Genetics

G

Genes in Development

Gene regulation helps individual cells within an organism function in a specialized way. Other regulatory mechanisms coordinate the genes that determine how cells develop. All of the specialized cells in an organism, including those of the skin, muscle, bone, liver, and brain, derive from identical copies of a single fertilized egg cell. Each of these cells has the exact same DNA as the original cell, even though they have vastly different appearances and functions. Genes dictate how these cells specialize.

Early in an organism’s embryonic development the overall body plan forms. Individual cells commit to a particular layer and region of the embryo, often migrating from one location to another to do so. As the organism grows, cells become part of a particular body organ or tissue, such as skin or muscle. Ultimately, most cells become highly specialized—not only to develop into a neuron rather than a muscle cell, for example, but to become a sensory neuron instead of a motor neuron. This process of specialization is called differentiation. At each stage of the differentiation process, specific genes known as developmental control genes actively turn on and switch off the genes that differentiate cells.

One class of developmental control genes, known as homeotic genes, directs the formation of particular body parts. Activating one set of homeotic genes instructs part of an embryo to develop into a leg, for example, while another set initiates the formation of the head. If a homeotic gene becomes altered or damaged, an organism’s body development can be dramatically disrupted. A change in a single gene in some insects, for instance, can cause a leg to grow where an antenna belongs.

Homeotic genes work by regulating the activity of other genes. Homeotic genes code for the production of a regulatory protein that can bind to DNA and thus affect the transcription of one or more genes. This enables homeotic genes to initiate or halt the development and specialization of characteristics in an organism.

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Nearly identical homeotic genes have been identified in varied organisms, such as insects, worms, mice, birds, and humans, where they serve similar embryonic development functions. Scientists theorize that homeotic genes first appeared in a single ancestor common to all these organisms. Sometime in evolutionary history, these organisms diverged from their common ancestor, but the homeotic genes continued to be passed down through generations virtually unchanged during the evolution of these new organisms.

IV

How Scientists Work with Genes

Scientists have developed a number of biochemical and genetic techniques by which DNA can be separated, rearranged, and transferred from one cell to another. Some of these laboratory methods help scientists study the properties of genes in nature—for example, by comparing DNA from different animals to find out whether those animals are closely related to each other or only distant relatives. Other DNA techniques provide tools for genetic engineering—the alteration of genes in an organism. These tools are used in industry to develop commercial products, such as hardier crops, microbes that can break down oil slicks or decompose garbage, and improved medicines.

A

Recombinant DNA

The DNA molecules of all life forms, from oak trees to sea horses, have the same structure and the same four bases. Scientists have made use of these similarities in a technology called recombinant DNA. In this laboratory method, one or more genes of an organism are introduced into a second organism. The new genes, sometimes known as foreign DNA, become functional in the second organism and produce a desired protein. In this way, scientists can create changes in the genetic makeup of an organism that would be unlikely to occur through natural processes.

Scientists use recombinant DNA when they want to obtain large amounts of a protein, such as insulin, produced by a gene. Insulin was once in short supply for diabetics, whose bodies lack adequate supplies. Insulin supplies were derived from cows in an expensive and time-consuming process. Today recombinant DNA techniques produce insulin cheaply and in abundance. The first step in creating insulin using recombinant DNA is to isolate the sequence of nucleotides in the DNA of a human cell that forms the insulin gene. Scientists use restriction enzymes, specialized proteins that act like molecular scissors, to cut the double-stranded DNA at the point where the insulin gene occurs. The isolated DNA can then be recombined, or spliced, with a vector, a fragment of DNA that is able to transport genes from one organism to another. A vector may be a plasmid, a small, circular segment of DNA found in bacteria. Bacteriophages, viruses that are parasites of bacteria, also act as vectors.

Scientists insert the vector containing the insulin gene into a bacterium, such as E. coli. Within just a few hours, a single E. coli will reproduce hundreds of times to make millions of cells, all containing exact copies of the insulin-producing gene inserted by the scientists. This process of making many cells with identical DNA is known as cloning.

B

DNA Libraries

A DNA library is a storehouse of genetic information maintained in bacteria instead of books. These bacteria are clones created by recombinant DNA, and the foreign DNA they hold is the library’s store of information. DNA libraries are helpful to scientists who require a plentiful supply of particular DNA segments to do their work. These repositories of genetic information are stored in small tubes, which can easily be shipped to other researchers for study.

Each library has a unifying theme. For example, a library may contain the entire chromosomal DNA, or genome, of a given organism, or it may consist of genes that are active within certain types of cells, such as heart cells. To create a library of the human genome, DNA from all the human chromosomes would be cut into many pieces. These pieces would be randomly inserted into vectors, such as plasmids, which would then be placed into a population of bacteria. Taken together, the entire population of bacteria would contain all the DNA of the human chromosomes.

C

Polymerase Chain Reaction

Polymerase chain reaction (PCR) offers an alternative to vector-based cloning as a means of generating numerous copies of DNA from a small initial sample. Performed in a test tube, PCR mirrors the way in which DNA is replicated within a cell. To perform PCR, scientists isolate the piece of DNA to be amplified (multiplied) in a test tube and heat it to separate the two strands of the molecule. As cooling occurs, short pieces of DNA called primers are added to the test tube. The primers attach to each strand, marking the segment that will be cloned. Free-floating nucleotides and an enzyme called DNA polymerase are then added to the mixture. DNA polymerase uses the free-floating nucleotides to build a complementary copy of each amplified DNA segment, resulting in two new double-stranded DNA molecules. Each cycle of heating and cooling doubles the amount of the desired DNA fragment in the test tube. In a matter of hours, scientists can obtain millions of copies of a desired piece of DNA. PCR enables scientists to amplify traces of DNA found at a crime scene or in a fossil animal to produce sufficient quantities to study.

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