Genetic control mechanism in eukaryotes

1. Genetic control mechanism in
eukaryotes
Prepared by Ganga Ram Kohar
Assistant professor IAAS Paklihawa

2. Structure of chromosome
• One micron of a metaphase chromosome
contains about 800μ of DNA helix
• The structure of chromosome is different
according to different model given by
scientists

3. A. Typical early model
• This is one of he earliest model of chromosome
structure based on light microscope
• According to this model the basic component of
chromosome structure is chromonema which
composed of chromatin and contains genes
• The variation in chromosome length and
thickness is proposed to be due to coiling and
uncoiling of chromosome
• Each chromatid of a chromosome may contain
two or more chromonemata which run across
through centromere

4. Conti..
• In the case of meta phase chromosome
,chromosome are surrounded by an
amorphous matrix the outer side of which is
enclosed in the membrane call pellicle
• However ,electron micrographs of metaphase
chromosome do not show any evidence for
existence of either matrix or pellicle
• So ,this model of chromosome structure is
inadequate and of historical interest only

5. Conti…

6. B. Recent modes of chromosome
1. Multistranded model
• According to this model , each chromatin fiber is
on an average 100Λ in diameter
• Each chromatin fiber is composed of two strands
• Each strand being 35-40 Λ in diameter
• Each strand consists of single DNA double helix (20
Λ in diameter) and the associated histone and non
histone protein ,thus a chromatin fiber contains
two DNA double helix (separated from each other
by a space of about 25 Λ) and associated protein

7. Conti..
• Four chromatin fibers(each composed of two DNA
double helix) coil around each other to from quarter
chromatid which is a smallest sub unit of the
chromosome visible under light microscope in many
organisms
• Association of two quarter chromatid (each of 400 Λ
diameter ) give rise to one half chromatids of 800 Λ
diameter ,which is composed of 16DNA double helix
• Finally ,two half chromatid coil around each other to
produce one chromatid which is 1600 Λ in diameter
and made up of 32 DNA double helix

8. Conti..
• Thus , a metaphase chromosome has 64DNA
double helix and strand would be about 3200
Λ in diameter
• The variation in chromosome thickness would
be due to coiling and uncoiling of chromatid
• The number of DNA double helices in quarter
,half and full chromatids are proposed to vary
from 8 to 64 depending upon species

9. Conti..

10. 2. Folded fiber model
• This model was proposed by Du Praw in 1965 and is
widely accepted
• According to this model, chromosomes are made
of chromatin fibers of about 230A° diameter
• Each chromatin fiber contains only one DNA
double helix which is in a coiled state; this DNA coil
is coated with histone and non-histone proteins
• Thus the 230A° chromatin fiber is produced by
coiling of a single DNA double helix, the coils of
which are stabilized by proteins and divalent
cations (Ca++ and Mg++)

11. Conti…
• Each chromatid contains a single long chromatin
fibers; the DNA of this fiber replicates during
interphase producing two sister chromatin fibres, it
remains unreplicated in the centromeric region so
that the two sister fibres remain joined in the
region
• Subsequently, the chromatin fibre undergoes
replication in the centromeric region as well so
that the sister chromatin fibre are separated in this
region also

12. Conti…
• During cell division the two sister chromatin
fibres undergo extensive folding separately in an
irregular manner to give rise to two sister
chromatids
• Folding of the chromatin fibres drastically reduces
their length and increases their stainability and
thickness
• This folded structure normally undergoes
supercoiling which further increases the thickness
of chromosomes and reduces the length

14. C. Organization of chromatin fibers
• Any model of chromatin fibre structure has to
account for
• (i) packaging of a very long DNA molecule into a
unit length of fibre
• (ii) production of very thick (230-300A0) fibres
from very thin (20A°) DNA molecules
• (iii) the beads-on-a-string ultrastructure of
chromatin fibres observed particularly during
replication
• Two clearly different models of chromatin fibre
structure have been proposed

15. I. Coiled DNA Model:
• This is the simplest model of chromatin fibre organization
and was given by Du Praw
• According to this model, the single DNA molecule of a
chromatin fiber is coiled in a manner similar to the wire in a
spring; the coils being held together by histone bridges pro-
duced by binding histone molecules in the large groove of
DNA molecules.
• Such a coiled structure that would be stabilized as a single
histone molecule would bind to several coils of DNA
• This coiled structure is coated with chromosomal proteins to
yield the basic structure of chromatin fibres (type A fibre)
which may undergo supercoiling to produce the type B fibre
of DuPraw which is akin to the beads seen in electron
micrographs of chromatin fibres

