The process of scientific discovery is what drives and motivates the scientist. The Double Helix, by James D. Watson (1968, A Mentor Book by New American Library, New York) is a brief yet enticing look into the process of science-- the sleuthing, the deduction, and the sheer joy of discovery by the man who helped deduce the structure of DNA. Assign students to read this book, or portions of it, and get their impressions.
Pose the following problem to your students:
If DNA replication errors occur at a frequency of 10-8 to 10-5 per cell division, then how frequently do mutations actually occur in each of us?
Different types of cells in our bodies divide at very different rates. Nerve cells do not divide again past a certain point in a person's development. Skin cells divide continuously, and the cells lining your small intestine are replaced every few days. A million new red blood cells are made every minute, which gives us a place to begin our calculations.
If we have 1 million new red blood cells every minute, how many are made in 24 hours?
(1 million x 60 minutes per hour x 24 hours = 1.44 x 109 new red blood cells every day)
So how many errors occur in the new red blood cells produced each day?
(1.44 x 109 red blood cells x 10-8 errors per cell division = 1.44 x 101 or 14 per day)
These calculations can be carried out further. For a year, multiply by 365. For a lifetime, multiply this last number by 75 or 80 years.
Note: Red blood cells are somewhat atypical of body cells because, in the last stages of differentiation, the portion of the red blood cell containing the nucleus is pinched off, presumably to make more room for hemoglobin. Red blood cells, therefore, do not divide again after they are manufactured, so any mutation is not going to be passed along directly. This exercise simply illustrates the number of DNA mutations that are likely to occur in a large, multicellular organism.