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Transcript of GENE MUTATION
HOW DO MUTATIONS OCCUR?
Gene mutations occur in two ways: they can be inherited from a parent or acquired during a person’s lifetime.
A change in the genetic code that results in the coding for a stop codon rather than an amino acid. The shortened protein is generally non-function or its function is impeded.
An insertion changes the number of DNA bases in a gene by adding a piece of DNA. As a result, the protein made by the gene may not function properly.
deletion changes the number of DNA bases by removing a piece of DNA. Small deletions may remove one or a few base pairs within a gene, while larger deletions can remove an entire gene or several neighboring genes. The deleted DNA may alter the function of the resulting protein(s).
Hereditary mutations (also called germline mutations) are gene defects that are passed from a parent to child. Hereditary mutations are present in the egg or sperm that join during fertilization and develop into a fetus. Because the mutation is present at the beginning, it exists in all cells of the body, including reproductive cells (the cells that make sperm in males or the egg cells in females). This means the mutation can be passed from generation to generation.
A hereditary mutation is a major factor in about 5% to 10% of all cancers. Some people are more likely to develop cancer than others simply because they are born with mutations in their genes. To learn more about this, see our document, Heredity and Cancer.
De Novo Mutation
Mutations that occur only in an egg or sperm cell, or those that occur just after fertilization, are called new (de novo) mutations. De novo mutations may explain genetic disorders in which an affected child has a mutation in every cell, but has no family history of the disorder.
New mutations have long been known to cause genetic disease, but their true contribution to the disease burden can only now be determined using family-based whole-genome or whole-exome sequencing approaches. In this Review we discuss recent findings suggesting that de novo mutations play a prominent part in rare and common forms of neurodevelopmental diseases, including intellectual disability, autism and schizophrenia. De novo mutations provide a mechanism by which early-onset reproductively lethal diseases remain frequent in the population. These mutations, although individually rare, may capture a significant part of the heritability for complex genetic diseases that is not detectable by genome-wide association studies.
Alterations in DNA that occur after conception. Somatic mutations can occur in any of the cells of the body except the germ cells (sperm and egg) and therefore are not passed on to children. These alterations can (but do not always) cause cancer or other diseases.
A mutation occurring in any cell that is not destined to become a germ cell.
A change in the genetic structure that is neither inherited nor passed to offspring.
Differences of Somatic between Germline
Mosaicism is a condition in which cells within the same person have a different genetic makeup. This condition can affect any type of cell, including:
Egg and sperm cells (gametes)
In genetic medicine, a mosaic or mosaicism denotes the presence of two populations of cells with different genotypes in one individual, who has developed from a single fertilized egg. Mosaicism may result from a mutation during development which is propagated to only a subset of the adult cell.
- usually caused by a post-zygotic mutation which affects a certain percentage of cells in an individual.
An example of this is one of the milder forms of Klinefelter syndrome, called 46/47 XY/XXY mosaic wherein some of the patient's cells contain XY chromosomes, and some contain XXY chromosomes. The 46/47 annotation indicates that the XY cells have the normal number of 46 total chromosomes, and the XXY cells have 47 total chromosomes.
Around 30% of Turner's syndrome cases demonstrate mosaicism, while complete monosomy (45 XO) occurs in about 50–60% of cases.
presence of more than one cell line in the gonads but not in the rest of body (somatic cells).
The cause is usually a mutation that occurred in an early stem cell that gave rise to all or part.
This can cause only some children to be affected, even for a dominant disease.he gonadal tissue.
A missense mutation is a point mutation in which one nucleotide in a codon is switched with a different nucleotide, which causes the codon to code for a different amino acid.
Many missense mutations are spontaneous, caused by errors in cell division which caused a transversion or a transition. Others may be caused by exposure to mutagens, such as hazardous gases.
When a missense mutation does not cause a functional change, it is sometimes known as a conservative or silent mutation.
DISEASE CAUSED BY MISSENSE MUTATION
forms of cystic fibrosis,
sickle cell anemia, and
amyotrophic lateral sclerosis
The nonsense mutation converts a codon (a triplet of bases) that encodes an amino acid into a stop codon, one that specifies the termination of translation. There are three nonsense codons (UAG, UAA, and UGA) in mRNA. One of them comes normally at the end of each polypeptide. A nonsense codon is a stop codon which is out of place.
Disease caused by Nonsense mutation
Duchenne muscular dystrophy (dystrophin)
Beta thalassaemia (β-globin)
In this example, one nucleotide (adenine) is added in the DNA code, changing the amino acid sequence that follows.
A deletion occurs when a chromosome breaks and some genetic material is lost.
Insertion and Deletion
extra copies of a chromosomal region are formed, resulting in different copy numbers of genes within that area of the chromosome.
If the two copies of a gene next to each other the process is known as tandem duplication, whereas if they are separated by nonduplicated regions, the duplication is said to be displaced.
The effect of duplications on the phenotype is generally dependent on their size
- Larger duplications tend to have an effect, while smaller ones do not
Foot deformities in a 16-year-old boy with Charcot-Marie-Tooth disease type 1A.
Charcot-Marie-Tooth disease type 1A DNA test showing duplication in the short arm of chromosome 17p12
Charcot-Marie-Tooth disease is a group of progressive disorders that affect the peripheral nerves.
