Cell Division in Relation to Reproduction - 51 



male and female of its parents, and conse- 

 quently this new individual derives its herit- 

 able qualities from both. Sexual forms of re- 

 production always involve fertilization, and 

 it is only in sexual reproduction that bi- 

 parental inheritance is found. Asexual repro- 

 ductive processes never involve fertilization, 

 and in asexual reproduction, inheritance is 

 entirely uniparental. Asexually produced off- 

 spring tend to resemble their single parent 

 and do not display as wide a variability as 

 those that are generated sexually. 



Fig. 3-8. Diploid chromosome group of the fruit fly, 

 Drosophila. The numbers I, II, III, and IV indicate the 

 four pairs of chromosomes. 



mosomes are really four pairs. Each indi- 

 vidual chromosome possesses a homologous 

 mate that displays an identical size and 

 shape. Similarly the 16 chromosomes of the 

 onion plant are really 8 pairs, and the 46 

 chromosomes of man actually represent 23 

 pairs. 



The reason why most of the body cells of 

 plants and animals possess a diploid set of 

 chromosomes is relatively simple. The fer- 

 tilized egg from which all the body cells 

 arise is always formed by the coming to- 

 gether and fusing of two cells. A sperm cell 

 from the male parent always fuses with the 

 unfertilized egg cell from the female parent. 

 Thus the diploid set of chromosomes pos- 

 sessed by the fertilized egg is really consti- 

 tuted of two single, or haploid sets (Fig. 3-9). 

 One of these haploid sets was present in the 

 nucleus of the unfertilized egg, and the other 

 was brought in by the nucleus of the sperm 

 cell. When the egg and sperm nuclei finally 

 fuse into one — and this is the essential event 

 of fertilization — the nucleus of the fertilized 

 egg becomes diploid, and it remains so 

 throughout all subsequent mitotic divisions. 



Fertilization. Fertilization is a very impor- 

 tant process found in the reproductive cycles 

 of a great majority of plant and animal or- 

 ganisms. The new individual that arises 

 from the fertilized egg possesses chromo- 

 somes (and genes) contributed by both the 



MEIOTIC CELL DIVISION 



The real problem concerning chromosome 

 numbers does not revolve around how 

 the body cells come to be diploid, but how 

 the eggs and sperm of animals, and the spores 

 of plants, come to be haploid. This difficult 

 question took many years to solve. Finally it 

 was learned that these reproductive cells are 

 produced not by mitosis, but by a highly 

 modified type of division, which is called 

 meiosis. 



One difference between meiosis and mito- 

 sis is that meiosis involves two succeeding 

 divisions, whereas each single mitotic divi- 

 sion is complete unto itself. In meiosis, when 

 the two divisions have been finished, four 

 cells (or at least four nuclei) have been pro- 

 duced. The parent cell at the outset of 

 meiosis is always diploid and the four final 

 daughter cells are always haploid. In mitosis, 

 on the other hand, the daughter cells always 

 possess the same number of chromosomes as 

 the parent cell, whether that original num- 

 ber was diploid or, as sometimes is the case, 

 haploid. This and other differences between 

 meiosis and mitosis serve to distinguish 

 clearly between the two processes. Neverthe- 

 less, it is important to realize that the two 

 types of division have much in common. In 

 both cases the same four stages — prophase, 

 metaphase, anaphase, and telophase — are 

 plainly distinguishable, although in meiosis 

 the steps occur twice. Also the same accessory 

 mechanisms, including spindles, asters, and 



