Why does mutation happen

Genes are made of deoxyribonucleic acid DNA , a long molecule composed of building blocks called nucleotides. Each nucleotide is built around one of four different subunits called bases. These bases are known as guanine, cytosine, adenine, and thymine. A gene carries information in the sequence of its nucleotides, just as a sentence carries information in the sequence of its letters.

One type of mutation is a change in a base. This is called a point mutation and it is like changing one letter in a word. Most genes carry instructions for making proteins. When a base is changed in a gene, different results are possible, depending on which base is changed and what it is changed into. The gene may produce an altered protein , it may produce no protein, or it may produce the usual protein.

Most mutations are not harmful, but some can be. A harmful mutation can result in a genetic disorder or even cancer. Another kind of mutation is a chromosomal mutation. Chromosomes, located in the cell nucleus, are tiny threadlike structures that carry genes.

A chromosome consists of a molecule of DNA together with proteins. Sometimes, a long segment of DNA is inserted into a chromosome, deleted from a chromosome, flipped around within a chromosome, duplicated, or moved from one chromosome to another.

Such changes are usually very harmful. One example of a chromosomal mutation is a condition called Down syndrome. In each cell, humans normally have forty-six chromosomes, consisting of two copies of the twenty-three kinds of chromosomes. In contrast, in a person with ovaries, all the DNA copying leading up to egg production is completed before that person is even born.

All those cell divisions in the cells that eventually lead to sperm provide many opportunities for copying mistakes to occur. Biologists Felix Wu, Alva Strand, Laura Cox, Carole Ober, Jeffrey Wall, Priya Moorjani, and Molly Przeworski work at different universities and research centers, but they all wanted to know, is it really the case that most new mutations in humans are caused by copying errors when cells divide?

Some evidence had already suggested that copying errors are not the whole story when it comes to heritable mutations. The research team set out to test the hypothesis that most de novo mutations occur because of copying errors.

If new mutations mostly come from copying errors, then the number of mutations contributed by eggs should not be much affected by the age of the person contributing the egg since, in primates, all the DNA copying that leads up to egg production occurs before that future parent is even born. However, the number of mutations contributed by the sperm should increase with the age of the genetic father since DNA copying and cell divisions leading to sperm are ongoing from puberty throughout adulthood.

The team decided to compare mutations in humans to mutations in olive baboons Papio anubis. To get data on mutations, the team sequenced the genomes of three generations of three different human families 26 people total and three generations of two baboon families 29 baboons total.

Then the researchers used additional sequence information to determine which parent mother or father had contributed each mutation. In these graphs, each offspring in which new mutations were identified is represented by a circle and a triangle. The circle indicates the number of mutations contributed by the sperm and the triangle indicates the number contributed by the egg. The lines show the overall relationship between age at conception on the x -axis and mutations on the y -axis.

Right away, we can observe a few things. First, the blue circles are always positioned above the red triangles, indicating that sperm contribute more mutations than do eggs. Second, in humans, older genetic parents do seem to pass on more mutations to offspring, as shown by the upward slope of the regression lines. This result goes against previous findings about mutations and age.

This means that the data vary a lot, making it hard to detect patterns without a really large sample. So it is possible that baboon fathers, like humans, do pass on more mutations to their offspring as they age, but there were not enough data in this experiment to detect the pattern.

Polymorphisms are responsible for many of the normal differences between people such as eye color, hair color, and blood type. Figure 2. Xeroderma pigmentosa is a condition in which thymine dimerization from exposure to UV is not repaired. Exposure to sunlight results in skin lesions. A well-studied example of a mutation is seen in people suffering from xeroderma pigmentosa Figure 2. Affected individuals have skin that is highly sensitive to UV rays from the sun.

When individuals are exposed to UV, pyrimidine dimers, especially those of thymine, are formed; people with xeroderma pigmentosa are not able to repair the damage. These are not repaired because of a defect in the nucleotide excision repair enzymes, whereas in normal individuals, the thymine dimers are excised and the defect is corrected.

Mutations , variations in the nucleotide sequence of a genome, can also occur because of damage to DNA. Such mutations may be of two types: induced or spontaneous. Induced mutations are those that result from an exposure to chemicals, UV rays, x-rays, or some other environmental agent.

Spontaneous mutations occur without any exposure to any environmental agent; they are a result of natural reactions taking place within the body. Mutations may have a wide range of effects.

Some mutations are not expressed; these are known as silent mutations. Point mutations are those mutations that affect a single base pair. The most common nucleotide mutations are substitutions, in which one base is replaced by another. These can be of two types, either transitions or transversions. 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.