BOTANY Higher Secondary Second Year - Textbooks Online

the eukaryotic chromosomes may be rod shaped (telocentric and
acrocentric), L-shaped (sub-metacentric) and V-shaped (metacentric). There
are two types of chromosomes based on their function. They are autosomes
and sex chromosomes.
Autosomes
They are present in all the cells of the organisms. They control the
somatic characteristics of an organism. In the human diploid cell, 44
chromosomes are autosomes whereas the rest two are sex chromosomes.
Sex chromosomes
In the diploid cells of animals and certain plants, one or more special
chromosomes are different from the autosomes in their morphological
structures and behaviour. These chromosomes are involved in the
determination of sex. They are called sex chromosomes. In human being,
male has XY and female XX chromosomes.
Unusual chromosomes
These chromosomes are abnormal chromosomes. They differ from
the basic structure of normal chromosomes. Eg. B–chromosomes and
Double minutes. B–chromosomes are also called supernumerary and
accessory chromosomes. They are additional chromosomes found in some
individuals in a population. eg. maize. They are common in plants and
they reduce viability.
Double minutes are unstable chromosome like structures. They have
no centromere and formal telomeres. They occur in cancer cells which
show resistance against drugs.
Special types of chromosomes
In Eukaryotic organisms certain chromosomes are found only in certain
special tissues and are not seen in other tissues. These chromosomes are
larger in size and are called giant chromosomes. In certain plants, they
are found in the suspensors of the embryo. There are two types of giant
chromosomes – polytene chromosome and lamp brush chromosome.
Polytene chromosomes were observed by C.G. Balbiani in 1881 in
the salivary glands of Drosophila. The characteristic feature of polytene
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