recap

15.1 recap

Mutations are uncorrected alterations in the nucleotide sequence of DNA. Mutations can involve changes in single nucleotides or extensive rearrangements of chromosomes. If they occur in somatic cells, they will be passed on to daughter cells; if they occur in germ line cells, they may be passed on to offspring.

learning outcomes

You should be able to:

  • Use a defined DNA sequence to illustrate how the following occur, and describe their effect on phenotype: transitions, transversions, missense mutations, nonsense mutations, frame-shift mutations, and mutations outside the coding region of a gene.

  • Compare the following chromosomal mutations: deletion, duplication, inversion, and translocation.

  • Compare and contrast spontaneous and induced mutations and give examples of each.

  • Explain why some regions of DNA are considered mutation hot spots.

Question 1

What are the differences between point mutations that cause phenotypic changes and those that don’t?

Point mutations that cause phenotypic changes could have resulted in a different amino acid in the encoded protein that consequently changes a protein’s function; changed a promoter so a gene’s expression is significantly altered; or created a stop codon that terminates expression prematurely, resulting in a shorter nonfunctional protein. Point mutations that are phenotypically silent may arise in codons where redundancy ensures no amino acid change; cause codon changes that result in amino acid changes that are not significant to protein function; or occur in noncoding regions of DNA, such as introns.

Question 2

What distinguishes the various kinds of chromosomal mutations: deletions, duplications, inversions, and translocations?

See Figure 15.4. Deletions are missing part of a chromosome; duplications have an extra copy (or copies) of a chromosomal region; in inversions, a chromosome region is out of sequence; and in translocations, a piece of one chromosome breaks off and attaches to another chromosome.

Question 3

What are the differences between spontaneous and induced mutagenesis? Give an example of each.

Spontaneous mutation occurs without an external agent causing it. Example: tautomeric shift of A, so that at DNA replication A base-pairs not with T but with C. In an induced mutation, an environmental agent changes DNA. Example: nitrous acid deaminates C to U, so that at DNA replication, instead of C pairing with G, it is U pairing with A.

Question 4

Why do many mutations involve G-C base pairs?

C can be methylated to 5-methylcytosine. When deaminated spontaneously or by a mutagen, this base forms T. This is a normal base and is not removed by DNA repair. Other base changes are repaired.

You have seen that there are many different ways in which DNA can be altered, in terms of both the types of changes and the mechanisms by which they occur. We will turn now to the ways in which mutations can cause disease.