Eukaryotic-Chromosome-chromatin | Notes-by-UK-Sir | Cell-Bio- 15
CHROMOSOME (Cell Biology)
https://uksirnotes.blogspot.com/2021/08/Eukaryotic-Nucleus-chromatin-uksir.html
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| Chromatin |
Chromosome
•
Thread
like condensed chromatin fiber, which take deep stain and store genetic
information.
•
They
only appear during Karyokinesis (Nuclear division).
•
These are rod shape.
•
It
contains the DNA, which is the hereditary vehicle.
•
Clearly
visible only in Metaphase.
•
Normally appear as two halves – Chromatids.
STRUCTURE
Chromosome
•
Two
chromatids attached at a point - centromere or Primary constriction.
•
At
this point a disc shaped protein group present called as Kinetochore (Spindle
fiber attachment point).
•
Two
parts present either side of Centromere -called arms.
•
At
the terminal part- telomere present (for protection).
•
Chromosome
may contain secondary constriction. It develop small out growth or fragment- called
satellite.
•
Satellite
containing chromosome – called sat-Chromosome or marker chromosome.
•
In
light microscope – a coiled filament present- chromonema , which bear genes.
•
Actually
formed by Condensation to Loop of Chromatin on Scaffold protein- which maintains
the shape.
•
Diameter
mostly- 30nm and formed by single DNA duplex.
•
Ratio
between two Arms- called as centromaric ratio.
•
Isobranchial
chromosome- if arms are equal.
•
HeteroBranchial
chromosome – if arms are unequal.
Types : ( Basis- position of centromere )
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| Chromosome Type |
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Telocentric-
centromere is terminal.
•
Acrocentric
– centromere is inner to telomere or sub terminal.
•
Sub
metacentric- centromere is sub median.
•
Metacentric-
centromere is median. (centromere is in middle)
Giant Chromosome
•
Giant
chromosomes are larger than the normal size-
for which the name is given.
These are of
2 types:
•
Polytene
chromosome
•
Lampbrush
chromosome
1.Polytene
Chromosome
•
1st
time Reported by- E.G. Balbiani 1881.
•
Commonly
called- Salivary Chromosome.
•
As
present in – salivary gland of insects.
•
Size-
upto length 2000 μm.
 |
| Polytene Chromoosome |
•
They
are multi stranded structure, remain in prophage stage.
•
Formed
due to somatic pairing of homologous chromosome.
•
The
number increases due to repeated replication (but can’t separate)
•
Remain
attached at common point-called chromo center.
•
The
nucleus becomes larger. But the cells can’t divide further.
•
Polytene
chromosomes bear many dark bands.
•
Light
area of it called- light inter bands.
•
Sometimes
show swellings called as – puffs.
•
Larger
swellings also present called - Balbiani rings.
•
Here
DNA become uncoiled- form mRNA.
•
Also
present in Endo sperm cell, antipodal cell, suspensor cell etc.
Polytene Chromosome
•
Highly
elongated, lamp brush like structure.
•
1st
time Seen by Flemming 1882 and described by – Ruckert 1892.
•
Can
be found in animal Oocytes, and in some unicelluar algae like Acetabularia.
•
These
are Mid prophage or diplotene chromosome bivalents.
•
Larger
then Polytene chromosome - upto 3 times more. Upto- 5900μm
•
Mostly
Occurs in pairs.
 |
| Lampbrush Chromosome |
•
Homologous
chromosomes attached at a point- Chiasmata.
•
The
chromosomes have lateral projections or loops.
•
This
gives a lamp brush like structure.
•
Loops
can be - upto 100μm
long.
•
Here
some mRNA produced- called Informosomes (m RNA + Protein) for – biochemical
production in early embryo.
Chromosome Function
•
Chromosomes
contain genes.
•
Control
all the Activity.
•
They
replicate themselves and copies transfer to generations.
•
Some
Sex chromosome –determine sex of organism.
•
Form
a link between Parent and offspring.
•
Can
undergo crossing over to from variation.
•
Can
show mutation.
Want to Know about Nucleus, chromatin :
https://uksirnotes.blogspot.com/2021/08/Eukaryotic-Nucleus-chromatin-uksir.html
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