Presentation on chromosome

1. Structural aberrations ofchromosomes

2.  Chromosome means: chroma - colour; some - body)
 A chromosome is a thread-like self-replicating genetic structure
containing organized DNA molecule package found in the nucleus
of the cell.
 Chromosomes are seen during metaphase stage of mitosis when the
cells are stained with suitable basic dye and viewed under light
microscope.
 E. Strasburger in 1875 discovered thread-like structures which
appeared during cell division.
 In all types of higher organisms (eukaryote), the well organized
nucleus contains definite number of chromosomes of definite size,
and shape.
Chromosome

3.  Waldeyer coined the term chromosome first time in 1888.
 The somatic chromosome number is the number of chromosomes found in
somatic cell and is represented by 2n (Diploid).
 The gametic chromosome number is half of the somatic chromosome
numbers and represented by n (Haploid).
 The two copies of chromosome are ordinarily identical in morphology, gene
content and gene order, they are known as homologus chromosomes.

4. Chromosomes are of two types
Autosomes: that control characters other than sex characters or carry genes for somatic
characters.
Sex chromosomes (Gonosomes) – Chromosomes involved in sex determination.
Humans and most other mammals have two sex chromosomes X &
Y,also calledheterosome.
Females have two X chromosomes in diploid cells; males have an X and a Y
chromosome.
In birds the female (ZW) is hetero-gametic and male (ZZ) is
homo-gametic.

5. Diploid and Haploid chromosome number
 Diploid cells (2N where N- chromosome number) have two
homologous copies of each chromosome.
 The body cells of animals are diploid.
 Haploid cells (N) have only one copy of each chromosome.
 In animals, gametes (sperm and eggs) are haploid.
 Chromosome number varies from 2n = 4 (n = 2) to 2n = >
1200. (n = gametic or haploid chromosome number 2n = somatic or
diploid chromosome number)
 The number of chromosomes varies from species to species.
 Normally all individual of a species have the same chromosome
number.

6.  Each chromosome has two arms - p (the shorter of the two) and
q (the longer).
 Chromosome shape is usually observed at
anaphase,
whenthe position of primary constriction (centromere)
determines chromosome shape.
 This constrictionor centromerecan be terminal, sub-terminal
or median in position.
 The size of chromosome is normally measured at mitotic
metaphase and may be as short as 0.25 µm in fungi and
birds, or as long as 30 µm in some plants like Trillium.

7. Homologous Chromosomes
 Diploid organisms have two copies of each chromosome
(except the sex chromosomes).
 Both the copies are ordinarily identical in morphology, gene
content and gene order and hence known as homologous
chromosomes.
 Each pair of chromosomes made up of two homologs.
 Homologous chromosome is inherited from separate parents;
one homolog comes from the mother and the other comes from
the father.

8. On the basis of position of centromere and length of chromosomal
arms, chromosomes can be classified into 4 types:
1.Telocentric chromosome: I shaped
2.Acrocentric chromosome: J shaped
3.Sub-metacentric chromosome: L shaped
4.Metacentric chromosome: V shaped

9. CHROMOSOMAL ABERRATIONS
Chromosomal changes are abnormal structural and numerical
changes
They are also called chromosomal aberrations or chromosomal
mutations
Gross structural changes of chromosomes are called structural
aberrations or chromosome re arrangements
Numerical changes are called ploidy changes or numerical
aberrations

10. STRUCTURAL CHANGES
Structural changes of chromosomes involve the gain , loss or relocation of chromosome
segments and genes
• Intrachromosomal aberrations
• Interchromosomal aberrations
Chromosomal aberration are of four basic types
1. Deficiency or deletion
2. Duplication
3. Inversion
4. Translocation

11. (1). DELETION OR DEFICIENCY
• Loss of a chromosome segment, together with the genes contained in
it
• Terminal deletion – loss of a terminal segment, with a single break in
the chromosomes
• Intercalary deletion- loss of an intercalary segment, with two breaks
*Homozygous deletion – a segment is lost from both the chromosomes
of a homologous pair
*Heterozygous deletion - a segment is lost only from one of the two
homologues

12. • Terminal deletion Intercalary deletion
An intercalary chromosome deletion occurs when a
segment of DNA is lost from the interior of a
chromosome.
Deletion involving the terminal part of a
chromosome and leading to an adhesiveterminus.

