12.8Species and Sexes
This book started with the diversity and uniformity of organisms in Chapter 1, and now ends with the passing of genetic information from parents to children in this chapter. All the knowledge presented demonstrates that organisms are amazing entities that, while sharing similar basic apparatus, have evolved in hugely diverse ways. An important function of organisms is their self-replication. However, if organisms that emerged in ancient times had simply continued to replicate themselves, the diversity of organisms we see today would not have developed. The biodiversity that currently exists on the earth is proof that organisms have not only replicated but have also modified their genes during the course of their evolution. Organisms originating in the sea altered the global environment and found their way onto land and to the sea bottom, where they used various strategies to secure species-specific niches (or habitats). To expand a niche in manifold environments, it is advantageous for organisms to increase their intraspecies diversity, and one strategy for it is to acquire sexes and develop sexual reproduction. The existence of sexes allows the mixing of genes among many individuals of the same species, while sexual reproduction allows genetic progeny, in which the genes of both parents are mixed through many combinations of homologous chromosomes and through genetic recombination by crossover.
Here in the 21st century, we are fortunate to have genetic blueprints created by decoding the complete genome sequences of many organisms, and such genetic information for many more species will be published in the future. By comparing the blueprints of multiple organisms, strategies to transfer specific genes to progeny using sexes may be decoded in the not-too-distant future.
Clones are groups of organisms with identical DNA sequences. It is easy to produce clones in plants (as demonstrated in potatoes and by grafting), and bees clone themselves by parthenogenesis. Is it then possible to clone humans? An adult human consists of 60 trillion cells; these include somatic cells, which last only for one generation, and germ-line cells, which survive to the next generation by producing ova and sperms. Gametes are not clones, since different gene sets are created through meiosis (as already discussed). Is it possible to produce clones using somatic cells without going through these specialized reproduction cells? It has long been known that individual animals can be produced by transplanting the somatic nucleus to a nucleus-removed, unfertilized ovum in frogs. In February 1997, the birth of a cloned sheep named Dolly was widely publicized. In this case, the nucleus of a mammary cell (a somatic cell) from a female sheep was implanted into a nucleus-removed, unfertilized ovum, which was then implanted into a surrogate female sheep, thus producing a cloned sheep with a gene composition identical to that of its mother (Column Fig. 12-1). Clones have now been successfully created in many animals including cattle and pigs, demonstrating that cloning is possible in mammals and indicating that there are few biological barriers to human cloning. Another important conclusion from these results is that even somatic cells have all the information necessary in their nucleus to produce a complete mammal. In other words, the various cells that make up an organism have all the information needed to produce the organism, but they express only part of this information, and the expression of other genes is masked (or suppressed) (see Chapter 4).
A knockout mouse is one in which certain gene functions have been genetically disrupted through a method called targeting. As shown in Column Figure. 12-2, using the characteristic of the recombination that tends to occur between DNA regions with similar gene sequences (homologous recombination), a normal gene in an ES cell (embryonic stem cell) is replaced with a gene in the knockout vector (targeting vector) prepared in advance. In the following cell selection, cells that did not perform homologous recombination and those in which the vector was incorporated into the wrong chromosome are removed, and only ES cells that had the gene inserted correctly and underwent homologous recombination are obtained. These cells are implanted into the early embryo, and it is then implanted into the uterus of a pseudopregnant mouse, thereby producing chimeric mice consisting of cells with and without the target gene. Homozygous knockout mice are then produced by crossing these mice with other mice. The functions of the removed gene are identified by observing the phenotypes (traits) of the mice thus created.
Recently, a method known as conditional knockout has also been used, in which gene functions are disrupted only during certain stages or in certain organs. This technique enables the production of mice that cannot be created using the constitutive knockout method (in other words, the target gene participates in development) as well as the observation of the gene functions in adult mice.
Column Fig. 12-2. Method to select cells that have performed homologous recombination
Cells that have not performed recombination die as a result of adding the antibiotic neomycin. Cells in which the targeting vector has entered the wrong chromosome also die from the addition of ganciclovir (gcv), which inhibits DNA polymerase, due to the existence of the thymidine kinase (TK) gene in the vector. Only cells that performed homologous recombination survive.