Biology for Kids
What are chromosomes?
Chromosomes are tiny structures inside cells made from DNA and protein. The information inside chromosomes acts like a recipe that tells cells how to function and replicate. Every form of life has its own unique set of instructions, including you. Your chromosomes describe what color eyes you have, how tall you are, and whether you're a boy or a girl.
Inside the Cell
Chromosomes are found in the nucleus of every cell. Different forms of life have a different number of chromosomes in each cell. Humans have 23 pairs of chromosomes for a total of 46 chromosomes in each cell.
Can we see them?
Normally we can't see chromosomes. They are so small and thin, we can't see them even with a powerful microscope. However, when a cell gets ready to divide, the chromosomes wind themselves up and become tightly packed. With a high powered microscope, scientists can see chromosomes. They are usually in pairs and look like short little worms.
What do they look like?
When a cell is not dividing (called the interphase of the cell cycle), the chromosome is in its chromatin form. In this form it is a long, very thin, strand. When the cell begins to divide, that strand replicates itself and winds up into shorter tubes. Before the split, the two tubes are pinched together at a point called the centromere. The shorter arms of the tubes are called the "p arms" and the longer arms are called the "q arms."
Different chromosomes carry different types of information. For example, one chromosome may contain information on eye color and height while another chromosome may determine blood type.
Within each chromosome are specific sections of DNA called genes. Each gene contains the code or recipe to make a specific protein. These proteins determine how we grow and what traits we inherit from our parents. The gene is sometimes called a unit of heredity.
When we talk about a gene we are referring to a section of DNA. One example of this would be the gene that determines the color of your hair. When we talk about the specific sequence of a gene (like the sequence that gives you black hair versus the sequence that gives you blonde hair), this is called an allele. So everyone has a gene that determines their hair color, only blondes have the allele that makes the hair blonde.
As we mentioned above, humans have 23 different pairs of chromosomes for a total of 46 chromosomes. We all get 23 chromosomes from our mother and 23 from our father. Scientists number these pairs from 1 to 22 and then an extra pair called the "X/Y" pair. The X/Y pair determines if you are a boy or a girl. Girls have two X chromosomes called the XX, while boys have an X and a Y chromosome called the XY.
Chromosomes in Different Animals
Different organisms have different numbers of chromosomes: a horse has 64, a rabbit 44, and a fruit fly has 8.
Interesting Facts about Chromosomes
- Some animals have lots of chromosomes, but much of the DNA is blank. This blank DNA is called "junk DNA."
- Nearly every cell in your body carries a complete set of chromosomes.
- Some chromosomes are longer than others because they contain more DNA.
- Humans have about 30,000 genes in their 46 chromosomes.
- The word "chromosome" comes from the Greek words "chroma", meaning color, and "soma", meaning body.
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