Chromosomal Behavior, Somatic Segregation, Chimeras, Evolutionary significance of Chromosomal Aberration

2. PRINCIPLES OF CYTOGENETICS
GP-502
PresentedBY submittedto
Viveksuthediya DR.S.S.desai
Regno. adpm/19/2680

3. Variation in Chromosomal Behavior
Somatic Segregation
Chimeras
Endomitosis
Somatic Reduction
Evolutionary Significance of
Chromosomal Aberration
Chromosomal Complex
Balance Lethal

4. Variation In Chromosomal Behavior
• Variation in chromosomal behavior is seen during
mitotic and meiotic cell division.
 Mitosis :-
1. Prophase :
 During early prophase the chromatin network condenses
and resolves into definite number of chromosomes.
 Initially each chromosome appears as a single stranded ,
thin and long.
 As the condensation progresses , each of them reveals
identical chromatids held together by centromere.
 Throughout the prophase , chromosome undergoes
dehydration and coiling to become thick and short.

5.  During metaphase the condensation of chromosomes is
completed and the thick chromosomes get organized along
equatorial plane of the cell.
3. Anaphase :
 During anaphase the centromeres divide into two, resulting in
the separation of chromatids.
 Each separated chromatid is now called daughter
chromosome.
4. Telophase :
 At the poles, daughter chromosomes uncoil and undergo
hydration to form chromatic network.
:
2. Metaphase :

6.  Meiosis :-
A. Prophase-I
1. Leptonene :
 During this phase the chromatin network condenses and
resolves into long and thin, thread like chromosomes.
 Each chromosome consists of two chromatids but appears
as a single thread.
2. Zygotene :
 It is a phase in which homologous chromosomes begin to
pair lengthwise.
 Such a pairing of chromosomes is called synopsis.
 The pairs at this stage are called bivalents.

7. 3. Pachytene :
 During this phase condensation of chromosomes progresses and
they become short and thick.
 Each of the homologous chromosomes now shows two
chromatids so that the bivalents now appear to be composed of
four chromatids, known as tetrad.
 The twisting of homologues become compact resulting in
breakages and rejoining of the chromatids.
 During this process crossing over takes place.
 The point where crossing over takes place is called chiasmata.
4. Diplotene :
 The homologues now start repelling each other and begin to
separate.
 However at the points of crossing over they remain attached and
thus chiasmata can be seen.
3. Pachytene :

8. 5. Diakinesis :
 During this phase chromosomes continue to condense and
shorten.
 The separation of homologues proceeds and the chiasmata
get shifted to the ends of chromatids.
 This process is called terminalisation.
B. Metaphase-I
 Chromosomes are arranged at equatorial plate.
C. Anaphase-I
 During this phase the centromeres do not divide.
 Homologous chromosomes which are still attached at
chiasmata finally get separated. This is called disjunction.
 Each chromosome is with two chromatids and one
centromere.
5. Diakinesis :

9. D. Telophase-I
D. Telophase-I
 Chromosomes uncoil to form chromatin.

10. Somatic Segregation And Chimeras
 Somatic Segregation :-
 The phenomenon of somatic segregation is defined as the
formation of two new diploid cell types within the body of an
ordinary monozygotic individual.
 In these new cell types, either two paternally derived
chromosomes or two maternally derived ones constitute a
homologous pair. By this mechanism, two kinds of cells
homozygous for either one or the other allele emerge within the
body of an individual heterozygous for a given genetic locus.

11. Fig. Somatic Segregation

12.  Chimera :
 Plant or plant part composed of genetically different layers.
 It is a single organism composed of cells with distinct genotypes.
 In plant chimeras, the distinct types of tissue may originate from the
same zygote and the differences is often due to mutation during
ordinary cell division.

13.  The Concept of Apical Organization :
 No discussion of the origin of chimeras would be complete
without a review of the organization of the shoot apex.
 The pattern of cell division, frequency of cell division, and
layered organization of the cells in the apex interact in
determining the type of chimera which is produced and the
stability of the pattern which results .
 The apex is organized into a layered region (the tunica) and a
region where layering is not evident (the corpus).
 The derivatives of the outermost layer (L.I) give rise to the
epidermis . The epidermal layer is continuous as an outer
covering over all tissues of the leaf, stem, flower petals, etc.
 Derivatives of layer II (L.II) give rise to several layers within the
stem and a large proportion of the cells in the leaf blade.
 Derivatives of layer III (L.III) give rise to most of the internal
tissue of the stem and a number of cells around the veins
within the leaf.

14. a

15.  Types of Chimeras :-
i. Periclinal Chimera
ii. Mericlinal Chimera
iii. Sectorial Chimera

16. 1. Periclinal Chimera :
 A mutation occurs in one or more layers at the top of the
apex.
 Due to its position, the cell division products of the muted
cells spread and cover the entire layer of the apex.
 The entire layer is mutated and they are stable.
 It is the most common type of chimera in horticulture.
 In this case the LI and LIII produce cells with normal
chlorophyll production while the LII does not produce
chlorophyll and is colourless resulting in a variegated leaf.

