Chromosome

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Organism No. chromosomes

Onion 16

Mold 16

Carrot 20

Tomato 24

Tobacco 48

Rice 24

Maize 20

Haploppus gracilis 4

Crepis capillaris 6

Slide 9

On the extreme, round worm shows only two chromosomes, while the other extreme is represented by Protozoa having 300 or more chromosomes.

However, most organisms have numbers between 12 to 50.

3-8 in fungi

From 8 – 16 in Angiosperms (Most common number being 12).

Slide 10

Chromosome Size

In contrast to other cell organelles, the size of chromosomes shows a remarkable variation depending upon the stages of cell division.

Interphase: chromosome are longest & thinnest

Prophase: there is a progressive decrease in their length accompanied with an increase in thickness

Anaphase: chromosomes are smallest.

Metaphase: Chromosomes are the most easily observed and studied during metaphase when they are very thick, quite short and well spread in the cell.

Therefore, chromosomes measurements are generally taken during mitotic metaphase.

Slide 11

The size of the chromosomes in mitotic phase of animal and plants sp generally varies between 0.5 µ and 32 µ in length, and between 0.2 µ and 3.0 µ in diameter.

The longest metaphase chromosomes found in Trillium - 32 µ.

The giant chromosomes found in diptera and they may be as long as 300 µ and up to 10 µ in diameter.

In general, plants have longer chromosomes than animal and species having lower chromosome numbers have long chromosomes than those having higher chromosome numbers

Among plants, dicots in general, have a higher number of chromosome than monocots.

Chromosomes are longer in monocot than dicots.

Slide 12

In order to understand chromosomes and their function, we need to be able to discriminate among different chromosomes.

First, chromosomes differ greatly in size

Between organisms the size difference can be over 100-fold, while within a sp, some chromosomes are often 10 times as large as others.

In a species Karyotype, a pictorial or photographic representation of all the different chromosomes in a cell of an individual, chromosomes are usually ordered by size and numbered from largest to smallest.

Slide 13

Can distinguish chromosomes by “painting” – using DNA hybridization + fluorescent probes – during mitosis

Slide 14

Karyotype: is the general morphology of the somatic chromosome. Generally, karyotypes represent by arranging in the descending order of size keeping their centromeres in a straight line.