So, how many of these bases are there? The ways you are different from your classmates, friends, and siblings is due to your DNA sequences being different from theirs. About half of your DNA comes from each of your parents.
Now that we understand some basics about DNA, we can talk about how the sequence can change. Sometimes our DNA sequence gets altered; this is called a mutation.
There are different types of mutations. For example, a base can be changed from what it was originally supposed to be to a different base substitution , a base or bases can be deleted from the DNA deletion , a base or bases can be added to the DNA insertion , or a piece of DNA can be flipped inversion or repeated duplication see Figure 2.
While mutations always change the DNA sequence, they do not always cause a change in the resulting protein or an obvious effect on the organism. This can occur because most amino acids can be coded by two or more different codons. Mutations that do not affect the protein are called silent mutations, because the DNA still makes the same protein that would be expected, and a person with a silent mutation would not even realize it.
Other times, the change in the DNA sequence does affect the protein. In this case, the amino acid glutamic acid would be replaced with valine. This specific sequence change is the mutation found in most people with sickle cell anemia, which is a very painful condition. Other times, a base is inserted into or deleted in the DNA sequence, which alters the way codons are read. This results in a large number of amino acids being altered, which is called a frameshift mutation.
Notice how none of the amino acids in the protein made from the mutated DNA are the same as the original sequence. A third possibility is that the mutated DNA sequence causes the protein production to stop early, so that the protein is shorter than normal.
This is referred to as a nonsense mutation. So, the resulting protein would be shorter than normal and would not function properly. Mutations can be passed down from the mother or father to the developing baby, and these are called inherited mutations. For example, if your mother had a mutation that caused her to be a lot shorter than average, you could inherit her mutation and be shorter than average yourself. If a person with an inherited mutation has a baby one day, that person would pass the mutation on to the next generation.
With the example above, if you gave your son or daughter the short stature mutation your mom gave you, your child could say he is short because of both you and his grandmother your mother. Other mutations happen after birth, and these are called acquired mutations. Acquired mutations are usually due to something in the environment and their effects are usually only present in the cells that were exposed to that environmental trigger.
So, some cells will have the mutation and other cells will have the normal sequence. For example, if you somehow got a mutation in the skin cells on your knee and then scraped your knee and had to make new cells to replace the ones that got hurt, those new cells would contain the mutation. However, the mutation would not be passed on to your future offspring, if you had a baby later.
Sunlight is one thing that can cause mutations. How does sunlight affect our DNA? Sunlight creates structures called thymine dimers , which means that two thymine T bases T on the same DNA strand become connected in an abnormal way, instead of correctly attaching to the complementary base adenine A on the opposite strand. Thymine dimers create kinks in the DNA shape see Figure 3 [ 2 ].
These kinks make DNA difficult to copy, which can cause a mutation. Insertion or deletion of one or more nucleotides during replication can also lead to another type of mutation known as a frameshift mutation. The outcome of a frameshift mutation is complete alteration of the amino acid sequence of a protein.
This alteration occurs during translation because ribosomes read the mRNA strand in terms of codons, or groups of three nucleotides. These groups are called the reading frame. Thus, if the number of bases removed from or inserted into a segment of DNA is not a multiple of three Figure 4a , the reading frame transcribed to the mRNA will be completely changed Figure 4b.
Consequently, once it encounters the mutation, the ribosome will read the mRNA sequence differently, which can result in the production of an entirely different sequence of amino acids in the growing polypeptide chain. To better understand frameshift mutations, let's consider the analogy of words as codons, and letters within those words as nucleotides. Each word itself has a separate meaning, as each codons represents one amino acid.
The following sentence is composed entirely of three-letter words, each representing a three-letter codon:. Now, suppose that a mutation eliminates the sixth nucleotide, in this case the letter "G". This deletion means that the letters shift, and the rest of the sentence contains entirely new "words":. This error changes the relationship of all nucleotides to each codon, and effectively changes every single codon in the sequence.
