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Stages of Meiosis
Cells of higher organisms possess a basic chromosome number or ploidy (x), the smallest number of chromosomes in a chromosome set. Multiple sets of chromosomes are usually required in order for cells to function normally during the growth and development of plants and animals. Diploid organisms have 2 chromosome sets (2x), while polyploids have 3 or more sets – for example triploids (3x) and tetraploids (4x). Each chromosome in a set is distinct in physical and genetic structure. In diploids, identical chromosomes from each set are called homologous pairs or homologues.
When organisms reproduce, they form specialized sex cells or gametes in which the normal chromosome number is halved by the process of meiosis. The reduction in ploidy is accomplished by cell divisions. In the case of diploid plants, pollen and egg cells are haploid and contain a single set of chromosomes. Upon fertilization, the diploid condition is restored to the embryo and the process is repeated in subsequent generations.
Most of the stages of meiosis are outlined below in a hypothetical diploid organism having a basic chromosome number x = 2. The chromosomes are distinguishable by length; one is short, the other long. The fundamental events in meiosis are 1) pairing of homologous of chromosomes 2) physical exchange of genetic material and 3) subsequent separation of homologues and reduction of cell ploidy level by one half.
|Stages of Meiosis