Eukaryotic Chromosomes



Eukaryotic Chromosomes:

In Eukaryotic  DNA occurs in nucleus and semi_ autonomous organelles ( viz. mitochondria, plastids). Nuclear DNA is organized into true or eukaryotic chromosomes while organelle DNA constitutes organelle prochromosome that resembles prokaryotic chromosome. Eukaryotic chromosomes are DNA_ histone complexes. Chromosomes are not visible during interphase as they are highly elongated and thin forming chromatin fibres. Chromatin fibres can be stained like the chromosomes . Chromatin fibres show overlapping  and form a network, called chromatin reticulum. The chromatin fibres condense during prophase. The best time to observe and to count the number of chromosomes is late prophase and metaphase.


Shape: Chromosomes  are rod__ shaped in outline. The smaller one  may appear as dots with rounded  or oval outline. The larger chromosomes  may be straight  or bent to become  arc__ like, horse__ shoe shaped or sigmoid.


Size: The size of chromosomes varies from 0.5 ųm __ 30.0 ųm in length  and 0.2 ųm __ 3.0 ųm in breadth. Chromosomes are usually larger in organisms having fewer of them. Chromosome is usually short in fungi and birds ( 0.25 ųm) and is quite long in plants like Trilium ( 30 ųm). Most metaphase chromosomes fa within a range of 3 ųm in Drosophila,  5 ųm in and  8__ 12 ųm in maiz. In some organisms ( e.g. Parascaris equorum) a single large chromosome breaks up into many smaller chromosomes in the metabolically active state. The various chromosomes  of an individual  show differences  in size, shape and other characteristics. The size of the same chromosome often differs in mitotic and meiotic metaphases. Unlike prokaryotes, eukaryotic chromosome structure and its organization is much more intricate.This complexity  is because  of greater amount of DNA per chromosome , more number of chromosomes and large number of proteins associated  with  DNA. Taken the distance between  two consecutive  base pairs as 0.34 nm, if the length of DNA double helixIn a typical mammalian cell is calculated ( by multiplying the total number of bp with distance between  two consecutive bp, that is 6.6× 10⁹ bp × 0.34× 10-⁹m/bp), it comes out  to be approximately  2.2m.In comparison to E.coli chromosome of about 1.2nm (  According  to NCERT 1.36mm), the 46 human chromosomes taken together  will measure  upto 2 meters. Such a large amount of chromatin material  is located inside a nucleus, which is not bigger than 5ųm in diameter . So the chromatin condenses almost 10,000 times to give the size of a mitotic  chromosome. The overall compression of the DNA has been described  as packaging  ratio. It is the length  of DNA divided  by the length  of unit that contains it. The packaging ration in eukaryotes reaches 1000__ 2000 in interphase nuclei and upto 10,000 in metaphase  nuclei.



Genomes:

Genome is the complete but single set of chromosomes. The condition of having a single genome or a set of chromosomes is called monoploid. It is written as 1n. The somatic cells of animal and higher plants possess two genomes. The condition is called diploid ( 2n). Several modern day crop plants have more than two sets of chromosomes in their somatic cells viz., triploid ( 3n e.g., Banana), tetraploid ( 4n e.g., Rice), hexaploid ( 6n, e.g., Wheat). The condition of having more than two sets of chromosomes is known as polyploid .It is quite common in ferns  and mosses. Polyploid seems to be causative factor of very large number of  chromosomes present in some organisms. 


   Gametes possess half the number of chromosomes found in zygote. This condition of having half the number of chromosomes is called haploid. The genetic number of chromosomes is monoploid (1n) in diploid organisms but in polyploid forms, it is more than monoploid, e.g., 2n,  3n. That means in diploid organisms monoploid and haploids are identical while in a tetra or hexaploid with 4n or 6n chromosomes, the haploids  will possess  2n or 3n chromosomes whereas  its monoploid will possess  only one set (n) of chromosomes. In order to avoid confusion  in their regard the gametic and zygotic conditions  are provided with separate  symbols of x and 2x. Male Honey Bee is haploid as it develops parthenogetically from an unfertilized  egg. The female bee is diploid.