16. Conti…..

17. II. Nucleosome-Solenoid Model
• This model was proposed by Romberg and Thomas
(1974) and is the most widely accepted
• According to this model, chromatin is composed of a
repeating unit called nucleosome
• Nucleosomes are the fundamental packing unit par-
ticles of the chromatin and give chromatin a “beads-
on-a string” appearance in electron micrographs that
unfold higher-order packing (Olins and Olins, 1974).
• One complete nucleosome consists of a nucleosome
core, linker DNA, an average of one molecule of H1
histone and other associated chromosomal proteins

18. Conti….

19. Nucleosome Core
• It consists of a histone octamer composed of
two molecules, each of histones H2a, H2b,H3
and H4. In addition, a 146 bp long DNA
molecule is wound round this histone octamer
in 13/4 turns; this segment of DNA is nuclease
resistant

20. Linker DNA
• Its size varies from 8bp to 114 bp depending
on the species
• This DNA forms the string part of the beads-
on-a string chromatin fibre, and is nuclease
susceptible; and the beads are due to
nucleosome cores
• Thus, linker DNA joins two neighbouring
nucleosomes

21. H1 Histone
• Each nucleosome contains, on an average, one
molecule of HI histone, although its uniform
distribution throughout the length of chromatin
fibres is not clearly know
• Some studies suggest that the molecules of H1
histone are involved in stabilizing the supercoils
of nucleosome chromatin fibres
• Other studies suggest that HI is associated on the
outside of each nucleosome core, and that one
H1 molecule stabilizes about 166 bp long DNA
molecule

22. Other Chromosomal Proteins:
• Both linker DNA and nucleosome are associated with other
chromosomal proteins
• In native chromatin, the beads are about 110A° in
diameter, 60A° high and ellipsoidal in shape
• Each bead corresponds to a single nucleosome core.
• Under some conditions, nucleosomes pack together
without any linker DNA, which produces the 100A° thick
chromatin fibre called nucleosome fibre which may then
supercoil to give rise to the 300A° chromatin fibre called
solenoid
• The nucleosome model of chromatin fibre structure is
consistent with almost all of the evidence accumulated so
far

23. Special Chromosomes
• Some tissues of certain organisms contain
chromosomes which differ significantly from
normal ones in terms of either morphology or
function; such chromosomes are referred to as
special chromosomes. The following types of
chromosomes may be included under this
category:
• (1) Lampbrush chromosomes,
• (2) Giant chromosomes or salivary gland
chromosomes and
• (3) Accessory or B chromosomes

24. Lampbrush Chromosomes:
• Lampbrush chromosomes are found in oocytes of
many invertebrates and all vertebrates, except
mammals; they have also been reported in human
and rodent oocytes
• But they have been the most extensively studied
in amphibian oocytes
• These chromosomes are most distinctly observed
during the prolonged diplotene stage of oocytes
• During diplotene, the homologous chromosomes
begin to separate from each other, remaining in
contact only at several points along their length

25. Conti…
• Each chromosome of a pair has several chromomeres distributed over its
length; from each of a majority of the chromomeres generally a pair of
lateral loops extends in the opposite directions perpendicular to the main
axis of the chromosome
• In some cases, more than one pair, even upto 9 pairs of loops may emerge
from a single chromomere
• These lateral loops give the chromosomes the appearance of a lampbrush
which is the reason for their name ‘lamp-brush chromosomes’
• These chromosomes are extremely long, in some cases being 800-1000^ in
length
• The size of loops may range from an average of 9.5ja in frog to 200|i in
newt
• The pairs of loops are produced due to uncoiling of the two chromatin
fibres (hence the two sister chromatids) present in a highly coiled state in
the chromosomes; this makes their DNA available for transcription (RNA
synthesis)

26. Conti….
• Thus each loop represents one chromatid of a chromosome and is
composed of one DNA double helix. One end of each loop is thinner (thin
end) than the other end (thick end)
• There is extensive RNA synthesis at the thin ends of loops, while there is
little or no RNA synthesis at the thick end
• The chromatin fibre of the chromomere is progressively uncoiled towards
the thin end of a loop; the DNA in this region supports active RNA
synthesis but later becomes associated with RNA and protein to become
markedly thicker
• The DNA at the thick end of a loop is progressively withdrawn and
reassembled into the chromomere
• The number of pairs of loops gradually increases in meiosis till it reaches
maximum in diplotene
• As meiosis proceeds further, number of loops gradually decreases and the
loops ultimately disappear due to disintegration rather than reabsorption
back into the chromomere

27. Conti..

28. Conti…
• Loops represent the sites of gene action
(transcription), and the function of lampbrush
chromosomes is to produce the large numbers
and quantities of proteins and RNA’s stored in
eggs

