Eugene Isman, age 12, of Fargo, N.D., for his question:

WHAT IS A CHROMOSOME?

A chromosome is a threadlike structure found in the nucleus of each plant and animal cell. It can be seen through a microscope as a particle only when the cell is ready to divide into two cells.

Before division starts, the chromosomes are duplicated. During division, each duplicate chromosome forms into a pair of rods. The new cells that are formed receive one rod from each pair.

The new cells that are formed then have a set of chromosomes exactly like those of the original cell.

Each species or kind of organism has a characteristic number of chromosomes in each body cell. Human beings typically have 46 chromosomes, or 23 pairs, in most of their cells.

Chromosomes are made up largely of DNA (deoxyribonucleic acid) and proteins. DNA is the coded information for every living thing's heredity. Chromosomes consist of large numbers of DNA units called genes.