Nucleus, diploid

Briggs and King were able to grow a differentiating embryo from an egg of Rana pipiens, the chromosomes of which were replaced with a single diploid nucleus from another embryo. What does this tell us about the state of the nucleus in the developing embryo ... 

Diploid—Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei. 

Cytokinesis—The physical division of the cytoplasm of a eukaryotic cell to form two daughter cells, each housing a newly formed nuclei. Diploid—Nucleus or cell containing two copies of each chromosome generated by fusion of two haploid nuclei. 

Figure 11.2-4. Comparison of therapeutic cloning and reproductive cloning. In therapeutic cloning (left panel), the diploid nucleus of an adult donor cell is introduced into an enucleated oocyte and it divides into cloned blastocysts that will provide lines of embryonic stem cells that can be differentiated in vitro into any types of cell for therapeutic purposes. In contrast, reproductive cloning (right panel) requires the transfer of the cloned blastocysts into surrogate mothers and a few of the cloned blastocysts will give rise to the cloned animals.

A Neurospora cell contains many nuclei they can be of the same type (a homokaryon) or of different type (a heterokaryon). A heterokaryon formed between two strains carrying different recessive lethal mutations will usually have wild-type phenotype. Thus, the heterokaryotic stage effectively mimics a diploid nucleus heterozygous for recessive genes. 

If haploid cells fuse, nuclear fusion follows and a diploid nucleus (one containing two sets, twice the haploid number, of chromosomes) is formed. The immediate product of such a fusion process is termed a zygote. From the zygote develop diploid cells which reproduce vegetatively by budding the nucleus divides mitotically and the cells are said to be in the diplophase. 

Direct measurements of the DNA content per diploid nucleus of various somatic cell types of an organism reveal equal amounts of DNA (Boivin et al., 1948 Mirsky and Ris, 1949 Vendrely, 1955). Only those tissues which contain polyploid or polytene cells show an increased amount of DNA compared to the 2C value present in normal diploid cells. These exceptional cases of increased DNA content have often been interpreted as a result of several additional rounds of replications of the entire genome. Provided that this assumption is correct, such cells would merely differ in quantity and not in quality from the DNA content of diploid somatic cells. 

In spite of the fact that in any differentiated cell many of the genes appear to be completely non-functional, all nuclei are equipotential. This was shown by some illuminating experiments carried out by Gurdon and his colleagues in 1966. They destroyed the nucleus of frogs ova by ultraviolet irradiation and introduced in its place a nucleus removed by microdissection from the columnar epithelium of the intestine of the same species (i.e. a diploid nucleus from a differentiated cell). Subsequently a number of these experimentally treated ova developed into normal adult frogs, some of which were shown to be fertile. 

The quantity of DNA contained in the ordinary nucleus usually exceeds these calculated values considerably. The spermatozoon of the trout contains 6.4 x lO mg DNA with a mean molecular weight of 6 X 10 (Sadron et al., 1957), equivalent to 640,000 DNA molecules. Because of the possible existence of surplus DNA, there is no direct relationship between the mass of DNA in the nucleus and the animaPs position in the scale of evolution. The DNA content in the human spermatozoon, for example, is less (about 2.8 X 10" mg) than in the spermatozoon of the trout. Du Praw (1965) has calculated that if all DNA molecules of one diploid nucleus of a human cell were to be joined together into a single chain its length would be about 180 cm. 

So far as changes in DNA in connection with differentiation are concerned, it is clear that no substantial changes can be expected in the qualitative composition of DNA because the somatic cells retain their diploid set of chromosomes and the DNA content per diploid nucleus usually is unchanged during differentiation. 

In subsequent experiments (McIQnnell, 1962), when the recipient oocyte received a diploid nucleus from an adrenocarcinoma cell, the tadpoles which were obtained, although possessing anomalies, were capable of swimming and possessed well-differentiated tissues skin, brain, muscles, notochord, and so on. In this case a nucleus which had previously been a nucleus of a kidney cell, when transplanted into the cytoplasm of the oocyte, led to the formation of many types of specialization. Its total genetic material had not undergone irreversible inactivation. 

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