juliana

increase of ploidity (4, 8, 16, 32, 64, etc.), the chromatids remain bound to
one another and the so-called polythene chromosomes are obtained.
Sometimes the chromatid number in them is so high, that it is almost
impossible to differentiate between the individual chromosomes in the
nucleus.
Polythene chromosomes represent a particular case of polyploidy
since they permanently exist in an interphase, that is unfolded state which
renders them ideal for the study of their structural organization and
functions. They consist of consecutively arranged intensively stained discs
and light interdisc spaces. At certain moments some of the discs swell and
form spheric swellings denoted as puffs, the most typical of them called
Balbiani rings after the name of their discoverer (Fig. 2–25). The
processes taking place in them are related to intensive RNA synthesis and
specific enzymes.

image

Figure 2–24. Set of four giant chromosomes from a cell of the salivary 
gland of Chironomus tentans (After Beermann and Clever, 1964). 

image

image

Figure 2–25. Chromosome puffs in giant chromosomes from a salivary 
gland cell of Chironomus tentans. The larger of them are called Balbiani 
rings (After Beermann and Clever, 1964).

The lampbrush chromosomes (Fig. 2–26) are observed during the
meiosis in oocytes of fish, crabs, etc. Recently they have been provoking
certain interest from an evolutionary point of view after it has been
established that they are devoid of DNA or its content is very low in them.

In the literature there are met over 5000 types of monogenic disorders
due to disturbances in the functions of only one gene are known by now.
More widely spread among them are thalassemia, phenylketonuria,
mucoviscidosis, hemophilia, etc.
Down’s syndrome is the most widely spread chromosome ailment in
the human population. It is caused by the trisomy of chromosome 21
(Fig. 2–23). The formula of Down’s syndrome is 47, XY, +21 for boys
and 47, XX+21 for girls.

image

Figure 2–23. Trisomy of chromosome 21. Arrous indicate the three 
copies of the chromosome (Courtesy of A. Andreev, Laboratory of 
Cytogenetics at the State University Pediatric Hospital, Sofia).

As was mentioned in Section 2. 5 giant and lampbrush type chromosomes
have been discovered in the cell nuclei from tissues specific of certain organisms
(Balbiani, 1881; Flemming, 1882). The interest towards them has considerably
grown in half a century when they were rediscovered by a number of authors
(Painter, 1933—35; Heitz, Bauer, 1933; King, Beams, 1934; Bridges, 1935; Bauer,
1935, 1936, etc.). Such chromosomes have been observed in the salivary glands
of Drosophila, Chironomus and some other two-winged insects of the Diptera
order.
The giant chromosomes (Fig. 2–24) are formed as a result from the
manifold replication cycles in them, which are not accompanied by release
and distribution of the chromatids in the daughter cells as it is normally
taking place in mitosis and cytokinesis. That is why there is a multiple

image

image

Figure 2–22. Normal karyotype in men (a) and in women (b). Arrows 
indicate the X- and Y-sexual chromosomes (Courtesy of A. Andreev, Laboratory
 of Cytogenetics at the State University Pediatric Hospital, Sofia). 

The term chromosomes was introduced by Wilhelm Waldeyer
(1888). The studies showed that these are complex nucleoprotein
structures differing in shape, size and functions. Each of them displays
specific configuration with a defined quantity of DNA, which is bound to
different basic proteins (protamins and histones). Some of these protein
molecules are wound around the DNA double helix or are situated in
its grooves, and replicate simultaneously with the DNA itself.

image

Figure 2–20. Chromosome set of 
Crepis capilaris at the time of the 
metaphase plate (After M. S. 
 Navashin; From Dubinin, 1976).

The number of chromosomes in the cells of a given species is
constant. Sexual cells normally possess a haploid chromosome set
(1n) while the somatic ones — diploid (2n). They are best observed under a
light microscope in the process of mitosis and meiosis especially at the
metaphase stage, when they are strongly spiralized and arranged in
the so-called metaphase plate. Owing to the high resolution of
electron microscopes, the replication of the chromatids has been proved in
an undisputable manner (Fig. 2–21). Human genome provokes an
understandable interest. The haploid chromosome set of the Homo sapiens
consists of 23 chromosomes and the diploid one, (called also karyotype) is
composed of 46 homologous chromosomes (Fig. 2–22).
Sex determination in the human also depends on combining the X
and Y-chromosomes. Changes in some of the chromosomes lead to
irreparable consequences — genetic and chromosome diseases, characterized
chiefly with physical abnor-malities and mental insufficiency.

image

Figure 2–21. Electron micrograph of 
human chromosome 12 with two 
chromatids (After DuPraw, 1970). 

