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Cells are the basic units of life. Everyone has millions of different types of cells. There are brain, blood, skin, liver, and bone cells. Cells have different sizes, shapes, and jobs to do. For example, some cells in your blood, called red blood cells, are responsible for carrying oxygen from the heart to other organs. Other blood cells, called white blood cells, are responsible for attacking and fighting germs that can make you sick.
Each cell comes from a cell that already exists. As they grow, cells copy themselves so they can maintain your body and repair damage. In this process, 1 (one) cell turns into 2 cells, 2 cells turn into 4 cells, 4 cells turn into 8 cells, and so on. Every human starts life with only 1 cell. When a cell divides, it copies its information onto the 2 new cells it creates. Cells usually copy information very well.
All cells contain chromosomes. These are packages of DNA, the material that contains genes. Humans have 23 pairs of chromosomes. Each pair has 1 (one) chromosome from each parent, with a copy of that parent’s DNA.
DNA is the material in your chromosomes that contains genes, the instructions needed to maintain life in humans and almost all other life forms. Genes are made of a series of chemicals called adenine, guanine, cytosine, and thymine, or A, G, C, and T. Each gene has these As, Ts, Cs, and Gs in a certain order. The order tells each gene what to make or do in your body. One way to think about this is to remember how the order of letters in a word determines what it means.
Because every cell in your body has the same DNA, each cell has a system that tells cells what do and when to do it. The system allows cells to make different types of tissue and organs, fight disease, and do other things your body needs. For example, this system told your body how to make the right number of fingers, eyes, and ears when you grew inside your mother. Some genes called master genes tell the other genes when to start and stop working.