4) of paternal origin (pat). This initiates chiasma for- mation and is the prerequisite for crossing-over and subsequent recombination. (Diagram after Watson et al., 1987). C. Chiasmata When a chiasma is formed, either of the two chromatids of one chromosome pairs with one of the chromatids of the homologus chromo- some (e.g., 1 and 3, 2 and 4 and so on). Chiasma formation is the cytological prerequisite for crossing-over and is important in the definitive separation (segregation) of the chromosomes. The centromere (Cen) plays an important role in chromosome pairing. D. Genetic recombination through crossing-over Through crossing-over, new combinations of chromosome segments arise (recombination). As a result, recombinant and nonrecombinant chromosome segments can be differentiated. In the diagram, the areas A\u2013E (shown in red) of one chromosome and the corresponding areas a\u2013e (shown in blue) of the homologous chromo- some become respectively a\u2013b\u2013C\u2013D\u2013E and A\u2013B\u2013c\u2013d\u2013e in the recombinant chromo- somes. E. Pachytene and diakinesis under the light microscope In themicrograph, pachytene chromosomes are readily visualized as bivalents (a). An unusual structure in pachytene is formed by the X and the Y chromosomes. They appear to be joined end-to-end. Actually, short segments of the short arms in the regions with homologous sequences (pseudoautosomal region, see p. 390) have paired. In later stages (b), it can be seen that they have separated for themost part. (Photographs from Therman, 1986). Today, electron micrographs are usually used for mei- otic studies. References Therman, E.: Human Chromosomes: structure and behaviour. 2nd ed. Springer, Heidel- berg,1986. Watson, J.D., et al.: Molecular Biology of the Gene. 3rd ed. The Benjamins/Cummings Publishing Co., Menlo Park, California, 1987 Fundamentals Passarge, Color Atlas of Genetics © 2001 Thieme All rights reserved. Usage subject to terms and conditions of license. 119Crossing-Over in Prophase I Passarge, Color Atlas of Genetics © 2001 Thieme All rights reserved. Usage subject to terms and conditions of license. 120 Formation of Gametes Germ cells (gametes) are produced in the gonads. In females the process is called oogene- sis (formation of oocytes) and in males, sper- matogenesis (formation of spermatozoa). The primordial germ cells, which migrate to the gonads during early fetal development, in- crease in number bymitotic division. The actual formation of germ cells (gametogenesis) begins withmeiosis. Meiosis differs in duration and re- sults between males and females. A. Spermatogenesis Diploid spermatogonia are formed by repeated mitotic cell divisions. At the onset of puberty, some of the cells begin to differentiate into pri- mary spermatocytes. The first meiotic cell divi- sion occurs in these cells. At the completion of meiosis I, a primary spermatocyte has given rise to two secondary spermatocytes, each of which has a haploid set of duplicated chromosomes (recombination is not illustrated here). Each chromosome consists of two sister chromatids, which become separated during meiosis II. In meiosis II, each secondary spermatocyte divides to form two spermatids. Thus, one pri- mary spermatocyte forms four spermatids, each with a haploid chromosome complement. The spermatids differentiate into mature sper- matozoa. Male spermatogenesis is a continuous process. In human males, the time lapse be- tween differentiation into a primary spermato- cyte at the onset of meiosis I and the formation of mature spermatocytes is about 6 weeks. B. Oogenesis Oogenesis (formation of oocytes) differs from spermatogenesis in timing and in the result. At first the germ cells, which have migrated to the ovary, multiply by repeated mitosis (formation of oogonia). In human females, meiosis I begins about 4weeks before birth. Primary oocytes are formed. However, meiosis I is arrested in a stage of prophase designated dictyotene. The primary oocyte persists in this stage until ovulation. Only then is meiosis I continued (recombina- tion is not shown here). In females, the cytoplasm divides asymmetri- cally in both meiosis I and meiosis II. The result each time is two cells of unequal size: a larger cell that will eventually form the egg and a small cell, called a polar body. When the pri- mary oocyte divides, the haploid secondary oo- cyte and polar body I are formed.When the sec- ondary oocyte divides, again unequally, the re- sult is a mature oocyte and another polar body (polar body II). The polar bodies do not develop further, but degenerate. On rare occasionswhen this does not occur, a polar body may become fertilized. This can give rise to an incompletely developed twin. In the secondary oocyte, each chromosome still exists as two sister chromatids. These do not separate until the next cell division (meiosis II), when they enter into twodifferent cells. Inmost vertebrates, maturation of the secondary oo- cyte is arrested in meiosis II. At ovulation the secondary oocyte is released from the ovary, and if fertilization occurs, meiosis is then completed. Faulty distribution of the chromo- somes (nondisjunction) may occur in meiosis I as well as in meiosis II (see p. 116). The maximal number of germ cells in the ovary of the human fetus at about the 5th months is 6.8!106. By birth this has been reduced to 2! 106, and by puberty to about 200,000. Of these, about 400 are ovulated (Connor & Ferguson- Smith, 1993). The long period between meiosis I and ovula- tion is presumably a factor in the relatively frequent nondisjunction of homologous chro- mosomes in older mothers. The difference in time in the formation of gametes during oogenesis and spermatogenesis is reflected in the difference in germline cell di- visions. In the female there are 22 cell divisions before meiosis, resulting in a total of 23 chro- mosome replications. In contrast, 610 chromo- some replications have taken place in the an- cestral cells of spermatozoa produced in a male at the age of 40 (380 at the age of 30), yielding 25 times as many cell divisions during sper- matogenesis (Crow, 2000). This probably ac- counts for the highermutation rate inmales, es- pecially with increased paternal age. References Connor, J.M., Ferguson-Smith, M.A.: Essential Medical Genetics. 4th ed. Blackwell Scien- tific, London, 1993. Crow, J.F.: The origins, patterns and implica- tions of human spontaneous mutation. Na- ture Reviews 1 :40\u201347, 2000. Hurst, L.D., Ellegren, H.: Sex biases in the muta- tion rate. Trends Genet. 14:446\u2013452, 1998. Fundamentals Passarge, Color Atlas of Genetics © 2001 Thieme All rights reserved. Usage subject to terms and conditions of license. 121Formation of Gametes Passarge, Color Atlas of Genetics © 2001 Thieme All rights reserved. Usage subject to terms and conditions of license. 122 Cell Culture Cells of animals and plants can live andmultiply in a tissue-culture dish (as a cell culture) at 37 !C in a medium containing vitamins, sugar, serum (containing numerous growth factors and hor- mones), the nine essential amino acids for vertebrate animals (His, Ile, Leu, Lys, Met, Phe, Thr, Tyr, Val), and usually also glutamine and cysteine. Cell cultures have been in wide use since 1965 and have become the basis for genetic studies not possible in the living mam- mals (somatic cell genetics). A great variety of growth media are available for culturing mam- malian cells and accommodating their require- ments for growth. The predominant cell type that grows from a piece of mammalian tissue in culture is the fi- broblast. Although fibroblasts secrete proteins typical of fibrous connective tissue, in principle they retain the ability to differentiate into other cell types. Cultured skin fibroblasts have a finite life span (Hayflick, 1997).