14.1 MUTATIONS ARE VERY RARE FOR ANY GIVEN NUCLEOTIDE AND OCCUR RANDOMLY WITHOUT REGARD TO THE NEEDS OF AN ORGANISM.
14.2 SMALL-SCALE MUTATIONS INCLUDE POINT MUTATIONS, INSERTIONS AND DELETIONS, AND MOVEMENT OF TRANSPOSABLE ELEMENTS.
14.3 CHROMOSOMAL MUTATIONS INVOLVE LARGE REGIONS OF ONE OR MORE CHROMOSOMES.
14.4 DNA CAN BE DAMAGED BY MUTAGENS, BUT MOST DNA DAMAGE IS REPAIRED.
Describe different effects a mutation can have on an organism.
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Mutations account for the differences we see in individuals in a population (e.g., eye color). A mutation can also be detrimental to the organism if the organism cannot survive or reproduce. A mutation may not affect the organism at all and thus may be carried in the population at no benefit or detriment to the population.
Explain the difference between the mutation rate for a given nucleotide and the mutation rate for a given cell.
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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 occurs 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 occur. The same goes for cells with larger genomes; the bigger the genome the higher the probability that a mistake will be made during replication.
Explain what it means to say that mutations are random.
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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.
Describe several types of small-scale mutation and their likely effect on an organism.
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Several types of small-scale mutations are: (1) Point mutations are changes in a single nucleotide. Point mutations can lead to non-functional proteins if the mutation changes a key nucleotide in the protein sequence. This is called non-synonymous or missense mutation. Point mutations can also be synonymous or silent. This usually happens when the third nucleotide of a codon is mutated to another nucleotide that still will give rise to the original amino acid. Thus the protein will not be affected. A nonsense point mutation creates a stop codon that terminates translation prematurely. In this case, the final protein, if even made, would likely be non-functional because it would be truncated. (2) Small insertions or deletions of nucleotides. If an insertion of three nucleotides occurs, then a new amino acid is added to the protein and may or may not affect the functioning of the protein. In the same way, a deletion of three nucleotides will delete an amino acid in the protein and would have the same effects. An insertion or deletion can also result in a frameshift mutation, where the reading frame of the mRNA is altered and different amino acids are added to the peptide change. This would drastically affect the functioning of the final protein of the frameshift that occurred toward the beginning of the nucleotide sequence. (3) Transposable elements are movable sequences of DNA that insert themselves into the genome of an organism. This insertion, depending on where it is, could cause mass interruption of transcription, cause errors in RNA processing, or disrupt the open reading frame.
Explain why the location of a small-scale mutation in the genome can make a difference in the phenotype.
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If a small-scale mutation happened in the non-coding region of the DNA, the mutation may go unnoticed by the organism because the mutation is not changing actively transcribed genes. These mutations can, however, make a difference in the phenotype of an organism if they happen in the coding region of the DNA. This is the region where mRNA is actively transcribed and proteins are actively translated. A change in this DNA could lead to a mutant protein and phenotype.
Describe the differences among silent, missense, nonsense, and frameshift mutations.
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Silent mutations happen when a nucleotide in a codon (typically the third nucleotide) is changed but the change does not affect which amino acid is coded for. For example CTC and CTT both code for glutamic acid. A missense mutation is one where the nucleotide change does affect which amino acid is coded for. For example, CTC codes for glutamic acid, but if the T is mutated to and A, the new amino acid would be valine. A nonsense mutation occurs when the codon is changed from coding for an amino acid to coding for a stop codon. A frameshift mutation occurs when one or two nucleotides are inserted or deleted from the DNA and results in the reading frame of the mRNA to be altered. The severity of the mutation usually depends on the position of the frameshift mutation because all the amino acids following the mutation are changed.
Describe several types of large-scale mutation and their likely effect on an organism.
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Several types of large-scale mutations are: (1) Duplications and deletions of chromosomes. This happens when a segment of the chromosome is either present in two copies or deleted altogether. Duplications are typically less harmful to the organism than deletions because deletions can remove whole genes essential to the organism’s survival. However, since chromosomes usually occur in homologous pairs, the deletion in one may not be detrimental to the organism right away. An example where duplication is beneficial to the organism is when the duplicated gene gives the organism a reproductive and survival advantage in the population. This is called duplication and divergence. (2) Reciprocal translocation occurs when two different (nonhomologous) chromosomes undergo an exchange of parts. Since this phenomenon only causes change in the arrangement of genes and not their number, most do not affect the survival of the organism. (3) Inversions occur when the normal order of a block of genes is reversed. Large inversions can cause problems during meiosis while small inversions usually do not affect the survival of the organism.
Explain how a gene family, such as the odorant receptor gene family, is thought to have evolved.
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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).
Name several common mutagens and their effect on DNA.
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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 the ones in cigarette smoke, that often add bulky side groups to the bases and hinder proper base pairing.
Describe a DNA repair mechanism.
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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 3’ hydroxyl of one end to the 5’ 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 is 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.