3.7 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.

- Chromosomes are visible during cell division.

- They are capable of self replication and play vital role in heredity, mutation, variation, and evolutionary development 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

monoploid/haploid (with one set of chromosomes where x=n),

diploids (2n-two sets of chromosomes),

triploids (3n-three sets of chromosomes),

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 chromatids joined together at centromere or primary constriction.

Primary constriction consists 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.

Depending upon the position of centromere there are four types (shapes) of chromosomes viz.

- Acrocentric (j shaped),

- Telocentric (i shaped),

- Submetacentric (L shaped) and

- 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 Y Chromosomes as sex chromosomes. X chromosome is straight, rod like and longer than 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 non-homologous regions. Homologous regions show similar genes while non-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 present on non-homologous region of X-chromosome while Y linked genes re present on non-homologous region of Y-chromosome.