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DIFFERENTIATION AND DEVELOPMENTAL MUTATIONS

Unicellular organisms have little or no differentiation, whereas, the multicellular higher plants and animals have a variety of cells which originate from a single cell (zygote) by a highly ordered and coordinated process of differentiation and development. The information for the differentiation and development of (ill organism from the zygote is contained in the DNA that is expressed in a regulated manner to produce different body parts. Many factors control the process of differentiation and development, these are:

1. Nucleus. Nucleus is the main factor controlling differentiation and development of an organism that can be explained by the examples of clones developed by various scientists. Dolly, the sheep, was a clone developed by Ian Wilmut in 1997. Ian Wilmut isolated the nucleus from the cultured udder (milkbag) cell (somatic cell) of a female sheep and transferred this nucleus into the protoplasm of an enucleate~ unfertilized egg obtained from a second female sheep. This egg was then introduced into the uterus of a third female sheep and after the development of this egg into a fully developed foetus, this female sheep gave birth to Dolly. This lamb, Dolly, was clonal (genetically similar) to the first female who donated the nucleus. This indicates that the primarily, the nucleus contains the information for the development of a whole organism (Fig. 10.12 see Appendix)

2. Regulation of Gene Expression. In multicellular higher organisms, though all the cells have the same complete set of genes, but, at early stage of development of the embryo, different cells lose the potential for the expression of all the genes due to regulation of gene expression. Thus, various genes express differently in various cells of the embryo to give different kinds of proteins that control the process of differentiation. For example, when the nuclei were isolated from the intestinal cells (somatic cells) of the larva and adult frog, and the isolated nuclei were transformed separately into the enucleated protoplasm of the unfertilized eggs, the eggs containing the nucleus from the larva readily developed into the clonal frogs; whereas, the eggs having the nucleus from the intestinal cell of the adult frog did not develop readily into the clones (Fig. 10.13 see Appendix). Thus, the pluripotency (the ability of an animal cell to develop into a complete organism is lost upon maturation of an organism, as due to regulation of gene expression some genes are turned on whereas others are turned off permanently as per requirement.

3. Cytoplasmic Factors. The extranuclear genes (organeller genes), that are contained in the mitochondria or animals, and mitochondria as well as plastids or plants, also control the early development of an organism to some extent (Fig.10.14). It was found that the unequal distribution of mitochondria in the amphibian fertilized eggs controls the process of differentiation.

Fig.10.14
Fig. 10.14 Unequal distribution of mitochondria and yolk in amphibian egg.

4. Maternal Effect. In the fertilized egg of an organism, though, the nucleus contains one set of chromosomes from the paternal parent and the other set from the maternal parent, the cytoplasm is always derived from the maternal parent, This cytoplasm, in addition to containing organeller genes, also contain some maternal nuclear products (mRNA or proteins) that regulate the early embryonic development that is known as maternal effect (Fig. 10.15 see Appendix).

4. Environmental Factors. The environment also affects the development of an organism, for example, in Drosophila the normal body colour is brown, but when the normal larvae are fed upon the silver salt containing food, they develop into yellow flies. Some mutant flies of Drosophila are also reported that have yellow body colour. There two types of yellow flies (yellow mutant and the yellow flies reared on silver salt containing food), which are genotypically different but phenotypically same, are called phenocopies (Fig. 10.16).
Fig.10.16

Developmental Mutants
The differentiation and developmental processes of the organisms have not been fully understood. The homoerotic genes in organisms determine the body plan and mutation in these genes may lead to the transformation of one body part into other. Many developmental mutants of a number of organisms have been studied that have helped in understanding the process of differentiation and development to some extent.

For example, in Drosophila the antp gene of the antennapedia gene complex after mutation in homozygous condition (both the homologous chromosomes affected) leads to the conversion of antenna into legs.

Similarly, in a plant, Antirrhinum, the homoetic mutations lead to the transformation of one flower part into other, e.g., in apetala mutants the petals get transformed into bracts or stamens (male flower part) or carpels (female flower part).

QUESTIONS

1. What do you understand by spontaneous and induced mutations?
or
Write in brief the possible source and mechanism of spontaneous mutations in nature.
or
Discuss different kinds of radiations and chemical mutagens utilized for induction of mutations.
or
How transitions and transversions can be induced by chemical and physical mutagens?

2. How can base substitutions be induced by a base analogue?
3. Discuss the mechanism of the mutagenic action of an alkylating agent.
4. How do ultraviolet radiations bring about changes in genetic material? How can the effect of uy be modified due to exposure to light?
5. Write short notes on:
a. Frame shift mutations
b. Acridine dyes
c. Somatic mutation