Resources

Activities to teach dilution series to middle schoolers

We developed an activity that teaches a basic concept in microbiology: dilution series. When I became an biologist, I had little idea how much time I would spend counting things like phages, rings on fish ear bones, and gene variants within populations. In microbiology, microbes can be found in incredibly high numbers in a drop of liquid. In order to count them, we  dilute the microbes by transferring a small sample into a larger volume. We do this repeatedly until we can count the number of microbes: a serial dilution.

While diluting microbes that are invisible to the naked eye can be hard to students to visualize, we used visual activities to make abstract concepts more concrete. First, we performed a dilution series using food dye so students could gain some training in using serological pipets with bulbs and micropipettes. Eye droppers, turkey basters, and measuring cups could be used at home. This paved the way for later experiments with bacteriophage (bacteria-infecting viruses). While we used spare materials from the lab, like the Falcon tubes shown here, these exercises could be substituted for any vessel that you may have: test tubes, small beakers, even Dixie cups or shot glasses. Here is a worksheet that you use in the classroom or at home.

We followed this up with an activity aimed at demonstrating the importance of dilutions for counting in microbiology. We filled a graduated cylinder or beaker 1 liter, 700ml, etc) with candy (M&Ms, Skittles, etc.) or other small uniformly sized objects like marbles. We ask the students to estimate the total number of candies, which you can record on the worksheet. We then pour a sample into another vessel, and the sample should be 10% the volume of the larger vessel, so (100ml or 70ml, respectively, following the example volumes above) in another beaker or smaller graduated cylinder. The students then count the number of candies in the sample. If you are doing this with several groups of students, you can have each student or group count a separate sample and average the counts. Finally, we walk through how to extrapolate the number of candies in large vessel based on the sample counts. Optionally, you can count the total number of candies, a number you’d already have if you take 10 samples.

Activities to teach transcription to 7th graders

(part of NJ core curriculum content standards grade 7 model unit 6: inheritance and variation of traits)

In conjunction with a local middle school, we developed an activity to explain the genetic code (how DNA is transcribed to mRNA, which is then translated to codons) from a code breaking perspective. We gave pairs of students a worksheet on “Biological Code Cracking” explaining these concepts and then asked to transcribe 15 letter mRNA (using only A, C, T, U) words to different five letter words spelled out in amino acids.

For the second half of the activity, we had students use the genetic code in reverse and determine the mRNA that encodes for a given five letter word given in codons. Since we did this activity at Edgar Middle School, we had the students reverse transcribe EDGAR into mRNA. Although there are 384 possible combinations of codons, there were conveniently only two possible combinations of 1st and 2nd codon positions, so checking student sequences was simplified. Using four colors (we used school colors) of pony beads to represent A, C, T, U , the students spelled out their mRNA sequence, which they threaded onto twine to make bracelets or keychains.

The first activity took ~15 minutes, and the second activity could be comfortably completed before the end of the period.