Section 5: The Book of Nature – Understanding Genes and Mutations
Genes, Proteins and Inheritance
The next section after this one compares human genes and DNA with that of chimpanzees and other primates. If you have studied biology and are familiar with the basics of genetics you should skip directly to that section. For many of you, however, it will be helpful to first give an outline of genetic principles so that you can fully understand the comparisons we are going to make, and so you should familiarise yourself with this section. You will have to indulge in a little delayed gratification, but the outcome will be worth it.
Proteins – the workhorses of the body
The body is composed of 76–100 trillion microscopic cells. These cells, which form the organs and tissues of our bodies, are connected by a framework of proteins, and use other proteins to perform most of their vital functions.
Some proteins are structural. Hair and nails are principally made of proteins called keratins. Another protein, collagen, helps to form the supple, strong framework – connective tissue – which binds cells together in organs, bones and fat and holds the body together with a series of layers of membranes of which the outermost is the skin.
The active substances that produce and control the functions of the body are also proteins of different kinds. Functioning proteins include many thousands of enzymes, which are catalysts of chemical reactions. Digestive enzymes break our food into molecules we can absorb, other enzymes store or release fat and carbohydrate from storage, yet others degrade toxins, attack bacteria, copy DNA and control the development of the embryo. Haemoglobin is an iron-containing protein that carries oxygen in our blood. Insulin is a hormone protein, regulating glucose metabolism. Actin and myosin are the proteins producing motive power in muscles, and antibodies are proteins that target bacteria and viruses for killing. There are also proteins that store memory.
So what are these multifunctional substances? Proteins are very long chains of molecules called amino acids. You can think of a protein as being like a string of beads of different colours.
Some amino acids carry small electrical charges that attract or repel other amino acids in other parts of the same protein. The sequence of these amino acids makes the protein chain fold into a complex 3-dimensional shape that gives the most stable arrangement of the electric charges, and by doing so gives the protein its special function. A different sequence of amino acids would result in a different 3-D shape and a different function. Proteins can fold into more shapes than origami, and they are more versatile than duct tape, or, as we say in New Zealand, a bit of 4-by-2 timber and number 8 fencing wire. Proteins have more applications than an iPhone has apps.
Let me give you one example of how the shape is important. Lysozyme, an antibacterial protein in saliva, has a complex folded shape with a deep fold or cleft part way along it. When it encounters a bacterium, lysozyme’s cleft enfolds a structural molecule called polysaccharide in the bacterial cell wall. The wedged polysaccharide is broken into two bits by its reaction with the charged amino acids in the cleft – it’s a bit like breaking a stick across your knee. The lysozyme releases the fragments and goes on to do the same to other polysaccharides in the same or other bacteria. With their cell walls breached, the bacteria lose their integrity and die. (You can see various animations of this reaction if you do a Google search on “lysozyme reaction animation”.)
Genes are the blueprints for making proteins
It is easy to see that whatever controls proteins, controls the structure and workings of the body. The master architects of the proteins are genes, responsible for producing and regulating all the quarter of a million or so proteins in the body.
Just as proteins are long chains of amino acids, so the genes that code for proteins are also long chains of information called ‘DNA’. The sequence of code in a gene determines the sequence of assembly of amino acids into the protein chain. A gene is a bit like a knitting pattern, which uses English words like ‘knit’ or ‘purl’ to code for stitches that are added one by one to the piece of garment. Genes are inherited, which means the characteristics of the body are passed from parent to child.
Am I My Keeper’s Brother? pp 167-169. Order your copy of the book here.Previous Next Section