03 Inheritance and Variation - part 07 - Chromosomes

03 Inheritance and Variation - part 07 - Chromosomes


Chromosomes : 
  • Chromosomes are filamentous bodies present in the eukaryotic nucleus. 
  • The term chromosomes (Gr., Chromo = colour, soma = body) was coined by W. Waldeyer(1888). 
  • The size of chromosome varies from species to species.
  •  Each metaphase chromosome varies from 0.1 to 33 mm in length and 0.2 to 2 mm in thickness. 
  • Visible during cell division. 
  • Capable of self replication.
  • Play vital role in heredity, mutation, variation, and evolutionary devlopment of eukaryotic species. 
  • Chemically eukaryotic chromosomes are made of DNA, histone and non-histone proteins.
Function :  
  • Chromosomes are carriers of heredity. 
Number of chromosomes : 

  • The number of chromosomes is specific and constant for a particular species. 
  • Therefore it is of great importance in the study of phylogeny and taxonomy of the species. 
  • The term Ploidy speaks for the degree of repetition of the primary basic number of chromosomes (i.e. `x') in a cell. 
  • When the chromosome number in a cell is the exact multiple of the primary basic number, then it is called euploidy. 
  • Euploids include  - 
  1. monoploid/ haploid (with one set of chromosomes where x=n)
  2. diploids (2n-two sets of chromosomes)
  3. triploids (3n-three sets of chromosomes)
  4. tetraploid (4n-four sets of chromosomes) and so on. 
  • When the chromosome number is not the exact multiple of the haploid set, it is described as Aneuploidy. 
  • Aneuploidy is either addition or deletion of one or more chromosome (s) to the total number of chromosomes in a cell . 
Structure of chromosome : 

  • Chromosomes are best visible under microscope, when the cell is at metaphase stage. 
  • It is because at this stage chromosomes are highly condensed. 
  • Typical chromosome consists of two chromatides joind together at centromere or primary constriction. 
  • Primary constriction constists of a disk shape plate called kinetochore.
  •  It is at the kinetochore, spindle fibres get attached during cell division. 
  • Besides primary constriction, some few chromosomes possess additional one or two constrictions called secondary constriction. 
  • At secondary constriction I, nucleolus becomes organized during interphase. 
  • A satellite body (SAT body) is attached at secondary constriction II, in very few chromosomes. 
  • Each chromatid in turn contains a long, unbranched, slender, highly coiled DNA thread, called Chromonema, extending through the length of chromatid. 
  • Chromatid consists a double stranded DNA molecule which extends from one end of chromosomes to other.
Types (shapes) of chromosomes
  • Depending upon the position of centromere there are four types (shapes) of chromosomes - 
  1. Acrocentric (j shaped)
  2. Telocentric (i shaped)
  3. Submetacentric (L shaped) and 
  4. Metacentric (V shaped). 
  • The ends of chromosome (i.e. chromatids) are known as telomeres. 
Sex Chromosomes : 

  • The chromosomes which are responsible for the determination of sex are known as sex chromosomes (Allosomes). 
  • Human being and other mammals have X and YChromosomes as sex chromosomes. 
  • X chromosome is straight, rod like and longerthan Y chromosome. 
  • X chromosome is metacentric, while Y chromosome is acrocentric. 
  • X chromosome has large amount of euchromatin (extended region) and small amount of heterochromatin (highly condensed region). Euchromatin has large amount of DNA material, hence genetically active. 
  • Y chromosome has small amount of euchromatin and large amount of heterochromatin, hence it is genetically less active or inert. 
  • Both X and Y chromosome show homologous and nonhomologous regions. 
  • Homologous regions show similar genes whilenon-homologous regions show dissimilar genes.
  • Crossing over occurs only between homologous regions of X and Y chromosomes. 
  • Non-homologous region of X chromosome is longer and contains more genes than that of non-homologous region of Y chromosome. 
  • X-linked genes are persent on non-homologous region of X-chromosome while Y linked genesare persent on non-homologous region of Y-chromosome.

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