It is favourable to observe chromosomes during metaphase when both chromatides are still joined. They appear as X- or V-shaped bodies and can thus easily be counted. Structural changes or missing chromosomes can be detected, too. Already the first light-microscopic observations of chromosomes showed that they differ structurally. Sometimes, structures occur at the ends of single chromosomes that are called satellites.
Usually, the chromosomes of a haploid set are
graded and numbered according to their decreasing length. Such a
characterization of a species' chromosomes is called a
karyotype. Chromosome 1 is
normally the longest. X- and Y-chromosomes are depicted last. In some
species, the X-chromosome is bigger than chromosome 1. The
Y-chromosome can be very small. But in most higher plants, the sex
chromosomes cannot be distinguished structurally.
B. McCLINTOCK (Carnegie Institution of Washington) clarified
during the late twenties and the beginning thirties the identity of
the maize chromosomes (n = 10). She found out that chromosome 1 has
more than twice the length of chromosome 10 and that the long and the
short arm of chromosome 5 are of nearly equal length. The long arm of
chromosome 6 is seven times as long as the short one. Chromosome 6
has at its end a knob-like, final structure, a satellite DNA. Several
chromosomes are regularly thickened in certain domains. Linkage group
1 (see section above) could be assigned to chromosome 9.
The confusion with the numbering is caused by the fact that
chromosomes are numbered according to their size, while linkage
groups were numbered according to the sequence of their
discovery.
Both the knowledge about the chromosomal structures and the gene
maps of maize grew over the years. Some much less complete maps of
Antirrhinum majus and several cereals (Triticum
aestivum and others) exist. But even the maps of maize are far
less detailed than those of Drosophila melanogaster or humans.
This may be astonishing regarding the earliness of the first plant
maps. But it is due to the fact that animal gene maps can nowadays be
drawn up with the help of cell cultures, like, for example, the
analysis of fusion products of human DNA with the help of mice cells.
The interest in human research is certainly bigger than that in plant
research and medicinal research is accordingly supplied with much
bigger financial means. The other reason is that the methods used for
the mapping of animal cells cannot be transferred to plant cells. The
situation is different with genetic methods and the sequencing of
DNA. The amount of data gained here is still growing exponentially
and the research into plant genomes is pursued with increasing
interest. The object of choice is a species with a rather small
genome, the crucifera Arabidopsis
thaliana.
The classic method of gene mapping has thus been superseded. Soon, Arabidopsis will be the first plant whose genome is completely known. Details of gene maps are collected in data bases and can be called up via internet at any time, for example under the addresses for:
maize - http://www.agron.missouri.edu.
Dyes, like carminic acetic acid or orceine can be used to stain
certain domains of a chromosome. The
resulting pattern is characteristic for the respective chromosome of
a species. During interphase, the chromosomal structure is usually
resolved. The intensity of the nuclear staining becomes feebler and
less uniform than that of the chromosomes. The stainable substance
has been called chromatin by E.
HEITZ (formerly at the Botanical Institute of the University of
Hamburg, 1927, 1929). He distinguished between
heterochromatin and
euchromatin. Heterochromatin are
all the intensely stained domains, euchromatin the diffuse ones.
Heterochromatin is usually spread over the whole nucleus and has a
granular appearance. It is known today that the heterochromatic
domains are those where the DNA is tightly packed (strongly
condensed) which is the reason for their more intense staining. The
euchromatic domains are less tightly packed.
Since several years, highly specific fluorescent dyes are used to stain chromosomes. They permit the study of chromosomal details, for example the distribution of euchromatin and heterochromatin within the nucleus. With their help, it was possible to show that heterochromatin is no uniform fraction but that several types of heterochromatin with different staining behaviour exist. Usually, clear bands are observable on the chromosomes after staining.
These bands are specific and allow the identification of single chromosomes. Chromosomal preparations are often treated with protein-degrading enzymes, proteases, before staining in order to make the bands appear stronger. The simultaneous use of two fluorochromes may also facilitate a better analysis of the results. One dye could, for example, be better suited to stain euchromatin, while the other has a stronger affinity to heterochromatin.
Modern methods are not only important for the characterization of single chromosomes, they do also allow the study of the fate of single domains during the transition of the chromosomal to the interphase state of the DNA. In this way, those parts of the nucleus can be found that correspond to certain chromosomes. Furthermore, changes of the chromosomal structure (inversions, duplications, etc.; see the following section) as well as numerical aberrations can easily be detected. It should be mentioned that some dyes are better suited for animal than for plant chromosomes.
During certain phases in the development of a few species, special chromosomes are formed. Their DNA is mostly decondensed and organized into loops that are arranged around a central axis. Such lamp-brush chromosomes are typical for the oocytes of amphibians. They occur also in the siphonal green alga Acetabularia that has especially large cells. It became the ideal specimen for the study of selective gene activity.
Polytene chromosomes are a peculiarity of rather unspecialized cells. They occur in some animal and plant groups (e. g. in some dipteran species, in protozoa and beans). Normal chromosomes consist of one or two, polytene chromosomes of a whole range of chromatides (around 1000). The polytene character is kept during interphase.
Picture to the left: Salivary glands of Drosophila melanogaster. The DNA of the nuclei was stained with the fluorochrome ethidium bromide. The difference in size of the rather few polytene nuclei in the salivary glands and the diploid nuclei of the neck region of the salivary gland can clearly be recognized (M. JAMRICH, Heidelberg, 1978).
Often, but not always, are homologous chromosomes paired (in Drosophila, for example). The X-chromosome of the males is easily spotted, since it has only half the thickness of the others due to the missing homologous chromosome. All polytene chromosomes of Drosophila are connected at their centromeres. The centromere domains are strongly heterochromatic and are also called chromocenters. Polytene chromosomes can be recognized by their clear and regular bands, the pattern is highly specific. In Drosophila , every band has been identified, classified and numbered. The bands of the polytene chromosomes have nothing to do with the bands of ordinary chromosomes about which we have just talked. They can be seen without any staining. The bands of normal chromosomes however are a kind of artefact: they can only be observed after staining. Polytene chromosomes became a suitable specimen for the study of changes of the chromosomal structure and its consequences.
More about the polytene chromosomes of plants can be found in the illustrated minireview of W. NAGL, University of Kaiserslautern:
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