1 in 2,500 people in the United States.
Damage to the peripheral nerves can result in loss of sensation and wasting (atrophy) of muscles in the feet, legs, and hands.
Onset of symptoms is most often in adolescence or early adulthood, but some individuals develop symptoms in mid-adulthood.
Frameshift mutations may be beneficial, deleterious, or lethal.
Example: induction of frameshift mutation has been used to make certain bacteria capable of producing nylonase, an enzyme that can degrade nylon.
Frameshift mutation has also been one of the possible causes of albinism. A shift in the reading frame can lead to formation of stop codon, hence, early terminating protein translation. An early termination of any of the enzymes necessary for the production of melanin can result in albinism.
another example of a disease caused by frameshift mutation is Tay sachs disease. The disease is caused by various mutations, including frameshift mutations, on chromosome 15 in the HEXA gene that codes for the alpha-subunit of the lysosomal enzyme beta-N-acetylhexosaminidase A.
Tay Sachs Disease
Nucleotide repeats are short DNA sequences that are repeated a number of times in a row.
A repeat expansion is a mutation that increases the number of times that the short DNA sequence is repeated.
TNR expansion diseases can be divided into two major subgroups:
(i) those involving large non-coding repeats (typically 100-1000 repeats), and
(ii) those involving short coding repeats (< 100 repeats, coding for polyglutamine or polyalanine).
These include fragile X syndrome (FRAXA), which is caused by CGG repeat expansion in the 5’-untranslated region (UTR) of the FMR1 gene (Verkerk et al., 1991),
myotonic dystrophy type I (DM1), which is caused by CTG repeat expansion in the 3’-UTR of The DMPK gene (Brook et al., 1992), and
Friedreich ataxia (FRDA), which is caused by GAA repeat expansion within intron 1 of the FXN gene
mental retardation syndromes
Fragile X syndrome (FRAXA)
most prominent of the fragile site disorders is Fragile X syndrome (FRAXA),
an X linked disorder that is recognized as the most common inherited form of mental retardation
Caused by CGG repeat expansion within the 5’UTR of the FMR1 (fragile X mental retardation 1) gene, which is located at the FRAXA fragile site on chromosome Xq27.3
Myotonic Dystrophy type 1 (DM1)
autosomal dominant inherited multisystem disorder characterized by clinical features such as muscle weakness, myotonia and heart conduction defects.
The molecular basis for DM1 is expansion of a CTG repeat sequence within the 3’-UTR of the DMPK gene
Friedreich Ataxia (FRDA)
autosomal recessive neurodegenerative disorder caused by homozygous GAA repeat expansion within intron 1 of the FXN gene.
The effect of the expanded GAA repeat is to reduce expression of the essential mitochondrial protein frataxin which results in progressive spinocerebellar neurodegeneration and cardiomyopathy
A chromosomal defect in which a segment of the chromosome breaks off and reattaches in the reverse direction.
radiation causing the chromosome to break
two chromosomes entangling causing one to break.
Types of Inversion
An inversion in which the breakpoints are confined to one arm of a chromosome; the inverted segment does not span the centromere
An inversion in which the breakpoints occur on both arms of a chromosome. The inverted segment spans the centromere.
Pericentric inversion 9
Most common inversion of chromosome seen on man
Generally considered to have no deleterious or harmful effects
there is some suspicion it could lead to an increased risk for miscarriage or infertility for some affected individuals
It is frequently observed in individuals with recurrent abortion, intrauterine fetal death and still birth.
A chromosomal aberration in which two daughter chromosomes are formed, each lacking one
chromosome arm but with the other arm doubled.
The chromosome arm is already copied during S phase of the cell cycle.
found in some girls with Turner syndrome, patients with the Pallister-Killian syndrome, and some tumors
In people with tetrasomy 18p, cells have the usual two copies of chromosome 18 plus an isochromoso
The extra genetic material from the isochromosome disrupts the normal course of development, causing intellectual disability, delayed development, changes in muscle tone, distinctive facial features, and other birth defects
Contains two centromeres.
Result from the abnormal fusion of two chromosome pieces, each of which includes a centromere. These structures are unstable and often involve a loss of some genetic material.
Lilian Vaughan Morgan in 1926.
Ring chromosomes usually occur when a chromosome breaks in two places and the ends of the chromosome arms fuse together to form a circular structure.
The ring may or may not include the chromosome’s constriction point centromere. In many cases, genetic material near the ends of the chromosome is lost. Many cancer cells also have changes in their chromosome structure. These changes are not inherited they occur in somatic cells , cells other than eggs or sperm during the formation or progression of a cancerous tumor.
Ring Chromosome 20 syndrome
Ring Chromosome 14 and ring chromosome 13 syndrome
Ring chromosome 15
Ring formation of an X-chromosome causes Turner Syndrome.
Complex rearrangements, including segmental microdeletions and microduplications, have been seen in numerous ring chromosomes, providing important clues regarding the mechanisms of their formation.
chromosome mutations in which chromosome segments, and the genes they contain, change positions.
can occur within a chromosome (intrachromosomal) or between chromosomes (interchromosomal)
a segment breaks off the chromosome and rejoins it at a different location.
- occur when the transfer of chromosomal material is one way.
- chromosomes swap parts
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