Biology, Eighth Edition (Raven)

Chapter 15: Genes and How They Work

Mutation by Base Substitution

Why is the changing of a single base the least serious form of mutation? Addition and deletion mutations generally produce nonfunctional proteins or no protein product at all. They are frameshift mutations. A frameshift mutation alters the ‘reading frame’ of the DNA sequence and changes all the amino acids in the protein product after the point of mutation. Substitution mutations merely replace one base with another. Because the genetic code is redundant some substitutions will have no effect at all. For example the substitution of a uracil for a cytosine in the codon CCU will have no effect on the protein produced as both CCU and CCC code for proline. Substitutions that replace one amino acid with another vary widely in their effect depending on the substitution and its location in the amino acid chain.

A substitution that produces the stop codon (AUG) is the most serious as it will end an amino acid chain prematurely. Other substitutions can have severe effects if the replacement of an amino acid radically changes the shape of the protein or the active site. Changes that have minimal effect on the protein configuration are less likely to dramatically effect the functioning of the protein.

View the animation below, then complete the quiz to test your knowledge of the concept.


1.	After a tautomeric shift in adenine

		A)	adenine bonds with thymine
		B)	adenine bonds with urasil
		C)	adenine bonds with cytosine
		D)	adenine is unable to bond with any molecule

2.	A mutation that causes a change in a single nucleotide in DNA

		A)	will have no effect on the resulting protein
		B)	changes the corresponding nucleotide in mRNA, resulting in a different codon
		C)	causes the codon to be correct, but the anticodon to be incorrect
		D)	causes protein synthesis to stop

3.	In a tautomeric shift

		A)	it is always adenine that is changed
		B)	final bonding of nucleotides remains unchanged
		C)	adenine is changed so it can no longer form base pairs
		D)	hydrogen atoms move to form a base with altered hydrogen properties
		E)	carbon atoms move to form a base with altered properties

4.	A mutation that changes a single nucleotide can result in a different amino acid being added into a protein.

		A)	True
		B)	False

5.	DNA mutations are passed on to a cell’s progeny.

		A)	True
		B)	False