|INDEX > Rearrangement of DNA|
|Key terms defined in this section|
|Amplification refers to the production of
additional copies of a chromosomal sequence, found as intrachromosomal or
Transgenic animals are created by introducing new DNA sequences into the germline via addition to the egg.
Although genomic DNA is usually unaltered by somatic development, there are some cases in which sequences are moved within a genome, modified, amplified, or even lost, as a natural event. In this chapter, we discuss a variety of such events in yeast, plants, and lower eukaryotes. Examples of rearrangement or loss of specific sequences are especially common in the lower eukaryotes. Usually these changes involve somatic cells; the germline remains inviolate. (However, there are organisms whose reproductive cycle involves the loss of whole chromosomes or sets of chromosomes.) We also discuss the introduction of new sequences into the genome by experimental means. Reorganization of particular sequences is rare in animals, although an extensive case is represented by the immune system. In 24 Immune diversity, we discuss the rearrangement and expression of immunoglobulin genes.
We may distinguish two broad consequences of a gene rearrangement:
Yeast mating type switching and trypanosome antigen variation share a similar type of plan in which gene expression is controlled by manipulation of DNA sequences. Phenotype is determined by the gene copy present at a particular, active locus. But the genome also contains a store of other, alternative sequences, which are silent. A silent copy can be activated only by a rearrangement of sequences in which it replaces the active gene copy. Such a substitution is equivalent to a unidirectional transposition with a specific target site.
The simplest example of this strategy is found in the yeast, S. cerevisiae. Haploid S. cerevisiae can have either of two mating types. The type is determined by the sequence present at the active mating type locus. But the genome also contains two other, silent loci, one representing each mating type. Transition between mating types is accomplished by substituting the sequence at the active locus with the sequence from the silent locus carrying the other mating type.
A range of variations is made possible by DNA rearrangement in the African trypanosomes, unicellular parasites that evade the host immune response by varying their surface antigens. The type of surface antigen is determined by the gene sequence at an active locus. This sequence can be changed, however, by substituting a sequence from any one of many silent loci. It seems fitting that the mechanism used to combat the flexibility of the immune apparatus is analogous to that used to generate immune diversity: it relies on physical rearrangements in the genome to change the sequences that are expressed.
Another means of increasing genetic capacity is employed in parasite- or symbiote-host interactions, in which exogenous DNA is introduced from a bacterium into a host cell. The mechanism resembles bacterial conjugation. Expression of the bacterial DNA in its new host changes the phenotype of the cell. In the example of the bacterium Agrobacterium tumefaciens, the result is to induce tumor formation by an infected plant cell.
Alterations in the relative proportions of components of the genome during somatic development occur to allow insect larvae to increase the number of copies of certain genes. And the occasional amplification of genes in cultured mammalian cells is indicated by our ability to select variant cells with an increased copy number of some gene. Initiated within the genome, the amplification event can create additional copies of the gene that survive in either intrachromosomal or extrachromosomal form.
When extraneous DNA is introduced into eukaryotic cells, it may give rise to extrachromosomal forms or may be integrated into the genome. The relationship between the extrachromosomal and genomic forms is irregular, depending on chance and to some degree unpredictable events, rather than resembling the regular interchange between free and integrated forms of bacterial plasmids.
Yet, however accomplished, the process may lead to stable change in the genome; following its injection into animal eggs, DNA may even be incorporated into the genome and inherited thereafter as a normal component, sometimes continuing to function. Injected DNA may enter the germline as well as the soma, creating a transgenic animal. The ability to introduce specific genes that function in an appropriate manner could become a major medical technique for curing genetic diseases.
The converse of the introduction of new genes is the ability to disrupt specific endogenous genes. Additional DNA can be introduced within a gene to prevent its expression and to generate a null allele. Breeding from an animal with a null allele can generate a homozygous "knockout," which has no active copy of the gene. This is a powerful method to investigate directly the importance and function of a gene.
Considerable manipulation of DNA sequences therefore is achieved both in authentic situations and by experimental fiat. We are only just beginning to work out the mechanisms that permit the cell to respond to selective pressure by changing its bank of sequences or that allow it to accommodate the intrusion of additional sequences.