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Learning Goals
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Describe the different types of point mutations
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Differentiate between different chromosomal mutations
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Explain how mutations play a role in evolutionary processes
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Discuss factors that can influence the rate of mutations
Introduction
A cell’s DNA is the repository of all the information needed to not only make the cell but to enable it to function. In addition, the chromosome(s) also contain the information to make any cell in the organism. Mutations are variations that occur in the DNA sequence which alter the genetic code and can be passed on through cell division (Fig. 1). It is the genetic variation within cells and within individuals that account in part for the physical differences that we see between individuals and between groups of organisms. Genetic variation is responsible for the diversity of organisms that are on this planet, from single-celled bacteria to the largest animal, the blue whale. This variation is a key factor in the ability of populations to evolve and ultimately adapt to their environments.
Mutations are the ultimate source of genetic variation
Small Scale Mutations
Mutations can also be the result of the addition of a base, known as an insertion, or the removal of a base, also known as deletion. These types of mutations can alter the reading frame (3 nucleotides code for 1 amino acid) of the amino acids, and thus the sequence of amino acids that are incorporated into the protein. The number of nucleotides that are inserted or deleted will determine how much of a change, and thereby how damaging the mutation is. If fewer than 3 insertions or deletions occur in a row, then a frameshift mutation can occur. This alters the reading of the triplet codons during translation of mRNA to proteins and most often results in a non-functional protein. Finally, sometimes a piece of DNA from one chromosome may get translocated to another chromosome or to another region of the same chromosome; this is also known as translocation.
Review Question:
Chromosomal Mutations
Smaller-scale duplications that include only one or a few genes can be maintained over many generations depending on the genes that are involved. Deletions can occur as a result of an error in replication or from the joining of breaks in a chromosome that eliminates a section. In some cases, deletions can persist in a population because chromosomes occur in homologous pairs in diploid organisms. This means that a copy of the deleted region is still present in one of the homologs and one copy may be enough for survival and reproduction. Some deletions, on the other hand, decrease the chance of survival and/or reproduction even when the homologous chromosome is normal.
Chromosomal Inversions and Translocations
In addition to duplications and deletions, the order of genes within a chromosome can also be altered through inversions and reciprocal translocations. An inversion occurs when a block of genes within a chromosome is reversed, typically when a s