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Simple Genetics

A human body is made up of fifty trillion cells. The cell is the basic structural and functional unit of all organisms. Cells form colonies or tissues in higher plants and animals. Cells have many different functions but inside almost every cell is a nucleus containing 99.9% of the genes and mitochondria containing a few more genes. Humans have approximately 20,000 genes. The genes are small parts of a long molecule called DNA. DNA is a double stranded molecule composed of sugar, phosphate and 4 different bases adenine, thymine, cytosine and guanine. Watson and Crick elucidated in 1953 the famous double-helical structure of the DNA molecule. The elegance of the DNA double helix probably helped make it the most popular notion to come out of molecular biology. But what made this discovery so important was not the helical shape, but the discovery that the DNA molecule consists of two complementary strands. By complementarity, we mean that a thymine (T) on one strand is always facing an adenine (A) (and vice versa) — and guanine (G) is always facing a cytosine (C). These couples, A-T and G-C, although not linked by a chemical bond, have a strict one-to-one reciprocal relationship. When you know the sequence of nucleotides along one strand, you can automatically deduce the sequence on the other one. This amazing property explains everything about DNA sequences. For instance, when living organisms reproduce, each of their genes must be duplicated. In order to do this, nature doesn’t go about it the way a photocopier would by making an exact copy. Rather, nature separates the DNA strands and makes two complementary ones, thanks to the magical two-sided structure of DNA molecules. DNA is coiled so tightly that it fits in just one cell nucleus. It is the number and order of these 4 bases (known as the genetic code) that determine a mouse, a strawberry, a human etc…

Most genes are recipes for making specific proteins. All these proteins are made up of the same basic building blocks, called amino acids. Amino acids are already quite complex organic molecules, made of carbon, hydrogen, oxygen, nitrogen, and sulfur atoms. Biochemists discovered that these amino acids are linked together as a chain and that the true identity of a protein is derived not only from its composition, but also from the precise order of its constituent amino acids. This ribbon of amino acids, however, is not what gives the protein its biological properties (for instance, its ability to digest sugar or to become part of a muscle fiber); those come from the three-dimensional (3-D) shape that the ribbon adopts in its environment. A protein molecule, once made, is not a chainlike, highly flexible object; rather, it’s more like a compact, well-bundled ball of string. The final 3-D shape of the protein molecule is uniquely dictated by its sequence because some amino-acid types (for instance, hydrophobic residues L, V, I) have no desire whatsoever to be at the surface interacting with the surrounding water — while others (for instance, hydrophilic residues D, S, K) are actively looking for such an opportunity. The protein chain is also affected by other influences, such as the electric charges carried by some of the amino acids, or their capacity to fit with their immediate neighbors.

Genes tell a cell how to function and what traits to express. The long molecules of DNA containing the genes are organized into pieces called chromosomes. Different species have different number of chromosomes. Humans have 2 sets of 23. Domestic pigs have 19 pairs, chimpanzees have 24 pairs and lab mice 20 pairs. Our genes come from our parents. We have two sets of 23 chromosomes one set from each parent. The X and Y chromosomes however, are special. Usually females have two X (XX) chromosomes while males have an X and an Y (XY). We get one of our X chromosomes from our mother. Whether we get our fathers X or Y chromosome determines our sex. Most adult cells contain 2 sets of chromosomes but sperm and egg cells have 1 set of 23 chromosomes each. When the body forms sperm or egg cells, a cell divides and pairs of chromosomes separate. A random member of each pair moves into each new cell. To form sperm or egg cells our chromosomes double. When the homologous pairs separate sometime they cross over and at seemingly random points exchange DNA. This is called genetic recombination. Because your genes get shuffled during recombination the chromosomes we pass along to our children are not exactly the same as the ones we inherited from your parents.

An entire set of 23 human chromosomes is called a genome. The human genome is composed of 3 billion base pairs. Variation at a single base pair is called a SNP (single nucleotide polymorphism). When the body makes new cells it does not make many mistakes but sometimes when the genome is copied to make a new cell a single base pair gets left out, added, or substituted. Single base pairs substitutions create SNPs. There are around 10 million SNPs in the human genome which account for many of the genetic differences between everybody on the planet. Since variants are passed on from one generation to the next the number of differences between our DNA and our friend’s for example can tell us how closely we are related to each other. Getting to know our genome can help us understand why we are the way we are and in what ways we are similar to or different from our family, friends and neighbors.

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