differential activities of the maternal and paternal copies of the lgf-2 gene arise from the melhyiation of an associated silencer DNA Uiat represses Igf-2 expression. During spermiogenesis. the DNA is methylated, and as a result, the Igf-2 gene can be activated in the developing fetus. The methylation inactivates the silencer. In contrast, the silencer DNA is not methylated in the developing oocyte. Hence, the Igf-2 allele inherited from the female is silent. In other words, the paternal copy of the gene is "imprinted"— in Ihis case, methylated—for future expression in the embryo. This specific example is discussed in greater detail in Chapter 17.

There are approximately 30 imprinted genes in mice and humans. Many of the genes, including the preceding example of Igf-2, control the growth of ¡he developing fetus, it has been suggested that imprinting has evolved to protect the mother from her own fetus. The lgf-2 protein promotes the growth of the fetus. The mother attempts to limit this growth by inactivating the maternal copy of the gene.

We have considered how every organism must maintain ant! duplicate its DNA to survive, adapt, and propagate. The overall strategies for achieving these basic biological goals are similar in the vast majority of oiganisms and, therefore, may be examined rati ¡or successfully using simple organisms. It is, however, clear thai the more intricate processes found in higher organisms, such as differentiation and development, require more complicated systems for regulating gene expression and

allele lgf-2 gene

allele lgf-2 gene thai these can be studied only in more complex organisms. We have seen that a wide range of powerful experimental techniques can be used with success to manipulate the mouse and to explore various complex biological problems. As a result, the mouse has served as an excellent model system for studying developmental, genetic, and biochemical processes that are likely to occur in more highly evolved mammals. The recent publication and annotation of the mouse genome has underscored the importance of the mouse as a model for further exploring and understanding problems in human development and disease.

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