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BASIC TERMS TO BE REMEMBERED
¢Karyotyping:
refers to a full
set of
chromosomes from
an individual, or
a photographic arrangement of
a set of chromosomes of
an individual.
¢Chromatid:
either of
the two
daughter
strands of the replicated chromosome that are
joined by
centromere and separate during
cell division.
¢Constituent of chromosome:
DNA, RNA, HISTONES (and some non-histone proteins)
¢No.
of
chromosomes
in human: 22 pairs of
autosomes & a pair
of sex
chromosomes.
Diploid: 46
(44+XX or 44+XY)
Haploid : 23
(22+X or 22+Y)
STRUCTURE OF CHROMOSOME
TYPES OF CHROMOSOMES
ABERRATION
—Any departure or deviation from normal.
CHROMOSOMAL ABERRATION
•Any aberration in the
shape, size or
structure of
a chromosome is called chromosomal aberration.
•It reflects an atypical number or a structure in one
or more
chromosomes.
TYPES OF CHROMOSOMAL ABERRATION
1. Structural aberration
2. Numerical aberration
STRUCTURAL
ABNORMALITIES
2.Duplication: Section of a chromosome is in duplicate, usually less harmful. Extra genetic material causes birth defects.
3.Inversion:
Two breaks
in
chromosome
&
then broken fragment reinserted in an inverted fashion.
4. Translocation: Transfer of
full or
a part of
a chromosome to another chromosome.
5. Fragile
site:
Constriction at sites other than centromere. There is more tendency to break. Ex: X linked mental retardation,
fragile X
syndrome.
6. Ring chromosome: Two
breaks
in
the
chromosome
and
these broken ends stick back together.
DELETION
•A missing chromosome segment is
referred to either as a deletion or
as a deficiency.
•Large
deletions
can
be detected cytologically
by studying the banding patterns in stained chromosomes,
but small
ones cannot.
•In
diploid organisms, the
deletion of a chromosome
segment makes
part of the genome hypoploid.
•May
be
associated with
a phenotypic
effect, especially if the deletion is large.
•In
case of a deletion heterozygote, where the dominant allele of a gene has been
deleted, the recessive allele of the deleted gene is generally expressed. This
is called pseudodominance.
•Example:
cri-du-chat
syndrome in
human
•Caused
by
a deletion in the short arm of
chromosome 5
•Individuals
heterozygous for the deletion
have the
karyotype 46 del(5)(p14)
•Bands
in region 14
of the short arm (p) of one of the chromosomes 5 is missing.
•These
individuals may be severely impaired, mentally as well as physically
DUPLICATION
•An extra chromosome segment is referred to as a duplication.
•The extra segment can be attached to one of the chromosomes, or it can exist as a new and separate chromosome, that is, as a “free duplication”.
•The organism
becomes
hyperploid for
part of its genome.
•May
be associated
with a phenotypic effect.
•Example:
Effect of duplications for region 16A of
the X chromosome on
the size of the eyes in
Drosophila.
•Tandem Duplications are
adjacent
to
each
other.
•Reverse
Tandem Duplications result
in genes
arranged
in opposite
order
of
the original.
•Tandem duplication
at
the
end of chromosome
is
a Terminal tandem
duplication.
INVERSION
•An
inversion
occurs
when a chromosome segment is detached, flipped around 180º, and
reattached to the rest of the chromosome.
•The
order
of the
segment’s
genes
is reversed.
•Pericentric inversions
include the centromere, whereas paracentric inversions
do not.
Pericentric inversion:
•Inverted segment includes the centromere
•May change the relative lengths of the two arms of the chromosome
•If an acrocentric chromosome acquires a pericentric inversion, it can be transformed into a metacentric chromosome and vice versa.
Paracentric inversion:
•Inverted segment does not include the centromere
•Does not change the relative lengths of the two arms of the chromosome
•If an acrocentric chromosome acquires a paracentric inversion, the morphology of the chromosome will not be changed.
Suppression of recombination in an inversion heterozygote
•An
individual in which one chromosome is inverted but its homologue is not is
said to
be an inversion heterozygote.
•During
meiosis,
the inverted and non-inverted
chromosomes pair
point-for-point along their length. Because of the inversion, the
chromosomes
must form a loop to allow for pairing
in the inverted region.
•Any
one
of
the chromosomes is looped, and the other conforms around it.
