National Human Genome Research Institute graphic
A representation of DNA’s double helix is shown.
As of Saturday, December 14, 2013
One way of viewing modern biology is to make a list of unifying ideas that tie such a huge field of science together. The shortest list would consist of evolution alone, for as a biologist named Theodosius Dobzhansky once said, “Nothing in biology makes sense except in the light of evolution.”
A longer list might also include cell theory, genetics and energy use by organisms. But whatever list you choose, genetics and DNA play a key role in all of biology.
From ancient archaea and bacteria, through insects, plants and mammals, all life on Earth shares the same genetic code. DNA is more than just heredity and reproduction, underlying nearly all cell growth and function combined.
Every cell of every creature, except for sperm and unfertilized eggs, contains a complete set of that organism’s DNA, carrying the blueprints for its development, growth and life itself.
DNA stands for deoxyribonucleic acid. Wound together like a twisted ladder in a pattern called a double helix, a DNA molecule consists of small repeating subunits made up of a sugar (deoxyribose), a phosphate group, and one of just four bases: adenine, thymine, guanine or cytosine.
These four base units, which link together in pairs, make up the entire alphabet that spells out every protein in our body. The order they occur in along the DNA strand ultimately guides the cell’s machinery, including when and how to make proteins.
Human DNA stretched out from a single one of our cells would measure over six feet in length, and consists of nearly three billion base pairs. In the cell’s nucleus, this genetic material is tightly packaged and wound up in shorter segments called chromosomes. We have 46 chromosomes per cell, 23 from each parent, with 25,000 individual genes scattered along their lengths.
Genes range from a couple hundred to two million base pairs in length, each gene coding for a single protein such as hemoglobin or insulin. But genes only account for a few percent of the total DNA in most organisms’ chromosomes.
For years the long stretches of non-gene DNA were considered “junk” DNA, possibly leftovers from evolution and millions of years of being passed along. Now, ever increasing attention is being paid to these areas, which seem to play a key role in how and when genes become active.
Such gene “regulation” plays a critical role from the time of conception. As a fetus develops, genes are constantly being turned off and on. These carefully timed changes direct cells not only into becoming specialized types like skin, heart or liver cells, but also how to form arms, legs and all our other structures.
Every cell in our body retains an entire copy of our DNA, but with different genes turned on or off to allow them to perform their necessary duties. Disorders of gene regulation appear to play an important role in a number of diseases, including the development of many cancers, and offer potential targets for new treatments.
Obviously, the system isn’t perfect, and genetic mistakes do arise regularly during cell reproduction and heredity. Human DNA adds between 30 and 50 new mutations each generation. The vast majority of these are harmless and involve non-gene segments, but studies show each of us carry several hundred silent, but potentially harmful, mutations in our DNA.
I’ve found genetics and DNA fascinating ever since first learning about them. It’s an amazingly complex and elegant way of communicating between cells and generations. Plus, the genetic mistakes combined with gene mixing during reproduction supply the variation that natural selection and evolution thrive on. It certainly helps explain why Dobzhansky felt evolution gives meaning to biology.
Lifelong Oregonian Fred Schubert, a The Dalles biologist, has a lifelong interest in general science and science writing. Feel free to submit any comments on this article or suggestions for new topics to fcscience @qnect.net.