Susan Boyd, age 12, of Fredericton, New Brunswick, Canada, for her question:
WHO DISCOVERED THE CHROMOSOME?
For more than 2,000 years men have debated the mystery of the human cell. Scientists decided a human being grew from a single egg cell. Some thought this c ell contained a tiny, completely formed human being while others argued that the heart, legs, and arms and other parts of the body developed successively. It took the microscope to begin to solve the mystery of the cell.
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. Before division, each duplicated chromosome forms into a pair of rods. The new cells that are formed receive one rod from each pair. The new cells then have a set of chromosomes exactly like those of the original.
Each kind of organism has a characteristic number of chromosomes in each body cell. Human beings typically have 46 chromosomes (23 pairs) in most of their cells.
Chromosomes are made up largely of DNA (deoxyribonucleic acid) and protein. DNA is the coded information for every living thing's heredity. Chromosomes consist of large numbers of DNA units called genes.
Cell research goes back to 1665 when an English scientist named Robert Hooke observed cells in a microscope. Then a German botanist named Matthias Schleiden in 1838 said that the cell was the basic unit of all life. Through the years the idea was advanced by others.
By the end of the 1800s a number of scientists argued that chromosomes must be the basis of heredity although even by this time the idea was not yet generally accepted.
In 1902 an American scientist named Walter S. Sutton pointed out that during cell division chromosomes behaved the way genes were thought to behave. And a few years later at Columbia University, a scientist named Thomas Hunt Morgan and his associates proved that genes are the units of heredity and that genes are arranged in an exact order on chromosomes.
Two other American scientists, George W. Beadle and Edward L. Tatum, discovered early in the 1940s that genes control chemical reactions in cells by directing the formation of enzymes. They found that there is a specific gene for each enzyme.
DNA had been discovered in 1869 by a Swiss biochemist named Friedrich Miescher. But scientists had dismissed DNA as unimportant, knowing that proteins were essential in the life process.
But then the turning point came in 1944. At the Rockefeller Institute, a team of scientists headed by Oswald T. Avery found that DNA alone was the substance that determined heredity.