The post Unravelling DNA: How physics helped solve the structure of DNA appeared first on physicsthisweek.com.

]]>Watson and Crick had determined the chemical compositions of DNA, but they couldn’t quite figure out how it could accomplish the genetic feats known by biologists. Rosalind Franklin’s work with x-ray diffraction provided the clues that led to the discovery of the Double Helix.

In this presentation, Dr. Johnson-Steigelman will use a laser to demonstrate diffraction and identify key features in DNA’s diffraction pattern.

2-3 p.m., Tuesday October 6, 2015 Room B245 Finger Lakes Community College 3325 Marvin Sands Drive, Canandaigua, NY 14424

This is a great demonstration of how the diffraction pattern contains information about the structure of the object producing the pattern. We’ll talk about how the diffraction pattern is produced and the measurements that we take.

If you like biology, chemistry, physics or most importantly lasers, you’ll enjoy this talk.

The diffraction pattern is produced using a visible laser and a special slide produce by ICE, the Institute for Chemical Education. You can order your copy of the slide and a set of lessons related to diffraction and DNA at the ICE website.

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]]>However, I was correcting labs the other day, and some of my students apparently believe that they were shooting small brass balls at speeds approaching 300 MPH!

Of course, they didn’t have this particular combination of number and units written down. They did find the speed to be 133 m/s. The problem is that students in the United States don’t have a feel for the metric system, so this number is essentially meaningless. We tend to not know speeds in the metric system.

I’m not currently the instructor for the lecture section of this particular class, so I haven’t shared the following with my lab group, but I will be doing so today. Maybe it will help you in your class.

Most people know that a reasonably fast “fastball” pitch is around 90 MPH, but only well trained athletes can throw a ball at this speed.

If we do a quick conversion, we can find the speed in meters per second. (I’m using 1 mile = 1600 m instead of 1609 m to help keep the math simple.)

$latex 90 MPH=90 \frac{miles}{hour}\times\frac{1600 m}{1 mile}\times\frac{1 hour}{3600 sec}$

$latex 90 MPH=90 \times\frac{16 m}{36 sec}$

$latex 90 MPH=90 \times\frac{4 m}{9 sec}$

$latex 90 MPH=10 \times\frac{4 m}{sec}$

$latex 90 MPH=40 \frac{m}{sec}$

So a 90 MPH fastball is travelling at about 40 m/s.

I like this number because students have a “feel” for the number. It is too fast to drive on the highway. It takes a lot of effort to throw a ball this fast. No one would expect to shoot brass balls this fast in a crowded classroom.

The numbers are fairly easy to remember. The derivation isn’t too hard to reproduce if a student doesn’t quite remember it.

Do you have any good markers for other quantities people should have a feel for in the metric system?

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