Concept 8 Sex cells have one set of chromosomes; body cells have two.
Theodor Boveri presents chromosomes' role in development.
Hello, I'm Theodor Boveri. I was interested in chromosomes and their role in development. In the late 1800's, I used worms as a model for study. I reasoned that since these worms undergo sexual reproduction, parental sex cells — sperm and egg — must have, and contribute material, chromosomes, to the progeny. [Ascaris sp. PARASITIC ROUNDWORM] This presented a problem. If sex cells, like sperm and egg, have the same number of chromosomes as regular body cells, then each time they combined, the progeny would have twice the number of chromosomes. This doesn't happen. So, there must be a process that halves chromosome number in sex cells. This process is called meiosis and there are two rounds of cell division. Let's look at the first round, meiosis I. As in mitosis, a cell that is not dividing is said to be in interphase. During interphase I, the cell duplicates its nuclear material. In this example, this cell has four chromosomes which replicate into eight. (In reality, chromosomes aren't visible until the end of prophase I.) In prophase I, the chromosomes condense and become visible underneath a light microscope. Each original chromosome and its copy are joined together at the centromere and are called sister chromatids. Later in prophase I, similar chromosomes — for example, the maternal chromosome#1 and the paternal chromosome#1 — pair up. Each member of this pair is called a homolog. In metaphase I, the pairs move and line up at the cell equator. HOMOLOGS In anaphase I, the homologs separate and move to opposite poles of the cell. Meiosis I ends with telophase I. Homologs collect at opposite poles of the mother cell, and the cytoplasm divides to produce two daughter cells, each with one set of chromosomes. These two daughter cells undergo one more round of division. In meiosis II, the centromeres divide, and the sister chromatids separate, as they do in mitosis. Interphase II and prophase II are much shorter than interphase I and prophase I. These two daughter cells undergo one more round of division. In meiosis II, the centromeres divide, and the sister chromatids separate, as they do in mitosis. Interphase II and prophase II are much shorter than interphase I and prophase I In summary, a cell with two sets of chromosomes at the beginning of meiosis will divide twice to give four daughters cells, each with one set of chromosomes. These can further mature into either sperm or egg. Chromosomes must be important for heredity. Otherwise, it wouldn't matter how many chromosomes a cell gets during mitosis or meiosis. I used sea urchins to prove that the right number of chromosomes is needed for correct development. Sometimes sea urchin eggs can be fertilized by two sperm. When this happens, the first cellular division is uneven. Instead of dividing into two cells, the fertilized egg divides into three cells. Each of these three cells continue to divide, and the divisions all look normal, but these doubly fertilized eggs eventually die. They never become sea urchins. I realized that when two sperm fertilize one egg, an extra set of chromosomes is introduced. The first division after fertilization is uneven, and each of these three cells receives an incomplete set of chromosomes. These cells then die. Chromosomes are the basis of heredity, and cells need a full set of chromosomes for proper development. I was not the only one who came to these conclusions about chromosomes. In 1902, Walter Sutton, an American student at Columbia University, published a paper on the number and shape of grasshopper chromosomes. [Grasshopper chromosomes adapted from Sutton's paper.] In his paper, Sutton lined up the grasshopper chromosomes to show the differences in sizes and shapes. Sutton realized, as I did, that similar chromosomes pair during meiosis and segregation of the homologous pairs reduces chromosome number. Based on our work, Sutton and I established chromosomes as the physical basis of the Mendelian laws of heredity. The segregation of chromosomes during meiosis was what Herr Mendel had predicted for the segregation of factors.
Polyploidy is the condition where an organism has more than the normal two sets of chromosomes. Polyploidy is fairly common in plants. Wheat, potatoes, bananas, coffee and tulips are examples of polyploid plants.
Why is polyploidy possible and common in plants?