Objectives Students will have opportunities to
1. Compare mitosis and meiosis briefly describing the activities of the chromosomes in each stage.
Approximate time one week.
Individuals have shown a marked difference in size and weight since the time of birth. A simple answer to this question is that cells reproduce by dividing. This cell division is controlled by the nucleus of the cell. During cell division, each chromosome splits down the middle to form a pair of twin chromosomes side by side. The pair of chromosomes lines up across the center of the cell and the nuclear membrane disappears. The twin chromosome of each pair separates and moves to opposite ends of the cell. The whole cell splits into two new cells. This division of the nucleus is called Mitosis. During this process the chromosome doubled before the cell is divided into two. Thus, each cell has a full set of chromosomes, DNA and genes.
Meiosis—refers to two successive nuclear divisions (with corresponding cell divisions) that produce gametes (in animals) or sexual spores (in plants) having half of the genetic material of the original cell.
In anaphase, the spindle pulls one chromosome from each homologous pair to one side of the cell and the other homologous chromosome to the opposite side. Eventually the two new cells that form contain one chromosome from each homologous pair.
During prophase, the replicate chromosomes of each homologous pair come close together and may twist around each other. In this position, parts of the chromatids of one chromosome may break off and exchange places with the identical parts of chromatids of the other homologous chromosomes—crossing over. After crossing over, the chromosomes are no longer identical to the original paternal and maternal chromosomes; they are a mixture of both.
In anaphase the homologous pairs separate as the spindle apparatus pulls the centromeres to opposite sides of the cell. The centromeres do not divide.
During metaphase, the homologous pairs move to the equator of the cell, and the centromeres attach to the spindle apparatus.
In telophase, the homologous chromosomes have separated and the reproductive cells are dividing.
The event of crossing over or genetic recombination is an event whereby genes recombine and are shuffled between the two homologous chromosomes, producing a new mixture of genes. Genetic recombination is a major contributor to variation among individuals of the same species. This is why one’s brothers or sisters do not look exactly alike (unless identical twins).
During meiosis, the chromosome number is halved which prepares the cells for fertilization when the diploid number is restored.
1. Students will use the Gameto-discs to construct a diagram showing Mitosis and Meiosis.
2. Students will also make simple drawings in their notebooks illustrating both processes, reviewing the differences and similarities.