Hi, I'm Jim Shields for About.com, and today I'm going to teach you about chromosomes. The genetic information needed by the cell is stored in the DNA, which resides in the nucleus of a eukaryotic cell. During mitosis, which is the process of cell reproduction, the DNA is tightly coiled into a strand called a chromosome. When I say tightly wound, this is what I mean: if you stretched the DNA in one human cell into a single long strand, it would be over 6 feet long. When tightly coiled in chromosome, it is less that one-tenth of a millimeter long. The DNA is coiled around a protein called a histone that both supports and helps organize the arrangement of chromosomes in the nucleus. The chromosome is actually two strands of identical DNA, tied together like a bow near the center of the strands by a centromere, which connects them and helps during the process of mitosis, or cell reproduction. Here's a way to visualize what a chromosome is. Imagine you've invited a friend over and you're going to teach your friend how to knit -- and you and your friend are going to make identical sweaters. When you go to open your knitting basket, you find that all your yarn -- all the different colors and textures -- are all mixed up into one big knot. The knitting basket is your nucleus. And the ball of all the loosely mixed-up yarn is chromatin.Now before my friend gets here, I'm going to need to pull those strands out and tightly wind them into balls of each different color and texture of yarn so that I can divide them up evenly so we both start with the same material to make the identical sweaters. The balls of yarn, which were once a disorganized pile of chromatin, are now discrete balls, which are like chromosomes. The chromosomes contain all the genetic information that cells need for the life of the organism. Chromosomes determine genetic traits such as eye color, sex, hair type and skin color. Thanks for watching. To learn more about biology, visit us on the web at About.com.
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