of genomes (see Appendix 2) without demonstrating a clear evolutionary
connection between the number of chromosomes and the specificity of
organisms or the complexity of their organization.
From the data of Appendix 2 it is seen that the haploid chromosome
set (1n) varies in large proportions in the various types of organisms from 1
to 520. The apprehensions of some authors regarding the virtual nonexistence
of a minimum chromosome set of only one chromosome (1n = 1)
in eukaryotes proved to be in vain. Crosland and Grozier (1986)
demonstrated the availability of such a genome in Australian ant Myrmecia
pilosula (Fig. 2–19). According to these authors a report of one
chromosome in the nematode Parascaris equorum univalens was
presented by T. Boveri even in 1887, but has remained unnoticed.

image

Figure 2–19. Chromosomes from prepupal cerebral ganglia: (A) Worker ant 
prometaphase chromosomes. Identical C-banding provides evidence for 
homology of the two chromosomes; (B) Male prometaphase chromosome. 
Chromosomes consistently display a large centromeric C-band on the short 
arm and a smaller centromeric C-band on the long arm. Most of the short 
arm C-band is not immediately adjacent to the centromere, though a very 
small portion of the short-arm C-band is centromeric. Arrows indicate 
position of centromere (After Crosland and Grozier, 1986).

Genomes consisting of two chromosomes (1n = 2) are known in the
parasitic round worms (Ascaris), the green alga spirogyra (Spirogyra weberi),
yeasts (Saccharomyces pombe), the scorpio (Tityus bahiensis), etc. The
genomes of the orders from 10 to 30 chromosomes are prevalent however.
Huge deviations are observed in amoeba (Amoeba proteus), the crawfish
(Astacus trowbridgei) and the snake fern (Ophioglossum petiolatum), where
the chromosome number is respectively 250, 188 and 520.
In Figure 2–20 is presented the cytological picture of a cell from
through hawksbeard (Crepis capilaris) with a diploid chromosome set
consisting of 6 chromosomes or 3 homologous pairs. Except diploid as was
already mentioned, the chromosome set can be triploid (3n), tetraploid (4n)
or polyploid.

1 mm long DNA molecule is so “packed” that it can easily be
accomodated by the much smaller recipient cell (2—3 x 1—1.5 μm).
DNA-molecule is structured in domains, some of them transcribing, others
— non-transcribing. The replication starts at a defined spot called origin
(oriC) and stops at another called terminus (terC), being attached to the
cytoplasmic membrane. The very site where at the given moment the act
of replication is taking place (the so-called replication fork) is also
attached to the cytoplasmic membrane. The mode of replication of the
circular bacterial chromosome is shown in Figure 2–18. The causes for
the uneven distribution of the genes in the bacterial chromosome, the
presence of plasmids, mobile elements and transpozons and their role in
the bacterial genome are treated in detail in Prozorov’s review (1989).

image

Figure 2–18. Autoradiography of a circular DNA-molecule of E. coli K-12, labeled with
³H-thymidine for two generations. Two circular 
structures are seen bound by a common segment (From Cairns, 1963). 

In the cells of eukaryotic organisms, most of which are multicellular, the
chromosomes are located in the nucleus separated from the cytoplasm by a
well-defined nuclear membrane. In them nature has created a greater variety

image a

image b

Figure 2–17. A — elongated dividing cell of Proteus vulgaris on which the 
bright areas shown by figures are nucleoids (After Peshkov, 1955); B — 
chromatin bodies of Bacillus cereus (After Robinow, 1962; From Hayes, 
1964).

It was until recently believed that in the formation of the bacterial
genome, in contrast to the chromosomes of eukaryotic cells, histones and
proteins of that kind do not take any part. This was even accepted as the
main distinction between the bacterial nucleoid and the nucleus of
eukaryotic cells. Now there are data available about the presence of
histone-like proteins in the prokaryote genome as well, that evidently
participate in the formation of structures resembling those of the
nucleosomes in eukaryotes (Rouvière-Yaniv, Gros, 1975, Rouvière-Yaniv
et al, 1979; Hübscher et al, 1980). Along with RNA they play a definite
stabilization role in the genome organization. In E. coli the approximately

definite number of chromosomes serving the purpose of reproduction. The
term genome was introduced by the German biologist Hans Winkler in 1920
for denoting the totality of all genes in a haploid chromosome set. At present
the term genome encompasses DNA as well as RNA, when it is the major
hereditary material, plus the extranuclear DNA-molecules located in certain
cell organelles autonomously replicating.

Figure 2–16. Scheme of crossing-over (Adapted after Janssens, 1909).

