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NEL Molecular Genetics 687

Mutations and Genetic Variation 20.420.4

Mutations are changes in the sequence of the DNA molecule and are the source of new

genetic variation that may be acted on by natural selection. A beneficial mutation gives

an organism a selective advantage and tends to become more common over time, leading

to new evolutionary changes. A harmful mutation reduces an individual’s fitness and

tends to be selected against. Harmful mutations occur at low rates in a species. Some

mutations are neutral, having neither a benefit nor a cost, and are not acted on by nat-

ural selection.

As scientists gained more knowledge about the nature of DNA and the genetic code,

they were able to more fully understand mutations.Point mutationsare changes in a

single base pair of a DNA sequence. They may or may not change the sequence of amino

acids.Gene mutationschange the amino acids specified by the DNA sequence, and they

often involve more than a single base pair.Figure 1summarizes the DNA changes that occur

in some common types of mutation.

Learning Tip

Review how mutations

contribute to the variability of

species and how natural

selection acts on mutations in

Chapter 6 of this book.

Met Asn Pro Thr stop

AUUGAACCCCAAAAC

Met

silent mutation

nonsense mutation deletion mutation (entire codon)

missense mutation insertion mutation

deletion mutation

normal mRNA strand

Asn Pro Thr stop

A U G A A C C C C A C U U A A

Met Asn Pro Pro stop

A U G A A C C C C C C A U A A

Met stop

A U G U A A C C C A C A U A A

A

Met Asn Pro His

U G C C C U

C

A A A C A A A

Met stop

A U G U A A C C C C A C A U A

Met Pro Thr stop

A U G C C C A C A U A A

A

A

C

mRNA 5
 3 


protein

Asn

5
3
5
3

5
3
5
3

5
3
5
3

Figure 1

A summary of different types of mutations that may occur in a DNA sequence, affecting the

transcribed RNA sequence.

point mutationa mutation at a

specific base pair

gene mutationa mutation that

changes the coding for amino acids