Structure of Chromosome// Chromosomes

 

STUCTURE OF CHROMOSOMES

Chromosomes were 1^st time observed by Hofmeister.

The term chromosome was given by W.Waldeyer.

During cell division the chromatin material gets condensed to form chromosomes.

Chromosome can be best studied at metaphase stage of the cell division.

Chromosome is called as Genetic carrierbecause it contains specific segments of genetic information and carrythis information to the next generation.

Number, size, and shape of chromosome in a particular species is fixed.

Minimum number of chromosome is found in Haplopappusgracilis(2n= 4) and maximum number of chromosome is found in Aulocantha (2n=1600).

Chromosome consists following parts:-

a.     Centromere-

Centromere is located in primary constriction.

b.    Chromatids-

During Metaphase the chromosome consist of two symmetrical structures, which called Chromatids.

Chromosomes may contain a single chromatid or may contain two chromatids.

Two chromatids of a chromosome are called sister chromatids.

c.      Chromonemata-

Internally chromosome contain a thread like structure which is highly coiled  calledchromonemataor chromonema.

d.    Chromomeres-

Some parts of chomonema are highly dense and these are called chromomeres.

Chromomeres are best seen in polytene chromosomes.

e.      Secondary constriction-

Sometime chromosomes have one more constriction, other than centromere. This is called secondary constriction.

In human secondary constrictions are found in 5 chromosomes – 13, 14, 15, 21, and 22. These chromosomes are called Sat chromosomes.

The portion of the chromosomes beyond the secondary constriction is called Satellite.

f.      Telomere-

Telomere is the extreme tips of the chromosomes.

Types of chromosomes-

A. on the basis of position of centromere-

Based on the position of the centromere chromosomes are four types-

1.     Telocentric:

In these chromosomes, the centromere is located at one end so that there is only one arm is found.

2.     Acrocentric:

In these chromosomes the centromere is present almost near one end so one arm is very long and while other is very short.

3.     Sub- metacentric:

When centromere is not exactly in the middle and isslightly towards one end.

Sub-metacentric chromosomes are L or J shaped.

4.     Metacentric:

When centromere is located exactly in the middle of chromosome and dividing it into two equal arms.

B. on the basis of characters Determination-

On the basis of determination of characters in eukaryotic cells there are two types of chromosomes are –

1.     Autosomes:

In human out of 23 pairs chromosomes 22 pairs of chromosomes are autosomes. These are not related to determine the sex of organism.

2.     Sex chromosomes:

In human 23 paired chromosomes are sex chromosomes. They are called X and Y chromosome. Sex chromosomes determine the sex of offspring.

                      Chromosome structure and types