In prokaryotic organisms (bacteria and blue-green algae) which do
not possess a well-defined nucleus with a nuclear membrane, the
genetic material represents a circular DNA-molecule forming the so-called
nucleoid. In bacterial cells for example, one, two or more
nucleoids are observed (Fig. 2–17 A, B) whose number varies depending
on their development stage and the cultivation conditions. In shape,
even in the same cell, they are very different — rounded, stick-like, V-shaped,
T-shaped, U-shaped, etc. Under the phase-contrast microscope
in non-stained living cells they look more diffuse rather than in the
processed stained preparations.

In some representatives of the Hymenoptera genus (bees, wasps, and
ants) the male individuals are normally haploid, while the female ones —
diploid. Sometimes the egg develops without any fertilization i.e.
parthenogenetically. All these and other deviations might be assumed
contradictory to the chromosome theory. They are possibly due to errors in
the process of meiosis or to genetic mechanisms remaining still unclear.
The emergence of new forms at the cell or organism level differing
from the ones existing in nature had made researchers think about their
genesis. One of the most accepted hypothesis at that time was that
genes can change, i.e. mutate thus generating new (mutant) genes. In
1908—1910 this hypothesis was subjected to a serious test in T.
Morgan’s Laboratory together with his collaborators C. Bridges, H. Muller
and A. Stertevent. As an object of the study they have used Drosophila
melanogaster which has only four homologous chromosomes. The first
mutant observed by them was with white eyes spontaneously emerged
in the collection of normally displaying red eyes. The gene determining
the red colour of the eyes was called wild type since it exists in nature
and the newly appeared gene controlling the white colour — mutant.
The mutant with white eyes was used for a crosses that have
yielded unexpected results. It proved that the mutant gene was
transmitted in the offspring with the X-chromosome, i.e. linked to the
sexual chromosome. The genes located in one and the same
chromosome transmitted to the offspring with the chromosome itself
were called linked genes. So in Drosophila there had been established
four groups of linked genes which correspond to the four different
chromosomes in its haploid chromosome set.
With the studies in that direction it became clear why in some cases an
independent distribution of the features, as established earlier by Mendel, is
not observed. It was found that the independent inheritance of the features
is due to the genes location on different chromosomes which in the process
of meiosis are distributed independently from one another, and when they
are linked and are on one and the same chromosome their distribution is
not independent.
Sometimes, however, two genes located on the same chromosome are
not distributed evenly in the process of meiosis. This has led researchers to
the thought of mechanisms, by the help of which an exchange of genes
between homologous chromosomes may be occurring. This process was
called crossing-over. The X-like figures thus formed were called chiasmata.
In 1909 the Belgian cytologist F. Janssens has made the suggestion that
chiasmata are related to the exchange of segments of the chromosomes. This
is presented in Figure 2–16. The results from the numerous studies in support
of the chromosome theory undoubtedly place the concept of the
chromosomes as bearers of heredity. Each cell both independently living or
included in the composition of a given organism has a specific genome with a

In this way, for the first time the link between a strictly defined
chromosome and a given hereditary feature has been shown. Besides, it
was clarified why male and female individuals are normally appearing in a
ratio approximately 1:1.
The problem of sex determination has been provoking a great interest
but it is not sufficiently clear up to day. The presented scheme is not
applicable to all species even within one and the same genus. Most often
supersex individuals appear displaying three X-chromosomes (XXX) or
intersex ones with a triploid (3n) instead of diploid (2n) chromosome
number. The intersex ones also called gynandromorphs (Fig. 2–15) are
characterized by that one half of them bears the features of the male sex
while the other half the ones of the female sex. In many plants and animals
the diferentiated sex is replaced by hermaphroditism, i.e. the combination
of the male and female sex in one organism. Grasshoppers, bugs, some
butterflies are deprived of the Y-chromosome, the heterogamete male or
female sex being (XO) and nevertheless, in some cases the ratio between
male and female individuals is roughly 1:1.

image a

image b

image c

image d

Figure 2–15. Gynandromorphs (intersexuals) in Drosophila melanogaster

A — first division of the egg; the elimination of the X-chromosome is
shown. B — a gynandromorph formed after the elimination; the left side
corresponds to the female sex (XX) and the right one — to the male sex
(XO); C — the head of the fly; the elimination of the X-chromosome in
the course of one of the last somatic mitosis has led to the emergence of
a red spot in the eye (After De Robertis et al., 1973). D — a bilateral
gynandromorph. The zygote had two X-chromosomes and should have
developed into a female. It was heterozygotic for the gene for white eyes
(w) and for shorter wings (m) with the structure ++/wm. In the first
division the X-chromosome with genes + and + has been eliminated in
one of the blastomers. As a result the left side of the fly has formed as a
male displaying mutant properties. The right side had two X-chromosomes
and has developed into a female (After Dubinin, 1976)