Chapter 14 Summary

Core Concepts Summary

14.1 Mutations are very rare for any given nucleotide and occur randomly without regard to the needs of an organism.

For an individual nucleotide, the frequency of mutation is very rare, though it differs among species. page 292

Mutations are common across an entire genome and across many cell divisions. page 293

Germ-line mutations occur in haploid gametes as well as in the diploid cells that give rise to them, and somatic mutations occur in nonreproductive cells. Only mutations in germ cells are transmitted to progeny and play a role in evolution. page 293

Mutations are random in that they occur without regard to the needs of an organism. This principle can be demonstrated by replica plating bacterial colonies in the absence and presence of antibiotic. page 295

14.2 Small-scale mutations include point mutations, insertions and deletions, and movement of transposable elements.

A point mutation, or nucleotide substitution, is a change of one base for another. page 297

The effect of a point mutation depends on where it occurs. If it occurs in noncoding DNA, it will likely have no effect on an organism. If it occurs in a protein-coding gene, it can result in a change in the amino acid sequence (nonsynonymous mutation), no change in the amino acid sequence (synonymous mutation), or the introduction of a stop codon (nonsense mutation). page 297

307

Small insertions or deletions in DNA add or remove one base or a few contiguous bases. Their effect depends on where in the genome they occur and on their size. An insertion or deletion of a single nucleotide in a protein-coding gene results in a frameshift mutation, in which all the codons downstream of the insertion or deletion are changed. page 298

Transposable elements are DNA sequences that can jump from one place in a genome to another. They can affect the expression of a gene if they insert into or near a gene. page 300

14.3 Chromosomal mutations involve large regions of one or more chromosomes.

A duplication is a region of the chromosome that is present two times, and a deletion is the loss of part of a chromosome. page 301

Duplications and deletions both affect gene dosage, which can have important effects on a cell or organism. page 302

Gene duplication followed by evolutionary divergence results in gene families made up of genes with related but not identical functions. page 302

An inversion is a segment of a chromosome in reverse orientation. page 303

A reciprocal translocation involves the exchange of a part of one chromosome with another. Reciprocal translocations do not affect gene dosage, but can lead to problems during meiosis. page 303

14.4 DNA can be damaged by mutagens, but most DNA damage is repaired.

Some types of damage alter the structure of DNA, such as single-stranded or double-stranded breaks, cross-linked thymine dimers, or missing bases. page 304

Other types of DNA damage, such as changes in the side groups that form hydrogen bonds or the addition of side groups that interfere with base pairing, affect the bases themselves. page 304

Many specialized DNA repair enzymes can correct DNA damage. page 304

DNA ligase seals breaks in DNA and is an important tool for molecular biologists. page 304

Postreplication mismatch repair provides a backup mechanism for mistakes not caught by the proofreading function of DNA polymerase. page 304

Base excision repair corrects individual nucleotides and involves several DNA repair enzymes working together. page 305

Nucleotide excision repair functions similarly to postreplication mismatch repair but excises longer stretches of damaged nucleotides. page 306

Self-Assessment

  1. Mutations in which types of cell are most likely to contribute to evolutionary change in a population of organisms? Why?

    Self-Assessment 1 Answer

    Mutations that contribute to evolutionary change must be capable of being transmitted from generation to generation. In animals, this means that they must be germline mutations; in plants, they must be mutations in cell lineages that give rise to reproductive cells. Beyond merely being able to be transmitted, the mutations most likely to contribute to evolutionary change are those that increase survival or reproduction, such as sickle-cell anemia in the presence of malaria or insecticide resistance in insects. Mutations that are neutral or nearly so can also be maintained in populations for long periods of time. However, mutations that are harmful tend to be eliminated from the population and usually do not contribute to evolutionary change.

  2. Explain the difference between the mutation rate for a given nucleotide and the mutation rate for a given cell.

    Self-Assessment 2 Answer

    The mutation rate for a given nucleotide varies depending on the organism, but is usually relatively low. The mutation rate for a given cell also varies depending on the number of times DNA replication has occurred and the size of the genome. If a cell’s DNA is replicated more frequently, then there is a greater chance that a mutation will take place. The same goes for cells with larger genomes; the bigger the genome, the higher the probability that a mistake will be made during replication.