13. EFFECTS OF DELETION
• Deletion involves the loss of genes so they have deleterious effect on
organisms
• If the lost genes are crucial for viability, sterile gametes or non
functional somatic cells may result
• In human genome deletion mutation
Granulocytic leukemia – deletion of long arm of chromosome 21
Cri-du-chat syndrome – heterozygous deletion in the short arm of
chromosome 5

14. (2.) DUPLICATION
• Duplication is the doubling or repetition of chromosome segment
during chromosome duplication
• As a result of it, a set of genes gets doubled or repeated
• The extra set of genes is generally called “repeat”
• Three types
1. Tandem Duplication – The extra segment and the parent segment
are next to each other and have the same order of genes
2. Reverse tandem duplication - The extra segmentlies next to
parent segment with reversed gene sequence
3. Displaced duplication- The duplicate segment lies some distance
away from the parent segment

15. Possible way in which a duplicate segmentis
incorporated

16. Homozygous &Heterozygous duplication
Heterozygous duplication: one of the homologues of
a pair of chromosomes will have homologous
duplicated sequences
Homozygous duplication: both of the
homologues of a pair of chromosomes
will have homologous duplicated
sequences

17. EFFECTS OF DUPLICATION
• Duplications are more frequent and less deleterious than deletions
• Duplications are believed to be the important raw materials of organic evolution
• Large duplications affect the regulation of gene activity and there by causes phenotypic alterations which in
turn promotes speciation and evolution

18. The Bar-eye phenotype inDrosophila
Wild type fruit flies have normal size eyes
Flies heterozygous for the Bar mutationhave
smaller bar shapedeyes
Flies homozygous for the Bar mutationhave
smaller bar shaped eyes
Flies with double bar have three copiesof
duplication and have much smaller Bar
shaped eyes

19. (3.) INVERSIONS
• Inversion the reversal of the linear order of a chromosome segment and its gene
sequence
• Belived to be occur in intercalary segments – intercalary inversion
• Breakage re union hypothesis: Intercalary inversion involves the breakage of a
chromosome segment and its subsequent re-insertion into the same location in a
reversed orientation
• In this case two breaks occur in a chromosome – inversion points
• *Paracentric Inversion –The rotation(180°) and reinsertion of an acentric
segment
• *Pericentric Inversion- The rotation(180°) and reinsertion of a centric segment
• Paracentric inversion is of two types
• (i) Intraradial or homobranchial inversion – single inversion in one arm
• (ii) InterradiaI or heterobranchial inversion- two inversions onein each arm

20. Origin of an inversion

21. • Paracentric Inversion Pericentric Inversion

22. Inversions
• Based on the no. of the inverted segments and the no. of breaks in
chromosome two major kind of inversions can be recognized
*Single Inversions – only one segment is inverted
*Complex inversions - more than one segment would be inverted
• Homozygous inversion – both the homologues are inverted
• Heterozygous inversion- one homologue is uninverted and its
companion is inverted

23. Complex inversions
• (i) Independent inversions:
Inversions occur in different regions of the chromosome and they are separated from one another by un-
inverted (normal) segment
• (ii) Direct tandem inversions:
There are two or more inverted segments which are directly adjacent to each other, i.e., the inverted
regions are not separated by normal regions
• (iii) Reversed tandem inversions:
The two inverted segments are adjacent to each other but their positions are mutually interchanged i.e.,
g. the first segment lies in place of the second and vice-versa
• (iv) Included inversions:
One inversion is confined within another inversion, i.e., a segment within an inverted segment is inverted
again; as a result, the second inverted segment possesses the normal gene sequence for the concerned
segment
• (v) Overlapping inversions:
Such inversions have a common segment, i.e., a part of an inverted chromosome segment is inverted
again together with an adjacent segment which was not included in the first inverted segment

24. • Single Inversions
Complex inversions
only one segment isinverted
more than one segment would beinverted

25. EFFECTS OF INVERSION
Helps to retain the original gene combination by reducing the
crossing over frequency
Inversion enhances the position effect & there by produces
phenotypic changes
Inversion causes chromosome polymorphism with in a population
leading to karyotype evolution
Inversion produces balanced gene complexes(super genes) through
reduction of crossing over, which in turn confers greater biological
fitness and adaptability & increases the evolutionary potentiality

