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When a cell reproduces itself, it also reproduces everything that is contained within it, including the chromosomes. Cells need to reproduce themselves frequently so that growth can occur, and to replace old and worn out cells. Normally this occurs through a process which scientists call mitosis. |
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In mitosis the chromosomes in each cell undergo a process that enables them to duplicate. Once the chromosomes have duplicated they then move to either end of the cell. |
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When the chromosomes are at either end the cell then splits into two. The final result is that you have two "daughter" cells that are identical to the parent cell. If you would like to learn more about mitosis please click here. |
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A simplified animation of mitosis |
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A baby also starts out as a single cell that reproduces itself. It is during these very early stages of the cell dividing that many chromosomal anomalies occur. Ova (female eggs) and sperm however undergo a different process in the beginning, called meiosis, to reproduce. |
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Unlike other cells in the human body, when a human egg or sperm cell are produced by the body, each pair of chromosomes in the egg or sperm divide into two and only one remains. So the "ripened" egg and the sperm each contain half the complement of chromosomes that the baby needs. |
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A diagram illustrating the process of meiosis |
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In the case of the male, all four cells will eventually develop into typical sperm cells. In the case of the female, three of the cells become non functional polar bodies and will typically abort, leaving a single cell to develop into an egg cell. |
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A simplified animation of meiosis |
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This means that in each ovum (or egg within the ovaries) there should be 23 of the mothers chromosomes (including the sex chromosome) and 23 in the fathers sperm. When an ovum containing 23 of the mothers chromosomes is fertilised by the sperm containing 23 of the fathers chromosomes, the resulting combination of 46 chromosomes form the basis of the new babies cells and the unique genetic blueprint for that individual baby. |
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It is thought that the majority of cases of trisomy occur during these stages of cell division. That is, it is thought, that an error occurs in the division of the cell leaving 24 chromosomes in either the egg or the sperm cell. |
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What is a Trisomy Index |
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| Home | What is a Trisomy? | What is Trisomy 18? | What is Trisomy 13? || Alex's Story | Stories From Special Friends | In Memory of Special Friends | | Special Occasions | Awards | Shapiro's Syndrome | SOFT Australia | Trisomy Listerv | |
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Visitors Since 19th November 2000 FastCounter by bCentral |
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