By: Mary-Kate Perrone
Revised August 20, 2006
Revised August 20, 2006
In order to truly understand how living organisms have evolved, one must look at all aspects of the theory of evolution. Most scientists will agree that there are two main factors that have influenced the evolution of biological organisms; they are common descent with modification and natural selection. This report will outline a specific variable in relation to natural selection, which is the notion where less favorable changes are removed from the gene pool while more beneficial modifications thrive. More specifically, this report will discuss one of the mechanisms of how and what causes the actual modifications that have been recorded and documented as evolutionary change.
Mutations are considered to be the raw materials for natural selection which lead to evolutionary change among living organisms. Mutations can be defined as a permanent change to the genetic makeup of an organism. In other words, they change or alter the base sequence of DNA or RNA, which are the crucial components to the formation and biological development of all cellular forms of life (Mutations n.d.). Mutations can be classified under three main categories: causes, types, and effects on biological organisms.
Causes of Mutations:
Mutations can be caused by spontaneous or induced alterations. Spontaneous or random mutations can happen as a result of either the deamination, or the removal of an amino acid group, of cytosine to uracil in the DNA double helix or from copying errors during replication (Lodish 2000). Spontaneous mutations occur unprompted and naturally within the genome of an organism. As well, mutations can occur from exposure to certain environmental factors (Krogh 2005). These agents that change the genetic information of a living organism are known as mutagens. Mutagens can be identified as induced alterations that result from exposure to certain kinds of chemicals, x-ray photographs, radioactive substances, and ultraviolet radiation from the sun (McGraw Hill 2002). These mutations lead to molecular decay and impairment of structure and function of the molecule within a biological organism.
Types of Mutations and Changes in the DNA Molecule:
Mutations can affect the arrangement of the DNA molecule, which represent gene alterations. According to Drake, these types of changes affect only a single gene and consist of chromosomal mutations, additions or deletions of base pairs, point mutations, and complex mutations combining any of the alterations listed above (2001). Furthermore, mutations are categorized in such a way that they are scaled according to size. On a small scale, scientists classify modifications by their subtle affect on one or a few nucleotides. This includes point mutations, which are defined as a variance in a single base pair in a genome during DNA replication (Krogh 2005). Point mutations can be divided into groups called transitions and transversions, where there is an exchanging of nucleotides. One example would be when the base adenine (A) is converted to cytosine (C) (Mutations n.d.)).
Moreover, there are various types of point mutations. The first type is referred to as missense alterations, which is the mechanism of coding for a protein where one amino acid is substituted for another (i.e.: Sickle Cell Anemia). Another type can be named a nonsense mutation, which codes for a stop on an amino acid codon and inevitably leads to execution of protein (i.e: Cystic Fibrosis can result when a nonsense mutation affects the CFTCR* chloride ion channel). Finally, silent mutations code for repetition of the same amino acids (Lodish et al 2000). These alterations occur outside of the gene and thus seem to have no profound effect on the organism. These tend to be dubbed “neutral” meaning they neither have a beneficial or harmful affect on the organism. In addition, insertion and deletion mutations can be considered small scale modifications within a genome. Here, changes occur within the DNA structure by adding or removing one or more nucleotides, which can disrupt the reading frame, or the grouping of codons in the nucleotides, and inevitably alters the gene product.
On the other hand, a type of large scale mutation can be identified as chromosomal modifications within an organism. Lodish et al describes these large scale changes as affecting numerous genes which in turn can produce significant phenotypic consequences. Such abnormalities in the genome can be attributed to deletion, insertion, or inversion of several genes on a single chromosome as well as the substitution and duplication among DNA fragments of non-homologous chromosomes (Lodish et al 2000).
As mentioned previously, chromosome mutations involve changing sections of DNA base pairs and can include partial losses, rearrangements, and additions of genes Rearranging genes in the sequence includes inversions, or reversing segments, and translocations, which can be defined as transferring segments to new locations on a chromosome (Drake 2001). Adding, or duplicating/ inserting, segments can lead to numerous repetitious copies of sections of chromosomes. Moreover, many of the mutations listed above impair the structure of the molecule, yet function of the cell is affected as well.
Effects of Mutations on Organisms:
Mutations come in many forms for various biological organisms and can occur in somatic cells, defined as not egg or sperm cells, and germ-line cells, which do become egg or sperm cells and can be passed from parent to offspring (Krogh 2005). Mutations can be classified by their effect on the structure of the molecule, and function or effect on phenotype (Mutations n.d.). Phenotypic disadvantages arise from semi lethal and sub lethal mutant genes. An organism carrying semi lethal or sub lethal genes most often has difficulty adapting to its environment and usually will not have a lengthy lifespan (McGraw Hill 2002). An organism which undergoes a large scale mutation may display phenotypic mistakes. For instance, a tree frog uses its green coloring for camouflage and protection from predators. If it undergoes a mutation in coding for pigment, its chances for survival in the environment are dismal. Needless to say, this type of genetic modification can be extremely harmful for this organism. On the other hand, though, some alterations can go virtually undetected in respect to physical characteristics; yet, these may weaken an organism’s ability to function adequately to compete with other genotypes (McGraw Hill 2002).
When modifications inside a cell occur, the function of that unit is affected in different ways. An amorphic mutation occurs as a result of an allele losing its ability to function normally; this is known as a null allele (Mutations n.d.). An example of this is when a protein is coded non-functional for human blood type O, which is expressed as a lack of antigens in the blood. Subsequently, anti-morphic mutations are produced when a gene product reacts antagonistically to a protein produced by normal alleles, which causes molecular malfunction or an inactive gene. An example of this type of function mutation is an organism who expresses Marfan syndrome, which is an autosomal dominant connective tissue disorder (Mutations n.d.).
Scientists have been studying mutations among organisms and its impact on evolutionary change in order to support the theory of natural selection. They are able to classify various types of mutations into causes, types, and the effects on structure and function or phenotype of an organism. They have categorized them into two scales, small and large in respect to how it affects a biological organism. Also, scientists have been able to link certain diseases and disorders to mutated molecules. Overall, scientists have made many discoveries in order to continue to make sense of and ask adaptive as well as mechanistic questions while studying the impact of mutations and the correlation to natural selection.
*CFTCR: Cystic Fibrosis Trans-membrane Conductance Regulator
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