The DNA in a cell is not a single long molecule. It is divided into a number of segments of uneven lengths. At certain points in the life cycle of a cell, those segments can be tightly packed bundles known as chromosomes. During one stage, the chromosomes appear to be X-shaped. Every fungus, plant, and animal has a set number of chromosomes. For example, humans have 46 chromosomes (23 pairs), rice plants have 24 chromosomes, and dogs have 78 chromosomes.
n the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.
Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing.
~~~ Francis S. Collins, M.D., Ph.D. ~~~