• Question: why do we end up with 3 chromosomes on some jeans in sted of 2

    Asked by skye14 to Anouk, Chris, Judith, Leisha, Seyyed on 20 Jun 2012.
    • Photo: Chris Kettle

      Chris Kettle answered on 20 Jun 2012:

      Hi Skye. Chromosomes are the appearance of DNA that has been folded and wrapped up lots. Like spaghetti around lots of forks. When the DNA is packed really tight (by metaphase in the cell cycle) we can see it using a light microscope. On each chromosome are hundreds of genes (not the other way around).

      Sometimes genes are surrounded by areas of DNA which are very similar to other areas of DNA elsewhere on the chromosome or on another chromosome. When DNA copies itself it usually makes an identical copy. However things can go wrong. If a copy begins at the wrong bit of DNA then we can get extra copies of that bit of DNA or lose that but entirely. If this region of DNA contains genes then these genes are duplicated (so we get three opines not two), or the genes are lost (one copy instead of two)

      Hope that helps explain how chromosomes (and dna) Can gain or lose genes

    • Photo: Judith Sleeman

      Judith Sleeman answered on 20 Jun 2012:

      Do you mean three copies of one particular chromosome in the cell? Like in Downes syndrome?

      Each chromosome is one enormous, long molecule of DNA containing lots of genes and packaged up with proteins to keep the DNA safe and to help squeeze it into the nucleus. Chromosomes are always present in pairs: one each from your two parents (two of chromosome number one, two of chromosome number 12 and so on). Every time a cell divides (mitosis) to make two new cells, it makes sure that each of the new cells has one of each of the chromosomes.

      When the ‘germ’ cells are made, though (the egg and the sperm), this is called meiosis and it works a bit differently. The cell ends up dividing into four eggs or four sperm and each only has one copy of each chromosome, rather than a pair. If a mistake is made during meiosis, one of the eggs or sperm can end up with two copies of one of the chromosomes, instead of just one. Then, when the egg is fertilized by the sperm and the two ‘half’ sets of chromosomes are added together, there will be a total of three copies of that one chromosome: two from one parent and one from the other. Then the cell has three copies of all of the genes on that chromosome, which can lead to problems with the development of the embryo.

      Hope that makes sense: it’s hard to explain without pictures!

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