29. Giant Chromosomes
• Giant chromosomes are found in certain tissues, e.g.,
salivary glands of larvae, gut epithelium, Malphigian
tubules and some Diptera, e.g., Drosophila, Chironomous,
Sciara, Rhyncosciara etc. These chromosomes are very long
(upto 200 times their size during mitotic metaphase in the
case of Drosophila) and very thick, hence they are known
as giant chromosomes
• They were first discovered by Balbiani (1881) in dipteran
salivary glands, giving them the commonly used name
salivary gland chromosomes
• The giant chromosomes are somatically paired.
Consequently, the number of these giant chromosomes in
the salivary gland cells always appear to be half that in the
normal somatic cells

30. Conti….
• The giant chromosomes have a distinct pattern of transverse
banding which consists of alternate chromatic and achromatic
regions
• The bands occasionally form reversible puffs, known as
chromosome puffs or Balabiani rings, which are associated with
active RNA synthesis
• The giant chromosomes represent a bundle of fibrils which arise by
repeated cycles of endo- reduplication (replication of chromatin
without cell-division) of single chromatids
• This is why these chromosomes are also popularly known as
polytene chromosomes and the condition is described as polyteny
• The number of chromonemata (fibrils) per chromosomes may
reach upto 2000 in extreme cases some workers placed this figure
as high as 16,000

31. Conti…
• 2000 in extreme cases some workers placed this figure as high as
16,000
• In D melanogaster, the giant chromosomes radiate as five long and
one short arms from a single more or less amorphous mass known
as chromocentre
• The chromocentre is formed by fusion of the centromeric regions
of all the chromosomes and, in males, of the entire Y chromosomes
• The short arm radiating from the chromocentre represents
chromosome IV, one of the long arms is due to the X-chromosome,
while the remaining four long arms represent the arms of
chromosome II and III
• The total length of D. melanogaster giant chromosomes is about
2000µ.

33. Accessory Chromosomes:
• In many species, one too many extra chro-
mosomes in addition to the normal somatic
complement are found; these extra
chromosomes are called accessory
chromosomes, B-chromosomes or
supernumerary chromosomes.

34. • About 600 plant species and more than 100 animal species
are reported to possess B- chromosomes. B-chromosomes
are generally smaller in size than the chromosomes of the
normal somatic complement but in some species they may
be larger (e.g., in Sciara).
• One of- the most important features of these chromosomes
is that their numbers may vary considerably among
individuals of the same species; in maize as many as 25-30
B-chromosomes may become accumulated in some
individuals without any marked effect on their phenotype
• These chromosomes are generally gained by and lost from
the individuals of a species without any apparent adverse
or beneficial effect

35. Conti…
• However, the presence of several B-chromosomes often leads to
some reduction in vigour and fertility in maize. In most cases, they
are largely heterochromatic, while in some species (e.g. maize) they
are partly heterochromatic, and in some other (e.g., Tradescantia)
they are entirely euchromatic
• They are believed to be generally inactive genetically, but they may
not be completely devoid of genes.
• The origin of B-chromosomes in most species is unknown
• In some animals they may arise due to fragmentation of the
heterochromatic Y chromosome
• In maize, morphological features and pairing behaviour of B-
chromosome clearly shows that they do not have any segment
which is homologous to a segment of any chromosome of the
normal somatic complement

36. Conti…
• B-chromosomes are relatively unstable; in
many species they tend to be eliminated from
somatic tissues due to lagging and non-
disjunction and they frequently change in
morphology through fragmentation
• Further, they may also show irregular
distribution during meiosis, but they are
invariably maintained in the reproductive
tissue

37. Functions of Chromosomes
• The role of chromosomes in heredity was suggested independently
by Sutton and Bover in 1902. This and various other functions of
chromosomes may be summarized as under.
• 1. It is universally accepted that DNA is the genetic material, and
that in eukaryotes almost all the DNA is present in chromosomes.
Thus, the most important function of chromosomes is to provide
the genetic information for various cellular functions essential for
growth, survival, development, reproduction, etc., of organisms.
• 2. Another very important function of chromosomes is to protect
the genetic material (DNA) from being damaged during cell division.
Chromosomes are coated with histones and other proteins which
protect it from both chemical (e.g., enzymes) and physical forces

38. • 3. The properties of chromosomes ensure a
precise distribution of DNA (genetic material) to
the daughter nuclei during cell division
• Centromeres of chromosomes perform an
important function in chromosome movements
during cell division which is due to the
contraction of spindle fibres attached to the
centromeric regions of chromosomes
• 4. Gene action in eukaryotes is believed to be
regulated through histone and non-histone
proteins associated with chromosomes

42. Genetic control mechanism in
eukaryotes