121.11 Homeotic genes in mammals are similar to those found in Drosophila. The complexes are arranged so that genes with similar sequences lie in the same column. See Figure 21.10 for the full names of the Drosophila genes.

The mammalian Hox genes are similar in sequence to the homeotic genes found In Drosophila, and they are in the same order.


Homeobox-containing genes are found in many organisms, in which they regulate development.

i" More information about Hox genes

Connecting Concepts]

The Control of Development

Development is a complex process consisting of numerous events that must take place in a highly specific sequence. The results of studies in fruit flies and other organisms reveal that this process is regulated by a large number of genes. In Drosophila, the dorsal-ventral axis and the anterior-posterior axis are established by maternal genes; these genes encode mRNAs and proteins that are localized to specific regions within the egg and cause specific genes to be expressed in different regions of the embryo. The proteins of these genes then stimulate other genes, which in turn stimulate yet other genes in a cascade of control. As might be expected, most of the gene products in the cascade are regulatory proteins, which bind to DNA and activate other genes.

In the course of development, successively smaller regions of the embryo are determined (I Figure 21.12). In Drosophila, first, the major axes and regions of the embryo are established by egg polarity genes. Next, patterns within each region are determined by the action of segmentation genes: the gap genes define large sections; the pair-rule genes define regional sections of the embryo and affect alternate segments; and the segment-polarity genes affect individual segments. Finally, the homeotic genes provide each segment with a unique identity. Initial gradients in proteins and mRNA stimulate localized gene expression, which produces more finely located gradients that stimulate even more localized gene expression. Developmental regulation thus becomes more and more narrowly defined.

The processes by which limbs, organs, and tissues form (called morphogenesis) are less well understood, although this pattern of generalized-to-localized gene expression is encountered frequently. Drosophila development, images of fruit-fly anatomy and development, images of mammalian embryos, and many resources on development biology

Single-celled embryo

Egg-polarity genes I

Determination of major body axes

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