Chromosomes The term chromosome was coined by W Waldeyer in 1888 Their structures varies in different organisms like they are circular in bacteria but linear in eukaryotic organisms however there number remains constant for a species ID: 913839
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Slide1
Chromosome Nomenclature, Types & Structure
Slide2Chromosomes
The
term chromosome was coined by W.
Waldeyer
in 1888
.
Their
structures
varies in
different organisms
like
they are circular in
bacteria
but
linear in eukaryotic organisms, however there number remains constant for a species.
Slide3Size of
Chromosomes
The
size
varies
from species to species
or
even within the same
species.
The
size
of chromosome is normally
measured at
mitotic metaphase
stage where they obtained their maximum
volume.
In
most of the organisms, their size range from
0.1 to 30
μm
in length
and
0.2 to 2.0
μm
in thickness
.
Slide4Organisms with
less number of chromosomes contain relatively
larger sized
chromosomes.
In general, chromosomes
in case of plants
are
larger in size
than in animals.
Among plants, cells of monocots contain larger chromosomes than cells of
dicot
plants.
Slide5Shape of chromosomes
Shape
is
changeable from phase to phase during cell growth and cell division
i.e. the
interphase
chromosomes occur in the form of
thin, coiled
, elastic, contractile thread like structure (a kind of chromatin threads).
However
,
during metaphase and anaphase
they become
thick and filamentous
.
Slide6A chromosome has two arms which are held together at a point called
centromere
.
The position of this centromere varies that provides different shapes.
If the centromere is present at the centre giving two equal arms then the chromosome will assume
'V' shape
.
But if the centromere is slightly away from the centre with one arm is shorter than the other arm assumes an
'L' shape
or
'J' shape
, however, when the centromere is present on the terminal point having only one arm and other arm is absent, they assumes a
'I' shape
.
Slide7Types of chromosomes
:
Based
on the
position of the
centromeres
chromosomes are of
four
types
:
1.
Telocentric
2.
Acrocentric
3
.
Sub-
metacentric
4.
Metacentric
Slide81.
Telocentric
where
centromere
present on the proximal end. They look like
rod shaped
2.
Acrocentric
where
centromere
present near one end, thus one arm is very short while the other arm is
ver
long. They also appear as rod shaped
3
.
Sub-
metacentric
where
centromere
occur near the centre resulting in two unequal arms, so they appear as L or J shaped)
4.
Metacentric
chromosomes where
centromere
occur in centre thereby forming two equal arms becoming V shaped.
Slide9Slide10Slide11Based on functions, they are two types:
(1)
Autosomes
and
(2)
Sex chromosomes.
Slide12(1)
Autosomes
:
F
ound in all the body cells
and they
decide the somatic characters
of the organism.
In humans, there are 46 chromosomes out of which 44 are
autosomes
and the remaining two are sex chromosomes.
Slide13(2) Sex chromosomes:
Involved in the
sex determination
.
In both
animals
and plants, these chromosomes are found different in both structure and functions.
In human males there are two sex chromosomes as XY and in human female there are two sex chromosomes as XX.
Slide14Unusual of Chromosomes
:
Beside these normal chromosomes, certain animals and plants possess one or more chromosomes which are quite different from the normal chromosomes in their basic structure, size and shape.
These
chromosomes are called unusual of chromosomes, examples
B-chromosomes
.
The
other name of B-chromosomes is accessory and supernumerary chromosomes.
These
chromosomes are actually extra chromosomes that are found in certain individuals especially in plants like in case of maize.
Slide15Special types of chromosomes
:
Found
in certain specialized eukaryotic cells.
They
are very large in size and hence called as giant chromosomes.
They
are of two types
(
1)
Polytene
chromosomes
and
(
2) Lamp brush
chromosomes
Slide16(1)
Polytene
chromosomes
First discovered by
C.G.Balbiani
in the year 1881 in the
salivary glands of drosophila
.
Hence called as
salivary gland chromosomes
.
They have an
extremely large puff in the center
called as Chromosomal puff or
Balbiani
ring.
Along their body, they have
alternat
light bands
(clear zones) and
dark bands
respectively like in a zebra.
Slide17(2) Lamp brush chromosomes
First discovered by
Flemming
in the year 1882.
They got this name as they resemble the
shape of a brush
.
They are generally seen in the
diplotene
stage
of the
meiotic prophase
in the
oocytes
of
Salamandor
and in the giant nuclei of the unicellular algae
Acetabularia
.
They have an extremely condensed central axis from which lateral loops made up of DNA extend outwards.
This is due to the intense RNA synthesis.
