Activity 1: Genes, DNA, and Mutations
Objective
Students will model two ways in which genetic mutations can cause genetic
disease.
Background Information
Genetic information is stored in a cell as deoxyribonucleic
acid, or DNA, which are strands that are paired, as in the rungs of a ladder,
and consists of pairs of four nucleotide
bases--adenine (A), guanine (G), cytosine (C), and thymine (T). Genes within
DNA can be hundreds or thousands of base
pairs long, with each gene having a specific sequence of nucleotide bases.
The DNA inherited by an organism directs the activities of each cell by
specifying the synthesis of proteins from amino acids. The formation of
proteins from the genetic information requires two main steps: transcription and translation.
During transcription, the double-stranded DNA partly unwinds. The individual
strands act as a template for the creation of messenger
RNA (mRNA). Messenger RNA is a single-stranded nucleic acid that contains
the sugar ribose. It consists of four nucleotide bases--adenine (A), guanine
(G), cytosine (C), and uracil (U). These bases follow the same base-pairing
rules that govern DNA replication (where
guanine pairs with cytosine, thymine pairs with adenine) except that uracil,
rather than thymine, pairs with adenine. Information for the sequence of
amino acids is contained in the mRNA in groups of three bases, known as
codons. Once the mRNA strand has been formed, it moves to the ribosomes
in the cytoplasm of the cell, where translation, or protein
synthesis, takes place.
 DNA
is transcribed into messenger RNA.
Translation involves another type of molecule known as transfer
RNA (tRNA), which is an L-shaped structure that has three bases on one
end (known as an anti-codon) and an amino acid attached to the other end. The
ribosomes in the cell link the anticodon on the tRNA with the complementary
codon on the mRNA. The amino acids on the tRNA detach from the tRNA and
link together in the specified order to form the protein. The tRNA then
moves away from the mRNA and is free to pick up another amino acid of the
same type to add to another protein chain.
Translation of messenger RNA into protein.
Many genetic disorders are due to mutations, or changes in the nucleotide
sequence of DNA. There are two main types of mutations within a gene: base-pair substitutions and base-pair insertions
or deletions. A base-pair substitution is the replacement of one nucleotide
and its partner with another pair of nucleotides. This type of mutation
may cause either no change in the protein; a small, insignificant change
in the protein; or a major alteration. For example, people with sickle-cell
anemia have an adenine-thymine pair instead of a thymine-adenine pair in
their hemoglobin gene; this substitution results in a major change in the
hemoglobin molecule. A base-pair insertion or deletion occurs when one or
more nucleotide pairs are inserted or deleted in a gene. Usually insertion
and deletion mutations cause more damage than the single-pair substitutions
because they may drastically change the sequences of the codons downstream
from the mutation.
Materials
For each group:
- Handouts:
- Scissors
- Paper
- Pen or pencil
Preparation
Duplicate Handouts to distribute to students.
Instructions
- Have students cut out the DNA sequence patterns from Handout 1 and
put pieces together side-by-side to form the following single-strand sequence:
C T T T T A T A G T A G A T A C C A C A A A G G
- Explain to students that they have just built a sequence for part of
the gene that can cause cystic fibrosis.
- Have the students cut out the mRNA and tRNA pieces from Handouts 1
and 2. Have them build a strand of mRNA by matching the ribonucleic bases
to the complementary bases on the DNA strand.
- Once students have created the mRNA strand, have them model translation
using the tRNA pieces. Students should match the tRNA pieces to the corresponding
codon on the mRNA strand. Tell them to
use the amino acid chart on the bottom of Handout 2 to determine the amino
acids that are carried by the tRNA. Students should record the sequence
of amino acids on a separate piece of paper.
- Next, explain that students will be making base-pair substitutions
in the original DNA sequence. Have them change the nucleotide in the 15th
position (from the left) of the original DNA sequence to cytosine. Have
students make the appropriate changes in the mRNA and tRNA strands, and
record the changes, if any, to the amino acid sequence. Then ask students
to change the nucleotide in the 15th position to adenine. Once again have
them record the corresponding change, if any, in the amino acid sequence.
- Now tell students that they will be making a deletion in the original
DNA sequence. Have them remove the nucleotides in the 13th, 14th, and 15th
positions. Then have students move the nucleotide pieces to the left to
close the gap that is created. Have them make the appropriate changes in
the mRNA and tRNA strands, and record the changes, if any, to the amino
acid sequence.
Discussion Questions
- Does changing the sequence of nucleotides in the DNA strand always
result in a different amino acid sequence? Why or why not?
- If the thymine in the 15th position of the sequence is changed to cytosine,
the person does not have cystic fibrosis. However, if the thymine in the
15th position is changed to adenine, the person does have cystic fibrosis.
What are some reasons this might occur?
- Suppose that in the original DNA strand the adenine in the 14th position
were changed to cytosine. Do you think a person with the change would have
cystic fibrosis? Why or why not?
- What happened to the amino acid sequence when you deleted three bases
from the original DNA strand?
- What do you think would happen if a single nucleotide were added somewhere
in the sequence. Explain your answer.
Links
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Resource Book Index: Promises and Perils
