information has to come from.
It was while observing cell reproduction, that scientists noticed the striking behavior of elongated structures in every cell. These are called chromosomes. In cells with a nucleus, these chromosomes occur in the nucleus. We can stain them by various chemical techniques so that they occur in bright colors in the cell, and therefore, they’re easier to observe when we look through our microscope. Certain characteristics of chromosomes, however, were very suggestive, such as, all chromosomes occur in pairs. Also, different species have different numbers of chromosomes.
Although humans have 23 pairs of chromosomes, some organisms only have one pair of chromosomes; some have more than 100 pairs.
Additionally, the chromosomes also show very striking behavior during cell division. In fact this was one of the things that led geneticists to focus in on chromosomes. Indeed, there are some parallels between the behavior of chromosomes and the aforementioned laws that Mendel proposed for classical genetics.
It was observed that most cells divide by a process called mitosis, in which one parent cell becomes two identical daughter cells. In the process, changes take place, too, with respect to the chromosomes. They thicken, just before cell division, and arrange themselves into pairs; each of these pairs looks a little like the alphabet ‘X’—two crossed chromosomes, if you will. These X-shaped pairs of chromosomes then line up as if they’re along the equator of a nucleus. Think of the nucleus as a globe, and the pairs of chromosomes, those X’s, line up along the equator.
Meanwhile, a bundle of fibers appears, and these fibers stretch from one pole to the other pole of the nucleus, like longitude lines in a globe.
What happens, then, is those X-shaped pairs of chromosomes are pulled apart, they split along the longitude lines; half the chromosomes go to one pole, and the other half of the chromosomes go to the other pole. Two new nuclei form where before there was only one. A nucleus forms at one pole, another nucleus forms at the second pole. A cell wall forms along the equator, and eventually we have two identical daughter cells, where before there was only a single parent cell.
During mitosis, the chromosomes have doubled, and each daughter cell has a full complement. There’s a very different process of cell division that’s observed during sexual reproduction, and these differences provide a key to understanding the genetic mechanisms at a cellular level. This process, by the way, is called meiosis. Mitosis is when two cells are produced from one, a simple process of cell division. Meiosis, on the other hand, is the sexual-reproduction side.
Common Questions about How Mendel Set the Stage for Discoveries on Cellular Level
Gregor Mendel’s laws state that, first of all, there exist genes, and these genes carry traits. Secondly, each individual carries two genes associated with each trait; one from the mother and one from the father.
Gregor Mendel’s laws say that genes come in different forms, or alleles. There are dominant alleles, and they are expressed in favor of the recessive allele. Finally, Mendel said that different alleles are sorted and distributed randomly, so that all combinations of alleles are equally likely.
Mitosis is when two cells are produced from one, a simple process of cell division. Meiosis, on the other hand, is the sexual-reproduction side.