Slide18Number of
chromosomes
Each species of plants and animals has a definite constant
somatic
and
gametic
number of chromosomes
.
The
somatic
or body cells contain
2 diploid set
of chromosomes which is represented as
2n
.
This
diploidy
is achieved by the union of two haploid male and female gametes
in sexual reproduction
.
Slide19The
Gametic
chromosome number
is the number of chromosomes found in the
gametes
of a species.
It is precisely
one half of the somatic number
and is
represented by n
commonly called as
haploid number
.
So somatic chromosome number is commonly known as the
diploid number.
The total number of genes present in a haploid set of chromosomes is called genome
.
Therefore one haploid set has one genome, whereas, the diploid cell will have two genomes.
Slide20Structure of Chromosomes:
A typical chromosome has similar structures called
chromatids
.
Since
they are similar, they are called sister
chromatids
.
The
chromatids
are connected to the center part called
centromere
.
A
centromere
is a spherical structure which is a network is protein
fibres
.
Slide21Slide22Slide23The
centromeres
vary in number in different chromosomes
.
A chromosome with only
one centromere is called
monocentric
and that one
with many
centromeres
is called polycentric
.
Slide24The structure of chromosome varies during various phases of cell cycle, thus, various term are used to define the structure of chromosomes:
1.
Chromonemata
:
During mitotic
prophase
, when the chromosomal material becomes visible as very thin filaments, they are called as
chromonemata
.
Chromonemata
is actually an earlier stage of
chromatid
that consists of a single linear DNA molecule with associated proteins. It is gene bearing portion.
Slide252.
Chromatids
:
At mitotic
metaphase
each chromosome consists of 2 symmetrical or similar structures called as
chromatids
.
Since they are similar, they are called sister
chromatids
.
Each
chromatid
contains a single DNA molecule.
Both
chromatids
are connected to the center part called
centromere
and become separated at beginning of anaphase when sister
chromatids
of a chromosome migrate to opposite pole.
Slide263.
Chromomere
:
These are regions of tightly folded DNA.
These are bead like accumulations of chromatin material visible along
interphase
chromosome.
Chromomere
become clearly visible in
polytene
chromosome where they become aligned side-by-side constituting chromosome
beeds
.
However, at metaphase, the chromosomes are tightly coiled therefore, these
chromomeres
become invisible.
Slide274.
Centromere
&
Kinetochore
:
There are of two types of constrictions, primary and secondary constrictions.
The
primary constriction is made up of
centromere
and
kinetochore
.
A
centromere
is a spherical structure contain a network is protein
fibres
.
Centromeres
vary in number in different chromosomes i.e. chromosome with only one
centromere
is called
monocentric
while with many is called polycentric
.
Centromere
consists of specific DNA sequences with special proteins forming
kinetochores
.
So,
kinetochore
is a plate or cup like disc situated upon primary constriction or
centromere
.
Centromere
is the region to which are attached the
fibres
of mitotic spindle and allows the
chromatids
to move.
So during mitosis, 4-40 microtubules of mitotic spindle become attached to
kinetochore
and provide the force for chromosomal movement.
If the
centromere
is damaged, then the chromosome cannot move in the anaphase stage.
Slide29Slide30Slide31Slide32Slide335.Telomeres:
Each extremity of chromosomes has a polarity and therefore it prevents other chromosomal segments to be fused with it.
The chromosomal ends are known as Telomeres. If a chromosome breaks, the broken ends can fuse with other chromosome due to lack of the Telomeres.
Slide346. Secondary Constriction:
In addition to primary constriction or centromere the chromosomes may have some more constrictions at any point on the chromosomes.
All such constrictions other than the primary constriction are called secondary constrictions.
Slide357.
Nucleolar
organizers:
There are certain secondary constrictions that contain genes coding for 5.8 s, 18 s and 28 s ribosomal RNA that induce formation of nucleoli.
Such
secondary constriction that gives rise to the nucleoli formation are called as
nucleolar
organizers.
In
human beings
nucleolar
organizers are located in secondary constrictions of chromosomes 13, 14, 15, 21, and 22 all of which are
acrocentric
and have satellites
.
Slide36A small structure attached to the lateral end of the chromosome by a secondary constriction is called
satellite
and such chromosomes with the satellite are called SAT-chromosomes.
Centromere
is another lateral structure attached to the chromosome.
It gives stability to the chromosome.
Slide37Slide38A typical chromosome has the satellite at its one end and the
centromere at the other end.
A chromosome is made up of
DNA
, RNA,
histone
and non-
histone
protons
and some of them also have metallic ions like Ca2+ and Mg2+.