26. (4.)TRANSLOCATION
 Translocation is the re-organization of chromosomes through the transfer or
change in position of chromosome segments with out gain or lose of genes
(i)Intrachromosomal translocation {chromosomal shift} -Change in position of a
segment within a chromosome, either from one arm to the other or from one
location to another in the same arm
(a)Intraradial- Translocation from one location to another in the same arm
(b)Extraradial- Translocation from one arm to the other
(ii) Interchromosomal Translocation– Transfer of a segment from one chromosome
to other
(a) Non reciprocal translocation- one way transfer of a chromosome segment
from one chromosome to another chromosome.
(b)Reciprocal Translocation -mutual exchange of segments between two
chromosomes

27. • (i)Intrachromosomal translocation (ii) Interchromosomal Translocation

28. (i)Intrachromosomal translocation
• (a)Intraradial Translocation (b)Extraradial Translocation
Translocation from one location to anotherin
the same arm
Translocation from one arm to the other

29. (ii) Interchromosomal Translocation
• Reciprocal Translocation
Mutual exchange of segments between twochromosomes

30. Reciprocal Translocation
1. Fraternal translocation- mutual exchange between homologous
chromosomes
2. External translocation- mutual exchange between non-
homologous chromosomes
3. Intercalation or insertional translocation- intercalary incorporation
of a chromosome segment by transposition
4. Asymmetrical or aneucentric translocation- Exchanged segments
are asymmetrical , produces a dicentric & an acentric chromosome
5. Symmetrical or Eucentric translocation- Exchanged segments are
symmetrical , produces two monocentric chromosomes

31. Reciprocal Translocation
Fraternal translocation External translocation
Segments are exchanged between
homologues
Segments are exchanged
between non -homologues

32. Translocation
• Intercalation or insertional translocation
• In the insertional translocation, a segment from the orange (donor) chromosome
is transferred to the purple (recipient) chromosome without any reciprocal
exchange.

33. Reciprocal Translocation in monocentric chromosomes
• Asymmetrical or aneucentric translocation
• Symmetrical or Eucentric translocation
Dicentric chromosome
Monocentric chromosomes

34. Whole arm translocations
• Translocation which involve the transfer of a whole chromosome arm
They take place by
Centric fusion- Two breaks in two acrocentric chromosome. One break is in the
short arm of a chromosome and the other one is in the long arm of the other
chromosome. Their subsequent fusion results in a metacentric chromosome
Dissociation- It is the reverse process in which a metacentric chromosome and a
sub metacentric chromosome give rise to two acrocentric chromosomes
Tandem fusion – it is the end to end fusion between an acrocentric
chromosomes and a metacentric chromosome with the loss of a centric segment
It produces a double length acrocentric chromosome.

35. Centric fusion

36. Dissociation
Metacentric chromosome
Sub metacentric
chromosome
Acrocentric chromosomes
a metacentric chromosome and a sub metacentric
chromosome give rise to two acrocentric chromosomes
 In this process an acentric segment is transferred from the former to the later

37. Tandem fusion
Metacentric chromosome
Acrocentricchromosome
Double length acrocentricchromosome
 end to end fusion between an acrocentric chromosomes
and a metacentric chromosome with the loss of a centric
segment
 Produces a double length acrocentric chromosome

38. Effects of Translocation
• Translocations & inversions change the linkage relationships of the
genes involved in the affected segments
• Translocation results in unusual linkage, reduced meiotic
recombination, low viability of chromosomes, familial patterns of
trisomy
• It may causes changes in chromosomes & also in gene combinations,
promoting variation, speciation, & evolution

39. CHROMOSOME STRUCTURE ABNORMALITIES

40. Isochromosome
• An isochromosome is an unbalanced structural abnormality in which
the arms of the chromosome are mirror images of each other. The
chromosome consists of two copies of either the long (q) arm or the
short (p) arm because isochromosome formation is equivalent to a
simultaneous duplication and deletion of genetic material.

41. Derivative chromosome
• A derivative chromosome is a structurally rearranged chromosome
generated either by a rearrangement involving two or more
chromosomes or by multiple aberrations within a single chromosome
(e.g. an inversion and a deletion of the same chromosome, or
deletions in both arms of a single chromosome)

42. Ring chromosomes
• A ring chromosome is a chromosome whose arms are fused together
to form a ring.
or
• A structurally abnormal chromosome in which the end of each
chromosome arm has been lost and the broken arms have been
reunited to form a ring