  3. Explain what it means to say that mutations are random.

    Self-Assessment 3 Answer

    Mutations occur randomly and without regard to the needs of the organisms. This is to say that an organism does not mutate to adapt to its environment. Rather, mutations happen spontaneously and randomly, and if they happen to give that organism a selective advantage in a particular environment, then the mutation will be propagated throughout the population.

  4. Describe the effects of nonsynonymous, synonymous, and nonsense mutations on a protein and the effects of small insertions or deletions in an open reading frame.

    Self-Assessment 4 Answer

    Nonsynonymous mutations alter a codon so that the result is a single amino acid replacement in a protein; they may affect protein function if the new amino acid causes protein misfolding or impairs interactions with other molecules. Synonymous mutations produce a codon that encodes the same amino acid as the nonmutant. Nonsense mutations result in stop codons that terminate polypeptide elongation. The effects of small insertions or deletions in open reading frames depend on whether their length is an exact multiple of three. If the length is a multiple of three, then the result is the insertion or deletion of amino acids in the polypeptide chain. Insertions or deletions that are not an exact multiple of three shift the reading frame so that amino acids attached to the polypeptide chain following the site of the insertion or deletion are incorrect.

  5. Explain how the location of a small-scale mutation in the genome can determine the effect it has on the functions of a cell.

    Self-Assessment 5 Answer

    If a small-scale mutation happens in the noncoding region of the DNA, the mutation may not affect the organism because the mutation is not changing actively transcribed genes. If these same types of mutations happen in a protein-coding gene, they may result in an impaired or inactive protein.

  6. Which type of chromosomal abnormality—deletion, duplication, inversion, or translocation—would you expect to have the greatest effect on the organisms that carry it, and why?

    Self-Assessment 6 Answer

    Inversions and translocations do not usually affect the individuals that carry them because they do not affect gene dosage, although they might result in chromosomally abnormal offspring. Hence, deletions or duplications are likely to affect the organism that carries them because of changes in gene dosage. For deletions and duplications of the same size, deletions are likely to have a greater effect than duplications because organisms are usually more sensitive to decreased gene dosage than they are to increased gene dosage.

  7. Explain how a gene family, such as the odorant receptor gene family, is thought to have evolved.

    Self-Assessment 7 Answer

    A gene family is defined as a group of genes with related functions. They are thought to occur through multiple rounds of duplication and divergence (when a mutation occurs in the extra copy of the gene that is beneficial to the organism), resulting in genes that are similar yet contain some differences that affect their function slightly (e.g., different odor receptors in the odorant receptor gene family).

  8. What is a mutagen? Name two common mutagens and their effects on DNA.

    Self-Assessment 8 Answer

    Mutagens are agents that increase the probability of mutation. Some common mutagens are: X-rays, which cause breaks in the sugar-phosphate backbone of DNA; ultraviolet light, which causes thymine bases to cross-link, resulting in thymine dimers; bleach or hydrogen peroxide, which causes the loss of a base from the DNA sequence; and chemicals that are highly reactive, such as those in cigarette smoke, that often add bulky side groups to the bases and hinder proper base pairing.

  9. Describe a DNA repair mechanism.

    Self-Assessment 9 Answer

    Since the preservation of the correct DNA sequence is so important to a cell, there are several repair mechanisms the cell has to combat mutagens and spontaneous mutations. DNA ligase repairs breaks in the sugar-phosphate backbone by joining the 3ꞌ hydroxyl of one end to the 5ꞌ phosphate of the other. The proofreading function of DNA polymerase corrects mispaired bases 99% of the time. The other 1% are corrected through a process called mismatch repair, in which one strand of the backbones in the vicinity of the mismatch is cleaved and then degraded, nucleotide by nucleotide, to a point at a distance past the site of the mismatch. Then the gap is filled by new synthesis and the mismatch is corrected. Base excision repair occurs where an improper base in DNA and its deoxyribose sugar are both removed and the resulting gap is then repaired. Nucleotide excision repair is similar to mismatch repair, with the difference being the number of mismatched bases. Nucleotide excision repair can recognize thousands of nucleotides that are mismatched or damaged, whereas mismatch repair can only recognize one mismatched base.