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Human Genetics

Human Genetics. Section 13.3: Mutations Section 14.1: Human Chromosomes Section 14.2: Human Genetic Disorders. Mutations. Section 13.3. Vocabulary. Mutation Germ Mutation Somatic Mutation Gene Mutation Chromosomal Mutation Point Mutation Frameshift Mutation. Deletion Duplication

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Human Genetics

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  1. Human Genetics Section 13.3: Mutations Section 14.1: Human Chromosomes Section 14.2: Human Genetic Disorders

  2. Mutations Section 13.3

  3. Vocabulary • Mutation • Germ Mutation • Somatic Mutation • Gene Mutation • Chromosomal Mutation • Point Mutation • Frameshift Mutation • Deletion • Duplication • Inversion • Translocation • Nondisjunction • Monosmy • Trisomy • Polyploidy

  4. Types of Mutations • Mutations are heritable changes in genetic information. • A mutation results from a mistake in duplicating genetic information (DNA replication).

  5. Types of Cells Affected • Germ Mutation - affects a reproductive cell (gamete or sperm/egg) • Does not affect the organism • Passed to offspring • Somatic Mutation – affects body cells (all cells except gametes) • Not passed to offspring

  6. Types of Mutations • All mutations fall into two basic categories: • Those that produce changes in a single gene are known as gene mutations. • Those that produce changes in a part of a chromosome, whole chromosomes, or sets of chromosomes are known as chromosomal mutations. Ameoba Sisters – Mutations(7 min)

  7. Mutagens • Mutations can be caused by chemical or physical agents (mutagens) • Chemical – pesticides, tobacco smoke, environmental pollutants • Physical – X-rays and ultraviolet light

  8. Gene Mutations • Mutations that involve changes in one or a few nucleotides are known as point mutations because they occur at a single point in the DNA sequence. They generally occur during replication. • If a gene in one cell is altered, the alteration can be passed on to every cell that develops from the original one.

  9. Gene Mutations • Point mutations include substitutions, insertions, and deletions.

  10. Substitutions • In a substitution, one base is changed to a different base. • Substitutions usually affect no more than a single amino acid, and sometimes they have no effect at all.

  11. Substitution - Silent Mutation • a change in one base pair has no effect on the protein produced by the gene. • This is allowed for by the redundancy in the genetic code. • Example (as shown in picture): • Both GGC and GGU code for the amino acid glycine. • Thus, the mutation is “silent,” i.e. causes no change in the final protein product.

  12. Substitution - Missense Mutation • a change in one base pair causes a single amino acid to be changed in the resulting protein. • The result is called “missense” since the code is now different. • In the following example, GGC has been changed to AGC, resulting in a different amino acid.

  13. Substitutions - Missense • In this example, the base cytosine is replaced by the base thymine, resulting in a change in the mRNA codon from CGU (arginine) to CAU (histidine).

  14. Sickle Cell Anemia • The effect of a missense mutation on the protein is unpredictable. • A missense mutation is the cause of the disease, sickle cell anemia. • a change in one base pair alters one amino acid • effects hemoglobin protein, causing red blood cells to take on a strange shape

  15. Sickle Cell Anemia

  16. Substitution - Nonsense Mutation • a change in a single base pair creates a stop codon. • Because this kind of mutation creates a stop signal in the middle of a normally functional gene, the resulting protein is almost always nonfunctional • hence the term “nonsense” mutation.

  17. Substitution Silent Mutation Missense Mutation Nonsense Mutation

  18. Insertions and Deletions • Insertions and deletions are point mutations in which one base is inserted or removed from the DNA sequence. • If a nucleotide is added or deleted, the bases are still read in groups of three, but now those groupings shift in every codon that follows the mutation.

  19. Frameshift Mutation • Insertions and deletions are also called frameshift mutations because they shift the “reading frame” of the genetic message. • Frameshift mutations can change every amino acid that follows the point of the mutation and can alter a protein so much that it is unable to perform its normal functions.

