Classroom Demonstration: MUTATIONS
DNA from an organism contains all the information needed to make another organism just like the original. We know that this information is coded in a chemical form within the DNA molecule made up of pairs of nucleotides. The nucleotides (now known by four letters, G, C, T, A) are always found in pairs in the intact DNA. "A" is always paired with "T." "G" is always paired with "C." Thus we find A-T and G-C pairs in DNA. A length of DNA that has the sequence "ATATAAGCGC" on one strand of its backbone structure would have the sequence TATATTCGCG on the opposite helix.
The process of unzipping the DNA molecule and allowing the unpaired nucleotides to find new partners and thus reform the DNA is the basis of genetic reproduction. Errors in this simple process may introduce new nucleotides into the established sequence of base pairs; or some pairs may be dropped from the sequence. Either of these errors alters the complete original sequence and is called a mutation. Small errors introduced into the DNA may cause lasting effects upon the organism and its progeny.
A simple classroom demonstration to begin discussion and understanding of the dynamics of mutation is described below. You will need to make the following:
On several (perhaps twenty) index cards use a marker to write four letters. Each card should be different and the letters should be T, A, G, or C.
Here are some examples:
TTTT, TTTA, TTTG, TTTC, TTAT, TTAA, TTAG, TTAC, TTGT, TTGA, TTGG, TTGC, TTCT, TTCA, TTCG, TTCC, TATT, TATA, TATG, TATC, TAAT, TAAA, TAAG, TAAC, TAGT, TAGA, TAGG, TAGC, TACT, TACA, TACG, TACC, TGTT, TGTA, TGTG, TGTC, TGAT, TGAA, TGAG, TGAC, TGGT, TGGA, TGGG, TGGC, TGCT, TGCA, TGCG, TGCC, TCTT, TCTA, TCTG, TCTC, TCAT, TCAA, TCAG, TCAC, TCGT, TCGA, TCGG, TCGC, TCCT, TCCA, TCCG, TCCC, ATTT, ATTA, ATTG, ATTC, ATAT, ATAA, ATAG, ATAC, ATGT, ATGA, ATGG, ATGC, ATCT, ATCA, ATCG, ATCC, AATT, AATA, AATG, AATC, AAAT, AAAA, AAAG, AAAC, AAGT, AAGA, AAGG, AAGC, AACT, AACA, AACG, AACC, AGTT, AGTA, AGTG, AGTC, AGAT, AGAA, AGAG, AGAC, AGGT, AGGA, AGGG, AGGC, AGCT, AGCA, AGCG, AGCC, ACTT, ACTA, ACTG, ACTC, ACAT, ACAA, ACAG, ACAC, ACGT, ACGA, ACGG, ACGC, ACCT, ACCA, ACCG, ACCC, GTTT, GTTA, GTTG, GTTC, GTAT, GTAA, GTAG, GTAC, GTGT, GTGA, GTGG, GTGC, GTCT, GTCA, GTCG, GTCC, GATT, GATA, GATG, GATC, GAAT, GAAA, GAAG, GAAC, GAGT, GAGA, GAGG, GAGC, GACT, GACA, GACG, GACC, GGTT, GGTA, GGTG, GGTC, GGAT, GGAA, GGAG, GGAC, GGGT, GGGA, GGGG, GGGC, GGCT, GGCA, GGCG, GGCC, GCTT, GCTA, GCTG, GCTC, GCAT, GCAA, GCAG, GCAC, GCGT, GCGA, GCGG, GCGC, GCCT, GCCA, GCCG, GCCC, CTTT, CTTA, CTTG, CTTC, CTAT, CTAA, CTAG, CTAC, CTGT, CTGA, CTGG, CTGC, CTCT, CTCA, CTCG, CTCC, CATT, CATA, CATG, CATC, CAAT, CAAA, CAAG, CAAC, CAGT, CAGA, CAGG, CAGC, CACT, CACA, CACG, CACC, CGTT, CGTA, CGTG, CGTC, CGAT, CGAA, CGAG, CGAC, CGGT, CGGA, CGGG, CGGC, CGCT, CGCA, CGCG, CGCC, CCTT, CCTA, CCTG, CCTC, CCAT, CCAA, CCAG, CCAC, CCGT, CCGA, CCGG, CCGC, CCCT, CCCA, CCCG, CCCC.
These are all of the four-letter permutations (combinations) of these four letters. You need not confine yourself to the length of four. In fact, a few three-letter or five-letter 'ringers' may make this demonstration even more productive.
2) Mark these original index cards in some way so that they may be distinguished from the ones that the students will produce during this demonstration. Place the 'marks' on the backs of the cards so that the students will not be obviously aware of the distinction between these original cards and the ones that they will produce.
3) Students at their desks should be arranged in a circle so that they may pass index cards from one student to the next. If the class is large, form two or three teams to heighten the feeling of competition and increase the speed of student participation.
4) Several blank index cards are given to each student. Each student is given a 'marked' index card containing the selected letters. The pairing schema T-A or A-T or G-C or C-G is explained.
5) Each student is expected to take the 'marked' card and produce a new card with the complimentary sequence of letters. This new card is then passed to the next student in the line who produces another new complimentary card. (Only the cards that a student produces are to be passed on to the next student) The cards that are given to the students, either originally or during the course of this exercise are to remain on the student's desk. Cards may not be 'taken back.' If the wrong card is passed it must continue on to the next student. The circle of students should have an even number of students. To make up an even team, allow one student to act as a cheerleader and encourage the other students to work as fast as they can. Speed correlates with mutation rate in this demonstration.
6) Stop the exercise after about five or six sets of cards have been produced.
7) Collect all of the original cards and post the original sequences.
8) Have the students search all the newly produced cards for any sequence that is not one of the original sequences or its compliment. If you begin with matched pairs of sequences and compliments you may only have to search for any sequence that deviates from these. Any card found to contain a sequence that does not have a match among the original sequences or their compliments is a mutation, essentially, an error in transcription. More advanced students may be able to use the knowledge of the order of the students within a team to determine which student introduced the mutation. There are several variations on this demonstration but this presentation is very straight-forward and requires a minimum amount of materials.