Biology  5/e   Raven/Johnson
	Student   Online Learning Center

Reproduction without sex is not uncommon in the living world. Many simple organisms reproduce by mitosis or binary fission, and never mate. The evolution of sexual reproduction required a new mechanism of chromosome number adjustment. If the nuclei of two organisms fuse, in a process called syngamy, their two genomes are combined in the fusion nucleus. The chromosome number would then be doubled. A doubling of chromosome number with each subsequent generation would cause insurmountable problems.

A process of chromosome number adjustment evolved. The process, called meiosis, involves two successive divisions. The first division reduces the number of chromosomes to half, a condition termed haploid. The second division reduces the chromatid number per chromosome from two to one. The process also includes events that enhance variability. As the first division begins, genes are exchanged between homologous chromosomes from the two parents. Each chromosome then carries genes from the chromosome of the other parent in place of the ones inherited from the prior generation. The altered chromosomes are then randomly sorted into the newly forming nuclei. These two events provide virtually infinite variability. It is believed that syngamy evolved as a mechanism for repairing breaks or deletions in chromosomes by providing an intact template from the mating partner.

Copyright ©1998 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.