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The Different Cells in the Body
There are a lot of different kinds of cells in the human body. There are skin cells, nerve cells, brain cells, several kinds of muscle cells, bone and teeth cells, red and white blood cells, liver cells, spleen cells, kidney cells, heart and lung and stomach lining cells. Each type does something different in the body and each is highly specialized for what it does. Each is important for the body to survive and grow. All of these cells - called somatic cells - have one thing in common: none of them pass on their genes to the next generation.
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Single-Celled vs. Multicellular Reproduction
In single-cell organisms like bacteria or protists (examples are amoebas, euglenas, or paramecium), reproduction is a simple matter of splitting into two equal-sized smaller cells. The organism makes a complete second copy of its genome, which then undergoes a separation process called "mitosis." One copy goes into each new cell. There is only one parent in this type of reproduction, which is called binary fission.
Most multicellular organisms, however, can't just split in two to make two new bodies that both live and grow. Instead, like all the other functions of a working body, reproduction is specialized. The cells specialized for it are called germinal cells. From a strictly genetics perspective, all the somatic cells of the body are basically just support structure for the germinal cells.
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In addition, many multicellular organisms reproduce sexually. Two parents combine their genes to produce each new offspring. All animals (including humans) are diploid, which means that each cell in the body has two complete copies of the genome - one from each parent. With mitosis alone, the total size of the genome would double with each generation. Therefore, each parent must first create cells that contain only one complete copy of the genome, or are haploid. This is possible via a process called "meiosis" and is what germinal cells do.
The haploid cells that germinal cells produce are called gametes. These are the only animal cells that are haploid. There are two kinds. Eggs are large and contain enough nutrients to get the offspring off to a good start. Sperm are small and highly mobile, just large enough to contain genetic information and find an egg. The two combine to produce a diploid zygote, which then develops into an offspring.
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How is a Gamete Different Than Other Cells in the Body?
To summarize, a gamete is different from all the other cells of an animal body in two important ways. First, it is haploid, with only one copy of the organism's genome. All others are diploid, and have copies.
Second, as part of the germ line descending from the germinal cells, it passes its genes to the next generation.
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Griffiths, Anthony J.F., Jeffrey H. Miller, David T. Suzuki, Richard C. Lewontin, and William M. Gelbart. 1993. An Introduction to Genetic Analysis 5th ed. W.H. Freeman and Company.
Alberts, Bruce, Dennis Bray, Julian Lewis, Martin Raff, Keith Roberts, and James D. Watson. 1989. Molecular Biology of the Cell 2nd ed. Garland Publishing, Inc.
Also see: Life Cycle of Plants: Alternation of Generations to compare how it works in plants.