Cancer is the most common human genetic disease. If you have any other comments or suggestions, please let us know at comment yourgenome. Can you spare minutes to tell us what you think of this website?
Open survey. In: Facts In the Cell. This results in changes in the proteins that are made. This can be a bad or a good thing. Mutations can occur during DNA replication if errors are made and not corrected in time. Mutations can also occur as the result of exposure to environmental factors such as smoking, sunlight and radiation. It is often impossible to tell exactly when a de novo variant happened.
As the fertilized egg divides, each resulting cell in the growing embryo will have the variant. De novo variants are one explanation for genetic disorders in which an affected child has a variant in every cell in the body, but the parents do not, and there is no family history of the disorder.
Variants acquired during development can lead to a situation called mosaicism, in which a set of cells in the body has a different genetic makeup than others.
As cells grow and divide, cells that arise from the cell with the altered gene will have the variant, while other cells will not. When a proportion of somatic cells have a gene variant and others do not, it is called somatic mosaicism. Depending on the variant and how many cells are affected, somatic mosaicism may or may not cause health problems.
When a proportion of egg or sperm cells have a variant and others do not, it is called germline mosaicism. In this situation, an unaffected parent can pass a genetic condition to their child. Most variants do not lead to development of disease, and those that do are uncommon in the general population. 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. Down syndrome usually results from the presence of one extra copy of a particular chromosome, or an extra portion of that chromosome. The presence of that extra chromosome leads to problems with certain organs of the body, such as the heart.
It can also lead to leukemia—a cancer of the blood-forming cells—and produce mental disabilities. Many people with Down syndrome also have distinct facial features.
Mutations can be inherited or acquired during a person's lifetime. Mutations that an individual inherits from their parents are called hereditary mutations. They are present in all body cells and can be passed down to new generations.
Once an acquired mutation is passed down, it is a hereditary mutation. Acquired mutations are not passed down if they occur in the somatic cells, meaning body cells other than sperm cells and egg cells. De novo mutations may explain genetic disorders in which an affected child has a mutation in every cell in the body but the parents do not, and there is no family history of the disorder.
Somatic mutations that happen in a single cell early in embryonic development can lead to a situation called mosaicism. As all the cells divide during growth and development, cells that arise from the cell with the altered gene will have the mutation, while other cells will not. Depending on the mutation and how many cells are affected, mosaicism may or may not cause health problems. Most disease-causing gene mutations are uncommon in the general population.
However, other genetic changes occur more frequently. Genetic alterations that occur in more than 1 percent of the population are called polymorphisms.
They are common enough to be considered a normal variation in the DNA. 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.
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