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Dr. R. Siva

VIT University, INDIA

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What Exactly is a chromosome?

What Exactly is a chromosome?

Chromosomes are the rod-shaped, filamentous bodies present in the nucleus, which become visible during cell division.

They are the carriers of the gene or unit of heredity.

Chromosome are not visible in active nucleus due to their high water content, but are clearly seen during cell division.

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Chromosomes were first described by Strausberger in 1875.

Chromosomes were first described by Strausberger in 1875.

The term “Chromosome”, however was first used by Waldeyer in 1888.

They were given the name chromosome (Chromo = colour; Soma = body) due to their marked affinity for basic dyes.

Their number can be counted easily only during mitotic metaphase.

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Chromosomes are composed of thin chromatin threads called Chromatin fibers.

Chromosomes are composed of thin chromatin threads called Chromatin fibers.

These fibers undergo folding, coiling and supercoiling during prophase so that the chromosomes become progressively thicker and smaller.

Therefore, chromosomes become readily observable under light microscope.

At the end of cell division, on the other hand, the fibers uncoil and extend as fine chromatin threads, which are not visible at light microscope

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Number of chromosomes

Number of chromosomes

Normally, all the individuals of a species have the same number of chromosomes.

Closely related species usually have similar chromosome numbers.

Presence of a whole sets of chromosomes is called euploidy.

It includes haploids, diploids, triploids, tetraploids etc.

Gametes normally contain only one set of chromosome – this number is called Haploid

Somatic cells usually contain two sets of chromosome - 2n : Diploid

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3n – triploid

3n – triploid

4n – tetraploid

The condition in which the chromosomes sets are present in a multiples of “n” is Polyploidy

When a change in the chromosome number does not involve entire sets of chromosomes, but only a few of the chromosomes - is Aneuploidy.

Monosomics (2n-1)

Trisomics (2n+1)

Nullisomics (2n-2)

Tetrasomics (2n+2)

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Organism No. chromosomes

Organism No. chromosomes

Human 46

Chimpanzee 48

Dog 78

Horse 64

Chicken 78

Goldfish 94

Fruit fly 8

Mosquito 6

Nematode 11(m), 12(f)

Horsetail 216

Sequoia 22

Round worm 2

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