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MUTATIONS HE.5.B.5 HE.5.B.6 What makes them different? Mutations • Now and then cells make mistakes in copying their own DNA. These mistakes are called mutations. • Change in a DNA sequence that affects genetic information. (pg. 307) There are 2 kinds of mutation • Gene mutation • Chromosomal mutation Gene Mutations Chromosomal mutations: Gene mutation • Mutations that produce changes in a single gene. Ex. Gene Mutation: Huntington’s Disease • Affects a person's ability to think, talk, and move. • Destroys cells in the basal ganglia, the part of the brain • • • • • that controls movement, emotion, and cognitive ability. Caused by a mutation in a gene on chromosome 4. Autosomal Dominant Inheritance. Normally, the coding region of this gene contains the DNA sequence "CAG" repeated again and again. The number of times this triplet is repeated varies from person to person, ranging from 10 to 26 times. People with HD have an abnormally high number of these CAG triplets, approximately 40 or more. This likely disrupts the function of the gene's protein product. Somehow the brain cells of HD patients accumulate clumps of protein that become toxic, resulting in cell death. Some patients lose more than 25% of their brain cells before they die. Chromosomal mutations • Produce changes in a whole chromosome. Gene mutations • Point mutations • Substitution • Insertion • Deletion Single Gene Mutations Out of the 3 gene mutations which 2 are the most dramatic? Why? Point mutations • Gene mutation involving changes in one or a few nucleotides. Frameshift mutations • Insertion and deletion cause a shift in the “reading frame” of the genetic message. • This kind of mutation may change every amino acid that follows the point mutation. Gene Mutations Activity Chromosomal mutations • Involve changes in the number or structure of chromosomes. 4 types of chromosomal mutations • Deletion • Duplication • Inversion • Translocation Chromosomal Mutations Chromosomal mutation: Deletion • Loss of all or part of a chromosome Ex of Deletion: Williams Syndrome • Rare genetic disorder that affects a child's growth, physical appearance, and cognitive development. • Missing genetic material from chromosome 7, including the gene elastin. • This gene's protein product gives blood vessels the stretchiness and strength required to withstand a lifetime of use. The elastin protein is made only during embryonic development and childhood, when blood vessels are formed. Because they lack the elastin protein, people with Williams Syndrome have disorders of the circulatory system and heart. Williams Syndrome • A deletion is caused by a break in the DNA molecule that makes up a chromosome. In most cases, the chromosome break occurs while the sperm or egg cell (the male or female gamete) is developing. When this gamete is fertilized, the child will develop Williams syndrome. • It is possible for a child to inherit a broken chromosome from a parent who has the disorder. But this is rare because most people with Williams syndrome do not have children. Chromosomal mutation: Duplication • Produces extra copies or parts of a chromosome. Chromosomal mutation: Inversion • Reverse the direction of parts of the chromosome. Inversion cont. • In most species small inversions go undetected. • Inversions usually do not cause any abnormalities in carriers as long as the rearrangement is balanced with no extra or missing DNA. • Individuals which are heterozygous for an inversion, there is an increased production of abnormal chromatids (this occurs when crossing-over occurs within the span of the inversion). This leads to lowered fertility due to production of unbalanced gametes. • The most common inversion seen in humans is on chromosome 9. This inversion is generally considered to have no harmful effects, but there is some suspicion it could lead to an increased risk for miscarriage or infertility for some affected individual. Chromosomal mutation: Translocation • Part of a chromosome breaks off and attaches to another. Ex. Of Translocation • Cancer: Several forms of cancer are caused by acquired • • • • translocations (as opposed to those present from conception); this has been described mainly in leukemia (acute myelogenous leukemia and chronic myelogenous leukemia). Infertility: One of the would-be parents carries a balanced translocation, where the parent is asymptomatic but conceived fetuses are not viable. Down syndrome is caused in a minority (5% or less) of cases by a translocation of the chromosome 21 long arm onto the long arm of chromosome 14.[5] Chromosomal translocations between the sex chromosomes can also result in a number of genetic conditions, such as XX male syndrome: caused by a translocation of the SRY gene from the Y to the X chromosome XX male syndrome (Translocation) • Caused by unequal crossing over between X and Y chromosomes during meiosis in the father, which results in the X chromosome containing the normally-male SRY gene. When this X combines with a normal X from the mother during fertilization, the result is an XX male. • This syndrome occurs in approximately four or five in 100,000 individual. • Symptoms usually include small testes and subjects are invariably sterile. Individuals with this condition sometimes have feminine characteristic. Non-disjunction • Happens in chromosomes, when cells are going through meiosis. Human Karyotype Trisomy Monosomy Mutations can be: • Neutral • Harmful • Beneficial Neutral (not good or bad Ø) • Most common. • Have little or no effect on the expression of genes or the function of the proteins for which they code. Harmful (bad -) • Often caused by dramatic changes in protein structure or gene activity. • Produce defective proteins that disrupt normal biological activities. • Often the cause of many genetic disorders. • Many cancers are due to harmful mutations. Beneficial (good +) • Source of genetic variability in a species. • Beneficial mutations may produce proteins with new or altered activities that can be useful to organisms in different or changing environment. Questions - Identify the mutations below as neutral (N), harmful (H) or beneficial (B) • A specific 32 base pair deletion in human CCR5 (CCR5-Δ32) confers HIV resistance to homozygotes and delays AIDS onset in heterozygotes. • Individuals with two alleles of the sickle-cell disease have sickle-cell disease. • Individuals with only one of the two alleles of the sickle-cell disease are more resistant to malaria • A mutation on the last nucleotide on GGU. • Color-blindness is caused by the mutation of a single gene.