  20. Frameshift Mutation: • Example: • Deletion: • THE FAT CAT ATE THE RAT • THE FAT ATA TET HER AT • Insertion: • THE FAT CAT ATE THE RAT • THE FAT CAR TAT ETH ERA T

  21. Insertions

  22. Deletions

  23. Muscular Dystrophy • Both Duchenne muscular dystrophy and Becker muscular dystrophy result from mutations of a gene on the X chromosome that codes for the dystrophin protein in muscle cells; this protein helps to stabilize the plasma membrane during the mechanical stresses of muscle contraction. • About 2/3 of cases are due to deletion mutations. • If the number of nucleotides deleted in the mRNA is not a multiple of three, this type of frameshift mutation results in a severely defective or absent version of the protein, resulting in more rapid breakdown of muscle cells and the more severe Duchennemuscular dystrophy. • If the number of nucleotides deleted in the mRNA is a multiple of three, the mutation does not cause a frameshiftand this typically results in a less defective version of the protein, less rapid breakdown of muscle cells, and the milder Becker muscular dystrophy. • Up to one-fifth of cases of Duchenne muscular dystrophy are caused by a nonsensemutation(a point mutation that results in a stop codon).

  24. Muscular Dystrophy • Because the dystrophin gene is on the X chromosome and because the alleles for defective dystrophin are recessive, both of these types of muscular dystrophy are much more common in boys than in girls. Duchenne muscular dystrophy affects one in every 3500 male babies.

  25. Gene Mutations:

  26. Chromosomal Mutations • Chromosomal mutations involve changes in the number or structure of chromosomes. • These mutations can change the location of genes on chromosomes and can even change the number of copies of some genes.

  27. Chromosomal Mutations • Deletion involves the loss of all or part of a chromosome.

  28. Chromosomal Deletion • Example: Cri-du-chat (5p minus) – a piece of chromosome 5

  29. Cri du chat (“cry of the cat”) • named for the distinctive cry affected infants make due in part to malformations of the larynx • intellectual disability/delayed development • small head size (microcephaly), • low birth weight • weak muscle tone (hypotonia) in infancy • widely set eyes (hypertelorism) • low-set ears • a small jaw • a rounded face • heart defect

  30. Chromosomal Mutations • Duplication produces an extra copy of all or part of a chromosome.

  31. Chromosomal Duplication

  32. Fragile X - Most people have 5-40 "repeats" at this end of their X-chromosome, those with Fragile X have over 200 repeats due to duplications

  33. Fragile X • FMR1 gene where a DNA segment, known as the CGG triplet repeat, is expanded • The abnormally expanded CGG segment inactivates (silences) the FMR1 gene, which prevents the gene from producing a protein called fragile X mental retardation protein. • Loss of this protein leads to the signs and symptoms of fragile X syndrome. • Both boys and girls can be affected, but because boys have only one X chromosome, a single fragile X is likely to affect them more severely. • Boys will have moderate mental retardation, a large head size, a long face, prominent forehead and chin and protruding ears, loose joints. • Affected boys may have behavioral problems such as hyperactivity, hand flapping, hand biting, temper tantrums and autism. Other behaviors in boys after they have reached puberty include poor eye contact, perseverative speech, problems in impulse control and distractibility. Physical problems that have been seen include eye, orthopedic, heart and skin problems. • Girls will have mild mental retardation. • Family members who have fewer repeats in the FMR1 gene may not have mental retardation, but may have other problems. • Women with less severe changes may have premature menopause or difficulty becoming pregnant.

  34. Fruit flies experience a change in eye size of when duplication occurs.

  35. Chromosomal Mutations • Inversion reverses the direction of parts of a chromosome.

  36. Chromosomal Inversion

  37. Inversions 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 individuals. An inversion does not involve a loss of genetic information, but simply rearranges the linear gene sequence.

  38. Chromosomal Mutations • Translocation occurs when part of one chromosome breaks off and attaches to another. Example: acute meyloid leukemia

  39. Translocation • Acute Meyloid Leukemia (AML) • Between chromosome 8 and 21 • is a cancer of the myeloid line of blood cells • characterized by the rapid growth of abnormal WBC that accumulate in the bone marrow and interfere with the production of normal WBC

  40. Chromosomal Translocation

  41. Nondisjunction • Chromosomal mutations that involve whole chromosomes or complete sets of chromosomes results from a process known as nondisjunction • This is the failure of homologous chromosomes to separate normally during meiosis.

  42. Nondisjunction

  43. Nondisjunction

  44. Nondisjunction

  45. Effects of Nondisjunction • If one chromosome is involved, the condition of one extra is called trisomy or one less is monosomy

  46. Trisomy 21

  47. Patau Syndrome (trisomy 13) - serious eye, brain, circulatory defects as well as cleft palate. 1:5000 live births. Children rarely live more than a few months

  48. Edward’s Syndrome (trisomy 18) - almost every organ system affected 1:10,000 live births. Children generally do not live more than a few months

  49. Trisomy X (XXX) • females. 1:1000 live births - healthy and fertile - usually cannot be distinguished from normal female except by karyotype

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