Pericentric inversion:
•As
a
result of crossing over
inside the loop,
daughter chromosomes with deletions/duplications for the
inverted region
are produced which are non-viable.
•Only
the gametes with parental chromosomes (one normal and one inverted) are recovered.
Paracentric inversion:
•Here,
as a
result of crossing over, one acentric and one dicentric
chromosomes are
formed.
•In
addition, the recombinant products contain deletions/duplications for the
inverted region and are non-viable.
•Only
the gametes with parental chromosomes are recovered.
TRANSLOCATION
•A
translocation occurs
when a segment from one chromosome is detached and reattached
to a different (that is, non-homologous)
chromosome.
•When
pieces
of two non-homologous
chromosomes
are interchanged
without any
net loss of genetic material, the event is referred to as a reciprocal translocation.
•During
meiosis, the
translocated
chromosomes
pair with
their untranslocated
homologues in
a cruciform, or crosslike, pattern.
•This
pairing configuration is diagnostic
of a
translocation heterozygote.
•Cells
in
which the translocated chromosomes
are homozygous
do not form a cruciform pattern
but pair normally with its
structurally identical
partner.
Altogether
there are three possible
disjunctional
events:
1.Adjacent disjunction I: If centromeres
2 and 4 (i.e.
centromeres from non-homologous chromosomes that are next to each other) move to
the same pole, forcing 1 and 3 to the opposite pole, all
the resulting gametes will be aneuploid—because
some chromosome segments
will be
deficient for genes, and others will be duplicated.
2.Adjacent disjunction II: If
centromeres
1
and 2 (i.e.
centromeres from the homologous
chromosomes that are next to each other) move to one pole and 3 and 4 to the other,
only aneuploid
gametes will be produced.
Each
of these cases is referred to as adjacent
disjunction because
centromeres that were next to each other in the cruciform pattern moved to the
same pole.
3.Alternate disjunction: If centromeres
1 and 4 (i.e.
centromeres from non-homologous chromosomes that are alternate to each other)
move to
the same pole,
forcing 2
and 3 to the opposite pole,
only euploid gametes
will be produced, and half
of them will carry only translocated chromosomes.
•Translocation heterozygotes are
therefore characterized by low fertility.
ROBERTSONIAN TRANSLOCATION
•Non-homologous
chromosomes
can
fuse
at their centromeres, creating
a structure
called
a
Robertsonian translocation.
•For
example, if two acrocentric chromosomes fuse, they will produce
a metacentric
chromosome; the tiny short arms of the participating chromosomes are
simply lost
in this process.
•Human chromosome
2, which is metacentric, has arms that correspond to
two different acrocentric chromosomes in the
genomes of the great apes.
•Chromosomes
can also fuse end-to-end to form a structure with two centromeres. If one
of the centromeres is inactivated, the chromosome fusion will be stable.
COMPOUND CHROMOSOMES
•Sometimes
one chromosome fuses with its homologue, or two sister chromatids
become attached
to each other, forming a single genetic unit.
•A
compound chromosome can
exist
stably in a cell as long as it has a single functional centromere.
•If there
are two centromeres, each may move to a different pole during division,
pulling the
compound chromosome apart.
•A
compound
chromosome may also
be formed
by the union of homologous chromosome segments. For example, the
right arms
of the two second chromosomes in Drosophila might
detach from their
left arms
and fuse at the centromere, creating a compound half-chromosome.
•This structure
is
called an isochromosome, because
its two arms are equivalent.
•Compound chromosomes differ from
translocations in that they involve fusions of homologous chromosome
segments. Translocations, by contrast,
always involve fusions between nonhomologous
chromosomes.
POSITION EFFECT VARIEGATION
•Gene action can be
blocked by
proximity to the heterochromatin regions
of
chromosome
and
this can result from translocation or inversion events.
•The
locus
for
white eye color in Drosophila
is
near the tip of the
X chromosome.
•The
allele w+ gives red color while
the recessive w allele gives white
color.
•If
a
translocation
occurs in
which the
tip of
an X chromosome carrying w+ is relocated next
to the heterochromatic region of
chromosome 4, w+
locus is inactivated.
•Position-effect variegation is observed in flies that are heterozygotes for such a translocation.
•The
w+ allele is not always expressed because
the heterochromatin
boundary is somewhat variable: in some cells it engulfs and inactivates the
w+ gene, thereby allowing the expression of w.
•If the position
of the w+ and w alleles is exchanged by a
crossover, then position-effect
variegation is not detected.
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