17. Fig. Periclinal Chimera

18.  A mutation occurs in one layer and along the side of the
apex.
 Due to its position, the cell division products of those
mutated cells occur as a layer on only one side of the plant
and they are not stable.
 Only a section of one of the layers is mutated.

19. Fig. Mericlinal Chimera

20. 3. Sectorial Chimera :
 A mutation occurs in multiple layers at the top of the apex.
 Due to its position, the cell division products of the mutated
cells give rise to a section of mutated cells.
 It is relatively unstable chimera type.
 Most common type of chimera in horticulture.

21. Fig. Sectorial Chimera

22.  Endomitosis And Somatic Reduction :-
 Endomitosis :-
 It is a sequence of changes in the nucleus resulting in division of
the chromosomes in mitosis but no separation of the chromatids
into daughter nuclei.
 The resulting nucleus is therefore polyploid.
 The process can be induced in isolated tissues by treatment with
colchicine, which prevents spindle formation, so the centromeres
of the daughter chromosomes are unable to move apart into
separate nuclei.
 It may occur as an error in some parts of a plant, for example- a
tetraploid branch on a diploid plant.

23.  It occurs as a normal feature in some tissues of higher
plants, for example- phloem cells of some leguminous plants
are polyploids.
 This type of polyploidy where some of the cells of a plant
have more than the normal complement of chromosomes
for the species is known as endopolyploidy.
 If endomitosis occurs in cells in the germ line or during the
second division of meiosis, the unreduced gametes may
result.

24. b

25.  Somatic Reduction :-
 Somatic reduction is the reduction in chromosome
number in somatic cells.

26.  Evolutionary Significance Of Chromosomal
Aberrations :
1. Numerical Chromosomal Aberrations :
i. Euploidy :
a) Monoploids and haploid :
 Development of pure lines : Pure lines can be obtained through
chromosomal doubling of haploids.
 Disease resistance : In tobacco double haploids obtained from
anther culture have been used to improve the disease resistance
of the Japanese flue cured cultivar Mc161.
 Production of inbreeds : It is useful tool in obtaining inbred lines
in dioecious plants.

27. b) Polyploids :
 Autopolyploids :
1. Autotriplods :
 Triplods are useful only in those plant species which propagate
asexually like banana, sugarcane, etc.
 Banana : Cultivated varieties of banana are triploids and
seedless. Such bananas have larger fruits than diploid ones.
 Sugarbeet : Triploid sugarbeets have higher sugar content than
diploids and are generally resistant to moulds.
 Water melon : Triploid water melons are seedless or have
rudimentary seeds like cucumber.

28. 2. Autotetraploids :
 Autotetraploids are larger and more vigorous than the diploid
species.
 Rye : Autotetraploid rye is grown in Sweden and Germany.
 They have larger heads and higher protein than diploids.
 Grapes : Tetraploid grapes have been developed in California,
USA, which have longer fruits and fewer seeds per fruit than
diploids.
 Alfalfa : Tetraploid varieties of alfalfa are better than diploids in
yield and have better recovery after grazing.

29.  Allopolyploids :
1. Natural Allopolyploids :
i. Wheat :
Triticum monococcum X Unknown sp.
↓
T. turgidum X T. tauschi
↓
T. aestivum

30. ii. Tobacco:
Nicotiana sylvestris X Nicotiana tomentosa
↓
Nicotiana tabacum
Nicotiana paniculata X Nicotiana undulate
↓
Nicotiana rustica
iii. Cotton :
Gossypium africanum X Gossypium raimundii
↓
Gossypium hirsutum

31. iv. Oats :
Avena barbata X Avena strigosa
↓
Avena sativa
v. Brassica :
Brassica nigra
B. carinata B. juncea
B. oleracea B. napus B. compestris

32. 2. Artificial alloploids :
i. Radish-Cabbage :
Raphanus sativus X Brassica oleracea
↓
Raphanobrassica
ii. Wheat-Rye :
Triticum aestivum X Secale cereale
↓
Triticale
iii. Mung-urid :
Vigna radiata X Vigna mungo
↓
Fertile amphiploid

33. vi. Cotton :
Gossypium herbaceum X Gossypium raimondi
↓
Like G. hirsutum

34. ii. Aneuploidy :
 Monosomics are used in transferring chromosomes with
desirable genes from one species to another.
 Aneuploids are used for developing alien addition and alien
substitution lines in various crops.
 Structural Chromosomal Aberrations :
 Deletions play an important role in species formation and
releasing variability through chromosomal mutations.
 Duplication lead to addition of some genes in a population
which after mutation play an important in evolution.
 Translocation and inversion also lead to evolution of new
species by changing the karyotype.

35.  Chromosomal Complex Rearrangement :
 Complex Chromosomal Rearrangements (CCRs) are
constitutional structural rearrangements involving three or
more chromosomes or having more than two breakpoints.
 Balanced lethal :
 It is defined as true breeding heterozygous organism
maintained in a stable state through the existence of
different lethals on each of a pair of homologous
chromosomes and resulting in loss of all homozygotes.

38. • References :
 Genetics – B.D. Singh
 Cytogenetics - P.K.Gupta

39. c
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