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Name: Cimara
Status: other
Grade: other
Location: CA
Country: USA
Date: Winter 2012-2913

How does an organism get an extra chromosome?

Hi Cimara,

Thanks for the question. Treating an organism with certain chemicals can induce the formation of an extra chromosome. If I recall correctly, treating the strawberry plant with a chemical called cholchicine will induce the formation of an extra chromosome and make the strawberry fruit more plump and juicy. Chemicals that do such transformations are called mutagens.

A mistake in meiosis can cause a human embryo (zygote) to have an extra chromosome. This occurs often in people with Down's Syndrome.

I believe that ionizing radiation may also contribute to the formation of an extra chromosome, but I do not have the time to search the scientific literature and find this out.

I hope this helps. Please let me know if you have more questions. Thanks Jeff

Instead of me trying to explain it, I suggest you look at the following website which explains it nicely in understandable terms.

Stephen R. Dunn Ass't Professor of Medicine (ret.) Kimmel Cancer Center & Division of Nephrology Thomas Jefferson University

Extra chromosomes are due to nondisjunction during meiosis.

In meiosis, DNA ("packaged" in chromosomes) is ordinarily doubled and then partitioned among four cells (either eggs or sperm). In nondisjunction, specific chromosomes don't get allocated correctly. This leaves one cell with an extra copy, while another cell is shortchanged.

In humans, two copies of every chromosome is expected (chromosomes #1 through #22, plus an XX or an XY, for a total count of 46). Individuals with Down's Syndrome have an extra chromosome #21 (three copies instead of two), while those with Turner's Syndrome have only one X chromosome (instead of the expected two). In both cases, the number of the other chromosomes is typically normal.

Dr. Tim Durham Office of Undergraduate Studies University Colloquium Department of Biological Sciences College of Arts and Sciences Florida Gulf Coast University


The most common way an organism can contain an extra chromosome is through what is called a nondisjunction event. Humans and many other organisms have cells that are called 'diploid', meaning every cell contains two copies of each chromosome, one from each parent. One copy is passed on from the mother's egg, and the other is passed on from the father's sperm. That means, of course, that in the process of generating an egg and sperm you must somehow split up the chromosomes so that each egg and sperm only contains one copy of each chromosome (called 'haploid').

The process by which this is achieved is called meiosis. Similar to mitosis, or normal cell division, it involves the duplication of DNA and the separation of half of the genetic material to each daughter cell. However, in meiosis there are two separate cell divisions, leading to four daughter cells, each containing half of the normal number of chromosomes (in humans, 23 instead of the normal 46). During this meiosis process, homologous chromosomes, one from each parent, are lined up in the middle of the cell and pulled apart by specialized cellular machinery. Sometimes this machinery malfunctions, and two of the homologous chromosomes do not separate. This is called a 'nondisjuction event', and means that one haploid daughter cell will have 24 chromosomes, rather than 23. When it combines with a sperm or egg, the resultant zygote (and, eventually, grown organism) will have 47 chromosomes, one more than the usual amount.

This can lead to many potential medical issues in humans, Having three copies of a chromosome can be lethal, or at least lead to significant disabilities. Down's syndrome, sometimes called trisomy 21, is normally due to a nondisjunction event leading to a child having three copies of the 21st chromosome. There are other organisms that can cope much better with extra chromosomes. In particular, many plants are what is called 'polyploid', often having more than two copies of each chromosome.

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