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Name ____________________________ _

• Da~ Period

What is a mutation?

A mutation is a permanent, heritable change in the DNA sequence of a gene. When a mutation occurs, one or several bases can be added, deleted, or replaced. Mutations in a gene's DNA sequence can alter the amino acid sequence of the protein encoded by the gene.

How does this happen? Like words in a sentence, the DNA sequence of each gene determines the amino acid sequence for the protein it encodes. This sequence is read, or "translated," in groups of three nucleotide bases, with each three-letter "word," or codon, specifying a single amino acid in a protein.

What causes mutations? Mutations often happen when mistakes occur during replication. Many mutations are caused by mutagens like UV light, radiation and certain chemical toxins. How often do mutations happen? Mutations are rare events. The average rate of mutations is one in a million replications. Are you a mutant? Since DNA replication takes place in your body so many times during your lifetime, at least one of your cells contains a mutation.

• Activity 1. Mutate a sentence! We can think about DNA gene sequence as a sentence made up entirely of three-letter words. In the DNA sequence, each three-letter word is a "codon," specifying a single amino acid in a protein. Have a look at this sentence:

Thesunwashotbuttheoldmandidnotgethishat

If you were to split this sentence into individual three-letter words, you would probably read it like this:

The sun was hot but the old man did not get his hat.

This sentence represents a gene. Each letter corresponds to a nucleotide base, and each word represents a codon. Only one of the three possible "reading frames" translates into an understandable sentence. In the same way, only one three letter "reading frame" within a gene codes for the correct protein. Now, go back to the original sentence.

The sun was hot but the old man did not get his hat.

What if you shifted the three-letter "reading frame?" In the space below, show how you can change the "reading frame" of the above sentence by inserting or deleting letters within the sentence. The result should be a "nonsense" sentence.

In the space below, make a mutation that maintains or changes the meaning of the sentence without creating eUCh nonsense.

mutationJA.doc 01117113

What types of mutations can occur?

Point mutations are single-base changes in a gene's DNA sequence. This type of mutation can change a protein in the following ways: •

• Missense mutations result in a single amino acid change within a protein.

• Nonsense mutations create a premature "translation stop signal" (or "stop codon"), causing the gene's protein product to be shortened.

• A third type of mutation, called a silent mutation, does not result in an amino acid change. We'll explore how this happens when we look at the "genetic code" below.

Frameshift mutations result from insertion or deletion of a more than one base into the DNA sequence. This alters the grouping of nucleotide bases into three-base codons, causing a shift of "reading frame" during the translation process. • Insertions and deletions add or remove chunks of DNA sequence containing many DNA bases. These mutations alter many amino acids in the protein at the same time.

Activity 2. Mutate a gene! It's time to try your hand at mutating a DNA sequence.

1. Observe the original DNA sequence listed below. Using your mRNA codon chart or the Figure 12.7 on page 188 of your textbook, write the amino acid sequence of the resulting protein.

Original DNA Sequence ATG TGC CTC

mRNA Sequence:

Amino Acid Sequence: • mutationJA.doc 01/17/13 2

2. Complete the table below with examples of the types of mutations described on the previous page. The underlined letter in the DNA sequence represents the nucleotide that has changed, inserted, or deleted.

e of Mutation Missense Mutation Nonsense Mutation Silent Mutation Insertion Mutation Deletion Mutation

DNA Sequence

AGG TGC CTC

ATC TGC CTC

ATA TGC CTC

ATI GTG CCT

TGT GCC TCC

mRNA Sequence Amino Acid Sequence Changed or Unchanverl ;

3. Let's imagine that this DNA sequence represents a gene that produces an enzyme required for the process of photosynthesis in an elm tree. Let's also imagine that the above mutations occur in a single cell that makes up a tiny leaf bud on the tree. What is likely to happen to the leaf after each type of mutation occurs? Will the leaf grow or die? Use complete sentences to explain why.

Missense Mutation:

• Nonsense Mutation:

Silent Mutation:

Insertion Mutation:

Deletion Mutation:

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