Chromosomes are long strands of DNA, or deoxyribonucleic acid. DNA -- the material that holds genes -- is considered the building block of the human body. The term "chromosome" comes from the Greek word for color, which is “chroma,” and the Greek word for body, which is “soma.” Chromosomes are threadlike structures that scientists stain using colorful dyes when performing research.
Location and Function
One primary characteristic of chromosomes is that chromosomes are located in the center of cells, called the nucleus. This characteristic applies to both animal and plant cells. Each chromosome actually features protein and a single DNA molecule. The DNA remains wrapped around histones, which are spool-like proteins, because of the unique structure of chromosomes. In addition, chromosomes are a key part of the process of accurately copying DNA and distributing it in many cell divisions, as cells constantly divide to produce new cells that replace old, wornout ones.
Chromosomes come in pairs. Each human body cell actually has 23 pairs of chromosomes, for a total of 46 of these DNA strands. Half of your chromosomes come from your mother, while the other half comes from your father. Other species have their own set number of chromosomes: a dog has 39 pairs, for example, while a rice plant has 12 pairs and a fruit fly has four pairs of chromosomes.
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X and Y
The fact that the X and the Y chromosomes -- two types of human chromosomes -- determine if an individual turns out to be a boy or a girl is another characteristic of chromosomes. The X and Y chromosomes are sex chromosomes. Females have two X chromosomes, while males have one X chromosome and one Y chromosome.
A characteristic of chromosomes is that a mother always contributes an X chromosome to her child, while the father of the child can contribute either an X chromosome or a Y chromosome. As a result, the father is the parent who determines the sex of a child. Still, a child inherits some traits from his mother and other traits from his father.
Outside of the X and Y chromosomes, the other chromosomes in the 23 pairs in the human body are called autosomal chromosomes. The autosomal chromosomes are considered chromosome pairs 1 through 22. Reproductive cells such as eggs and sperm must have the right number of chromosomes in order to have offspring that develops correctly. For example, individuals with Down syndrome have three copies of chromosome 21 instead of the two copies found in other individuals -- something considered an autosomal abnormality.