Our genome in a nut shell
Imagine that the human genome was an encyclopaedia collection. It would contain 23 books, with two copies of each, to give a total of 46 books. Opening a book would reveal pages with paragraphs – where each paragraph can be broken down into sentences, words, and individual letters. If this collection of books was our genome, it would contain about 22,000 paragraphs, and 6 billion individual letters. Now, imagine that you could shrink all 46 books to fit into a library that is one tenth the size of a human hair. Nature has managed this feat – packing 46 chromosomes, 22,000 genes, and 6 billion letters of DNA into each one of the trillions of cells that make up our body.
Passing on genetic information
In order to reproduce, we need to pass on genetic information. The way in which we do this, is via the sperm and egg. Each sperm and egg cell should contain half of the genetic information from each biological parent. When making sperm and eggs, a cell containing 46 chromosomes divides into two daughter cells (to make sperm or eggs). Each pair of chromosomes is divided, so that each daughter cell receives one copy of each chromosome. When a sperm cell carrying 23 chromosomes fertilises an egg cell carrying 23 chromosomes, the chromosome pairs unite and an embryo is created with 46 chromosomes (23 pairs).
Mistakes can happen
While nature has perfected packaging our genetic material into individual cells, when it comes to making more copies of this information and passing it on, errors can occur. This is particularly so when splitting the chromosome pairs during the production of sperm and eggs. If the mechanism that separates chromosome pairs during egg production mistakenly places two copies of the same chromosome in one egg, the other egg will not receive a copy of this chromosome. When a sperm cell containing one copy of that chromosome fertilises the egg with two chromosomes, the resulting embryo will contain an extra chromosome (three copies, instead of two). Conversely, if the egg that is missing that chromosome is fertilised by a sperm cell containing one copy of that chromosome, the resulting embryo will have a missing copy of that chromosome (only one copy). Missing or extra copies of chromosomes can have two outcomes: (1) the abnormal amount of genetic information is not compatible with life, and the embryo (or baby) will fail to develop and result in a miscarriage, or (2) the abnormal amount of genetic information allows for development, but will result in a genetic condition such as Down Syndrome.