Chromosomes
DNA is packaged in the cell into structures called chromosomes.
Chromosomes are the form by which genetic information is passed from old cells to new cells, one generation to the next, resulting in the successful and reliable inheritance of traits.
Nucleotides make up DNA. DNA makes up genes, and genes are small segments of chromosomes. (Image by P. Hain)
Every cell in an organism contains two copies of every chromosome present in that organism. For example, humans have 46 chromosomes in their body, 23 were inherited from the father and 23 from the mother. Gametes, the reproductive cells of an organism, (egg or sperm), have only one set of chromosomes. When the two gametes unite, they form a living embryo with two sets of genetic information. Therefore, we actually have two copies of the genetic information for each trait. Sometimes one copy controls trait expression, and other times both copies influence a trait. As a result, the offspring will have characteristics of both the mother and the father.
Chromosomes from brome grass (Bromus inermis). Photograph taken through a light microscope at 1000X magnification. (Image by Metin Tuna)