Chromosomes: Structure and Function

Chromosomes

• The nucleus of every cell contains threads called chromosomes
• Chromosomes were discovered in the nineteenth century
• Scientists first had sufficiently powerful microscopes to see very small things, such as the structure of cells
• When viewed looking at cells using a microscope, chromosomes are not visible unless the cell is dividing
• Chromosomes become visible when stained with special stains
• This photograph shows a plant cell that is just beginning to divide into two cells
• The cells have been stained with a special dye that colours the chromosomes dark red
• You can't really see other structures in the cell
• This photograph shows a chromosome from a fruit fly. Fruit flies have especially thick chromosomes, called giant chromosomes, which are much easier to see than most chromosomes
• Different species of organisms have different numbers of chromosomes in the nuclei of their cells
• Human cells have 46 chromosomes
• Fruit fly cells have eight chromosomes
• Mango trees have 40 chromosomes
• The longest chromosome in a human cell is chromosome 2
• We have two of each kind of chromosome in a human cell
• The photograph shows all the chromosomes in a human cell, lined up in size order
• If you look very closely at this photograph, you may be able to see that each chromosome looks like a cross shape
• Each chromosome makes a copy of itself just before a cell divides. The two copies stay joined together part-way along their length

Chromosome numbers of different organisms

• Potato - 48 chromosomes
• Shark - 28 chromosomes
• Lion - 38 chromosomes
• Parrot - 80 chromosomes
• Dog - 38 chromosomes
• Sunflower - 17 chromosomes
• Lily - 24 chromosomes