 Number:

The minimum number of chromosomes found in eukaryotes is 2n/ 2× = 4 ( Haplopappus gracilis, a plant and Ophryortocha puerilis, a polychaete) or n/x= 2 ( Mucor hiemalis).A single chromosome breaks up into as  many as 190 chromosomes in the somatic cells. The maximum  number  of chromosomes in plants is found in Adder's Tongue fern ( Ophioglossum reticulum). This number is 1262. The maximum  number  of chromosomes is found  in radiolarian protist,  Aulocantha which is 1600. Amoeba proteus contains 250 chromosomes while the diploid number of chromosomes is 20 in Maize and 46 in man. Thus, the number of chromosomes has no relation to the size, form and complexity of the organism. The similarity in number does not indicate  any phylogenetic relationship.

For example, both fowl ( a bird) and dog ( a domesticated  carnivore mammal) have the same chromosome number  of 78. However,  similar morphology  and banding pattern  act as an indicator  of  phylogenetic relationship. Thus the chromosomes of man ( 23 pairs) and apes ( 24 pairs) are quite similar  in size, shape and banding. 





Sex Chromosomes and Autosomes:

Sex chromosomes  determine  the sex of the individual  in unisexual  organisms. They are called allosomes or idiochromosomes or hetersomes. A sex chromosome that determines male sex is termed androsome. The chromosomes,  other than the sex chromosomes are known  as autosomes. Sex chromosomes may be simi in one sex and dissimilar  in the other. The two conditions  are respectively  called homomorphic and heteromorphic .Individual having homomorphic sex chromosomes produce only one type of gametes. They are called homogametic (e.g., human female). Individual  having heteromorphic sex chromosomes produce two types of gametes  ( e.g., X and Y condaining). They are termed as heterogametic ( e.g.,  human male).



Physical  Structure of Eukaryotic Chromosomes:

In prophase and metaphase, a chromosome possess two chromatids. They are attached  to each other at a narrow and nonstainable area called centromere  or primary  constriction.  Anaphasic chromosomes consist of a  single chromatid. The two parts of the chromosome on  either side of centromere are called arms. The two arms may be equal ( isobrachial) or unequal ( heterobrachial). The position of centromere is fixed for a chromosome.




 The centromere may be present  at the centre ( metacentric), near the centre ( submetacentric), subterminal ( acrocentric) or terminal  ( telocentric) position.  During anaphase the chromosome bends in the region of centromere.  Depending  upon the position of centromere, an anaphasic chromosome can thus have V_ shaped, L_ shaped, J_ shaped or I _ shaped .The common  type  of  chromosome  based on number of centromeres is as follows:


(i) Monocentric : Monocentric with one centromere. 

(ii) Dicentric : Dicentric with two centromere. 

(iii) polycentric with more than two centromeres. 

(iv) Acentric without centromere. Such chromosomes  represent  freshly broken segments  of chromosomes which do not survive for long.

(v)  Holocentric or diffused with indistinct centromere  diffused throughout  the lenght of chromosome. They are found in some insects, some algae ( Spirogyra)  and  some  plants  like  Luzula.



In Eukaryotic  chromosomes , two types of proteins  are  present  alongwith  the  DNA__ histones and  nonhistones  chromosomal ( NHC) proteins. 


Histones: 

Histones are low molecular  weight basic proteins  rich in  amino acids , lysine and arginine.  These proteins are positively charged at neutral pH. The positive charge of histones ( because  of exposed NH3+ group of arginine and lysine) helps to bind DNA ( which is  anionic  due  to negatively  charged phosphate group). The histones thus play a  crucial part in packing long DNA molecules. Histones are present in all Eukaryotic cell types although a few exceptions exist, most notably some sperms, where histones are replaced by another  class of small basic proteins  called protamines. ( The further details  of histrones are discussed under the Radial Loop Model).



Non_ Histone:


Non_ Histone  proteins  are high molecular weight acidic proteins  that possess abundant amount of tyrosine and tryptophan.  They are required for packaging  of chromatin at higher levels. These proteins are of three types__ structural,  enzymatic  and regulatory. 

● Structural proteins are also called scaffold proteins  as they constitute  the core or  exis of the chromosome. 

● Enzymatic proteins act as enzymes  which are involved  in chemical transformations ( e.g.  DNA polymerase , RNA polymerase, phophatase) .Regulatory  proteins  regulate gene expression. A type of regulatory  proteins  are HMG ( high mobility  group) proteins  which associate with histones  to allow gene expression.  The heterogeneous  nature of non__ histone proteins suggest that these proteins  are not as conserved in evolution  as histones. These proteins  differ even between  different  tissues  of same organism suggesting that they regulate  the activity of specific  genes.  RNA,  Ca²+  and other ions seem to be required  for organization  of chromosome.




































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