21.1 Many plants can be cloned from isolated single cells.

Thus none of the original genetic material is lost during development.

Steward concluded that each phloem cell contained the genetic potential for a whole plant; none of the original genetic material was lost during determination.

The results of later studies demonstrated that most animal cells also retain a complete set of genetic information during development. In 1952, Robert Briggs and Thomas King removed the nuclei from unfertilized oocytes of the frog Rana pipiens. They then isolated nuclei from frog blastulas (an early embryonic stage) and injected these nuclei individually into the oocytes. The eggs were then pricked with a needle to stimulate them to divide. Although most were damaged in the process, a few eggs developed into complete tadpoles that eventually metamorphosed into frogs.

In the late 1960s, John Gurdon used these methods to successfully clone a few frogs with nuclei isolated from intestinal cells of tadpoles. This accomplishment suggested that the differentiated intestinal cells carried the genetic information necessary to encode traits found in all other cells. However, Gurdon's successful clonings may have resulted from the presence of a few undifferentiated stem cells in the intestinal tissue, which were inadvertently used as the nuclei donors.

In 1997, researchers at the Roslin Institute of Scotland announced that they had successfully cloned a sheep by using the genetic material from a differentiated cell of an adult animal. To perform this experiment, they fused an udder cell from a white-faced Finn Dorset ewe with an enucleated egg cell and stimulated the egg electrically to initiate development. After growing it in the laboratory for a week, they implanted the embryo into a Scottish black-faced surrogate mother. Dolly, the first mammal cloned from an adult cell, was born on July 5, 1996 (IFigure 21.2). Since

21.2 In 1996, researchers at the Roslin Institute of Scotland successfully cloned a sheep named Dolly. They used the genetic material from a differentiated cell of an adult animal.

the cloning of Dolly, other sheep, mice, and calves have been cloned from differentiated adult cells.

These cloning experiments demonstrated that genetic material is not lost or permanently altered during developmentā€”development must require the selective expression of genes. But how do cells regulate their gene expression in a coordinated manner to give rise to a complex, multicellu-lar organism? Research has now begun to provide some answers to this important question. __

Concepts B

The ability to clone plants and animals from single specialized cells demonstrates that genes are not lost or permanently altered during development.

www.whfreeman.com/pierce Information about cloning, nuclear transfer research, and about the ethics of cloning

The Genetics of Pattern Formation in Drosophila

One of the best-studied systems for the genetic control of pattern formation is the early embryonic development of Drosophila melanogaster. Geneticists have isolated a large number of mutations in fruit flies that influence all aspects of their development, and these mutations have been subjected to molecular analysis, providing much information about how genes control early development in Drosophila.

The development of the fruit fly An adult fruit fly possesses three basic body parts: head, thorax, and abdomen (I Figure 21.3). The thorax consists of three segments: the first thoracic segment carries a pair of legs; the second thoracic segment carries a pair of legs and a pair of wings; and the third thoracic segment carries a pair of legs and the halteres (rudiments of the second pair of wings found in most other insects). The abdomen contains nine segments.

When a Drosophila egg has been fertilized, its diploid nucleus (IFigure 21.4a) immediately divides nine times without division of the cytoplasm, creating a single, multinu-cleate cell (IFigure 21.4b). These nuclei are scattered throughout the cytoplasm but later migrate toward the periphery of the embryo and divide several more times (iFigure 21.4c). Next, the cell membrane grows inward and around each nucleus, creating a layer of approximately 6000 cells at the outer surface of the embryo (I Figure 21.4d). Four nuclei at one end of the embryo develop into pole cells, which eventually give rise to germ cells. The early embryo then undergoes further development in three distinct stages: (1) the anterior-posterior axis and the dorsal-ventral axis of the embryo are established (I Figure 21.5a); (2) the number and orientation of the body segments are determined (I Figure 21.5b); and (3) the identity of each individual

| Within a few hours, segmentation appears.

^ The embryo develops into a larva that passes through three stages.

| Within a few hours, segmentation appears.

^ The embryo develops into a larva that passes through three stages.

9 days

21.3 The fruit fly, Drosophila melanogaster, passes through three larval stages and a pupa before developing into an adult fly.

The three major body parts of the adult are head, thorax, and abdomen.

segment is established (I Figure 21.5c). Different sets of genes control each of these three stages (Table 21.1).

Egg-polarity genes The egg-polarity genes play a crucial role in establishing the two main axes of development in fruit flies. You can think of these axes as the longitude and latitude of development: any location in the Drosophila embryo can be defined in relation to these two axes. There are two sets of egg-polarity genes: one set determines the anterior-posterior axis and the other determines the dorsal-ventral axis. These genes work by setting up concentration gradients of morphogens within the developing embryo. A morphogen is a protein whose concentration gradient affects the developmental fate of the surrounding region.

The egg-polarity genes are transcribed into mRNAs during egg formation in the maternal parent, and these mRNAs become incorporated into the cytoplasm of the egg. After fertilization, the mRNAs are translated into proteins that play an important role in determining the anterior-posterior and dorsal-ventral axes of the embryo. Because the mRNAs of the polarity genes are produced by the female parent and influence the phenotype of their offspring, the traits encoded by them are examples of genetic maternal effects (see p. 000 in Chapter 4).

Egg-polarity genes function by producing proteins that become asymmetrically distributed in the cytoplasm, giving the egg polarity, or direction. This asymmetrical distribution may take place in a couple of ways. The mRNA may be localized to particular regions of the egg cell, leading to an abundance of the protein in those regions when the mRNA is translated. Alternatively, the mRNA may be randomly distributed, but the protein that it encodes may become asymmetrically distributed, either by a transport system that delivers it to particular regions of the cell or by its removal from particular regions by selective degradation.

Determination of the dorsal-ventral axis The dorsalventral axis defines the back (dorsum) and belly (ventrum) of a fly (see Figure 21.5). At least 12 different genes determine this axis, one of the most important being a gene called dorsal. The dorsal gene is transcribed and translated in the maternal ovary, and the resulting mRNA and protein are transferred to the egg during oogenesis. In a newly laid egg, mRNA and protein encoded by the dorsal gene are uniformly distributed throughout the cytoplasm but, after the nuclei migrate to the periphery of the embryo (see Figure 21.4c), Dorsal protein becomes redistributed. Along one side of the embryo, Dorsal protein remains in the cytoplasm; this side will become the dorsal surface. Along the other side, Dorsal protein is taken up into the nuclei; this side will become the ventral surface. At this point, there is a smooth gradient of increasing nuclear Dorsal concentration from the dorsal to the ventral side (I Figure 21.6).

diploid zygote 0 sPerm and egg nuclei fuse to ---create a single-celled diploid zygote.

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