Giant chromosomes structure

Nuclear Differentiation Starts in Early Development Chromosome Structure Varies with Gene Activity Giant Chromosomes Permit Direct Visualization of Active Genes... 

Puffs are not fixed structures. In the course of development puffs that are already in existence on a given giant chromosome contract and new ones are formed. Differences in puff formation on any given giant chromosome are also found between one tissue and another. In puff patterns of this kind, which are specific for the tissue and stage of development, differential gene activity becomes microscopically visible. 

A giant chromosome from the suspensor of Phaseolus coccineus. The local loosenings of the structure correspond to the formations of puffs (from Nagl 1970). 

The biochemical functions of the nucleus are related primarily to its content of genetic material, i.e. the chromosomes, containing deoxyribonucleic acids. The fine structure of the chromosomes can best be studied in the giant chromosomes of cer-... 

The nuclear content of a normal cell (without giant chromosomes) does not reveal any details with the electron microscope it consists merely of granular or fibrillar structures. Deoxyribonucleic acid usually cannot be distinguished. The nuclear membrane consists of two lamellae which enclose the so-called perinuclear space. The latter is in contact with the cisternae of the ergastoplasm (see below). In the nuclear membranes pores can be seen that are usually closed by a very fragile diaphragm. 

With the exception of certain viruses, the blueprint for all organisms is contained in code by deoxyribonucleic acid (DNA), a giant macro-molecule whose structure allows a vast amount of information to be stored accurately. We have all arisen from a single cell, the fertilized ovum containing two sets of DNA (packaged with protein to form chromatin), one set from our mother, resident in the nucleus of the unfertilized ovum, the second set from our father via the successful sperm. Every cell in the adult has arisen from this one cell and (with the exception of the germ cell and specialized liver cells) contains one copy of these original chromosome sets. 

Moreover, it is giant molecules, presumably molecules of nucleoprotein, that determine the characters of individual living organisms and that are involved in the transmission of these characters to their progeny. These giant molecules are the genes, which are usually present in structures in the cell called chromosomes. 

The part of the band which contains the structural gene is located near the interband. Part of the band contains repeated sequences which separate the unique sequences from each other. The giant palindrome which corresponds to the 5 S RNA genes in Drosophila melanogaster was demonstrated to be associated with a band on the polytene chromosomes (Cohen, 1976). 

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