Figure 22.4. Exogenous genes that integrate into chromosomes can be stably transmitted to all daughter cells, unlike episomal (extrachromosomal) genes.

Figure 22.4Exogenous genes that integrate into chromosomes can be stably transmitted to all daughter cells, unlike episomal (extrachromosomal) genes

The figure illustrates two possible fates of genes that have been transferred into nucleated cells. If the cells are actively dividing, any genes which integrate stably into chromosomal DNA can be replicated under the control of the parent chromosome (during the S phase of the cell cycle). Following each cell division, an integrated gene will be stably inherited by both daughter cells. As a result, all cells that descend from a single cell in which stable integration took place, will contain the integrated gene. Gene therapy involving chromosomal integration of exogenous genes offers the possibility of continued stable expression of the inserted gene and a permanent cure, but carries certain risks, notably the possibility that one of the integration events may result in cancer (see text). By contrast, episomal genes which do not integrate but replicate extrachromosomally (under the control of a vector origin of replication) may not segregate to all daughter cells during subsequent mitoses. As a result, this type of approach has been particularly applied in gene therapies where the target tissue consists of nondividing cells (see text).

From: Chapter 22, Gene therapy and other molecular genetic-based therapeutic approaches

Cover of Human Molecular Genetics
Human Molecular Genetics. 2nd edition.
Strachan T, Read AP.
New York: Wiley-Liss; 1999.
Copyright © 1999, Garland Science.

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