Consequently, there is a widespread change in the amino acid sequence of the protein. Lets consider an example with an RNA sequence that codes for a sequence of amino acids:. With the triplet code, the sequence shown in figure 5 corresponds to a protein made of the following amino acids: Methionine-Lysine-Leucine-Arginine-Arginine-Methionine-Methionine-Methionin Figure 5: This sequence of mRNA codes for the amino acids methionine-lysine-leucine-arginine-arginine-methionine-methionine-methionine.
Gray horizontal cylinders represent ribose sugar molecules. Green, yellow, blue, and orange vertical rectangles that are twice as long and half as wide as the gray cylinders point upward from the gray cylinders, representing nitrogenous bases in each nucleotide. The left-hand end of the mRNA strand is labeled 3-prime, and the right-hand end is labeled 5-prime. Each nucleotide is labeled with a letter representing the identity of the nitrogenous base. Three-nucleotide units, labeled below the letters as codons, are enclosed in brackets from left to right.
An arrow points from the codon to a colored sphere representing the corresponding amino acid coded for by the three-nucleotide-long mRNA sequence. Each amino acid is identified by its three-letter abbreviation. Because there are 24 nucleotides, there are eight codons, each containing three nucleotides.
The first codon, AUG, codes for the amino acid methionine, represented by a beige circle. The following codon, AAA, codes for the amino acid lysine, represented by a gray circle. The codon CUU codes for the amino acid leucine, represented by a brown circle. The last three codons all have the nucleotide sequence AUG, and therefore all code for the amino acid methionine, represented by a beige circle. The final amino acid sequence coded for by the given mRNA sequence is methionine-lysine-leucine-arginine-arginine-methionine-methionine-methionine.
Mutations can arise in cells of all types as a result of a variety of factors, including chance. In fact, some of the mutations discussed above are the result of spontaneous events during replication, and they are thus known as spontaneous mutations. Slippage of the DNA template strand and subsequent insertion of an extra nucleotide is one example of a spontaneous mutation; excess flexibility of the DNA strand and the subsequent mispairing of bases is another.
Environmental exposure to certain chemicals, ultraviolet radiation, or other external factors can also cause DNA to change. These external agents of genetic change are called mutagens. Exposure to mutagens often causes alterations in the molecular structure of nucleotides, ultimately causing substitutions, insertions, and deletions in the DNA sequence. Mutations are a source of genetic diversity in populations, and, as mentioned previously, they can have widely varying individual effects.
In some cases, mutations prove beneficial to an organism by making it better able to adapt to environmental factors. In other situations, mutations are harmful to an organism — for instance, they might lead to increased susceptibility to illness or disease. In still other circumstances, mutations are neutral, proving neither beneficial nor detrimental outcomes to an organism.
Thus, it is safe to say that the ultimate effects of mutations are as widely varied as the types of mutations themselves. This page appears in the following eBook. Aa Aa Aa. Where do mutations occur? Germ-line mutations occur in gametes or in cells that eventually produce gametes.
In contrast with somatic mutations, germ-line mutations are passed on to an organism's progeny. As a result, future generations of organisms will carry the mutation in all of their cells both somatic and germ-line. What kinds of mutations exist? Base substitution. Base substitutions are the simplest type of gene-level mutation, and they involve the swapping of one nucleotide for another during DNA replication.
For example, during replication, a thymine nucleotide might be inserted in place of a guanine nucleotide. With base substitution mutations, only a single nucleotide within a gene sequence is changed, so only one codon is affected Figure 1.
Figure 1: Only a single codon in the gene sequence is changed in base substitution mutation. The nitrogenous bases are paired so that blue and orange nucleotides are complementary and red and green nucleotides are complementary. However, the 5 th nucleotide from the right on both the bottom and top strand form a mismatched pair: an orange nucleotide pairs with a red nucleotide.
There are many different ways that DNA can be changed, resulting in different types of mutation. Here is a quick summary of a few of these:. A substitution is a mutation that exchanges one base for another i. Such a substitution could:. Since protein-coding DNA is divided into codons three bases long, insertions and deletions can alter a gene so that its message is no longer correctly parsed. These changes are called frameshifts. In frameshifts, a similar error occurs at the DNA level, causing the codons to be parsed incorrectly.
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