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Genetic Mutations SDK October 8, 2013

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  1. Genetic MutationsSDKOctober 8, 2013

  2. OBJECTIVES By the end of this session the student should be able to: • Define Mutation • Frequency of mutations in normal individuals • Classify different types of mutation • Explain the mechanism of mutation • Explain the role of mutation in biodiversity • Explain how mutations can cause severe diseases • Give examples of deletions, duplications, and insertions in genes • Define trinucleotide repeat expansions and how they cause neurological diseases SDK 2012

  3. What is a gene mutation? • Replacement or change of a nucleotide base with another, in one or both strands, or addition or deletion of a base pair in a DNA molecule . • Mutations are changes in genetic material(Nitrogenous bases) – changes in DNA code – thus a change in a gene(s) • In gene mutations, the DNA code will have a base (or more) missing, added, or exchanged in a codon. SDK 2012

  4. Gene mutation out come • Mutations can lead to missing or malformed proteins, and that can lead to disease. SDK 2012

  5. Types of Mutations • Germ-line mutations .Mutations that are inherited from parents are called germ-line mutations. • Acquired mutations. Mutations that are acquired during your lifetime are called acquired mutations • Some mutations happen during cell division, when DNA gets duplicated. • Still other mutations are caused when DNA gets damaged by environmental factors, including UV radiation, chemicals, and viruses. SDK 2012

  6. How common are mutations? • Mutations occurs at a frequency of about 1 in every 1 billion base pairs • Everybody has about 5-10 potentially deadly mutations in our genes- in each cell of our body! SDK 2012

  7. When everyone has mutations, Why they are not always seen They are not always seen because the mutation may have occurred in a section of DNA that doesn’t make a protein. • Diseases caused by just one copy of a defective gene are not manifested with the exception of • Huntington's disease, which is rare and afflicted carriers are more likely to die before reproducing. • Most inherited genetic diseases are recessive, which means that a person must inherit two copies of the mutated gene to inherit a disorder. • This is one reason that marriage between close relatives is discouraged; two genetically similar adults are more likely to give a child two copies of a defective gene. SDK 2012

  8. Mutations Outcome • The affected gene may still function. • Mutations may be harmful. • Mutations may be beneficial. • Mutations may have no effect on the organism. • Mutations are a major source of genetic variation in a population increasing biodiversity. SDK 2012

  9. Mutations a cause of Biodiversity SDK 2012

  10. Does all mutations passed on to next generation? NO • Only mutations in gametes (egg & sperm) are passed onto offspring(Germline Mutation). • Mutations in somatic cells (body cells) only affect the body in which they occur and are not passed onto offspring. SDK 2012

  11. Brain Work 1 A mutation may happen in any gene. TRUE OR FALSE? TRUE SDK 2012

  12. Spontaneous and Induced Mutations • Spontaneous: Occur spontaneously without obvious reason. • Induced mutations: caused by mutagens. • Mutagens are the agent that causes the DNA code to change (mutate) • X-Ray, • Chemicals, • UV light, • Radiation, etc SDK 2012

  13. Brain Work 2 Which of the following may cause mutations? • Coffee • UV light (sun light) • Hair gel • Vaccines • UV Light (Sun Light) SDK 2012

  14. Types of Mutations • Point mutations. A point mutation is a simple change in one base of the gene sequence. 2. Frame shift mutations. one or more bases are inserted or deleted • Original The fat cat ate the wee rat. • Point Mutation The fat cat ate the wet rat. Original. The fat cat ate the wee rat Frame Shift The fat caatet hew eer at SDK 2012

  15. Morphological Types of Point mutations • Transitions. Transitions occur when a • Purine is converted to a purine (A to G or G to A) • Pyrimide is converted to a pyrimidine (T to C or C to T) • 2. Transversion. A transversion results when • Purine is converted to a pyrimidine (A to C or G to T) • Pyrimidine is converted to a purine. (T to A or C to G) SDK 2012

  16. Types of Mutations according to their effects on the protein (or mRNA). Silent Mutations. Mutation in a codons that produce same amino acid. These mutations affect the DNA but not the protein. Therefore they have no effect on the organism’s phenotype. CUU CUC Missense Mutations. Missense mutations substitute one amino acid for another. Example. HbS, Sickle Cell Hemoglobin, is a change in the beta-globin gene, where a GAG codon is converted to GUG. GAG GUG Nonsense mutations. convert an amino acid into a stop codon. The effect is to shorten the resulting protein. Sometimes this has only a little effect, however, often nonsense mutations result in completely non-functional proteins. UUU UAA\ UGA SDK 2012

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  18. Frame-shift • In a frameshift mutation one or more bases are inserted, or deleted. • Because our cells read our DNA in three letter words, adding or removing one letter changes each subsequent word. • This type of mutation can make the DNA sequence meaningless . • For example: • Original= THE FAT CAT ATE THE WEE RAT • FRAMESHIFT= THE FAT CAA TET HEW EER AT. SDK 2012

  19. Brain Work 3 Mutations are a natural part of the cellular process reproduction. The cell has tools that catch and repair 99.9% of mutations. TRUE OR FALSE? TRUE SDK 2012

  20. Brain Work 4 Most mutations are caught and repaired in the cell. TRUE or FALSE? TRUE SDK 2012

  21. Point mutation occurs when the base sequence of a codon is changed. (ex. GCA is changed to GAA) There are 3 types: Also called frameshift mutations Classical Types of Point mutation Mutations • Substitution • Deletion • Insertion SDK 2012

  22. Substitution • A substitution is a mutation that exchanges one base for another (i.e. a change in a single “chemical letter” such as switching an A to G. • For example: CTGGAG CTGGGG SDK 2012

  23. Substitution Mutations Normal DNA: CGA – TGC –ATC Alanine – Threonine - stop Mutated DNA: CGA – TGC –TTC Alanine – Threonine - Lysine What will happen to the amino acids? This is a substitution mutation What has happened to the DNA? The adenine was replaced with thymine SDK 2012

  24. The cat ate the rat The hat ate the rat SDK 2012

  25. Clinical Examples SDK 2012

  26. Sickle Cell Anemia • Sickle cell anemia is the result of a (substitution) point mutation in codon 6 of the -globin gene resulting in the substitution of amino acid glutamic acid by valine SDK 2012

  27. Sickle Cell Anemia • Under conditions of low oxygen tension, such as • Following exercise or • In an atmosphere containing a low oxygen level, The following changes occur: • The haemoglobin agglutinates to form insoluble rod-shaped polymers • Red blood cells become distorted and sickle-shaped • The sickle-shaped cells rupture easily causing haemolytic anaemia • The sickle shaped cells tend to block capillaries interfering with the blood flow to various organs. SDK 2012

  28.  Thalassaemia • Substitution of C by U in mRNA that is coding  globin chain of 146 amino acid. • Resulted in the formation of a stop signal UAG in place CAG of glutamate in codon number 39. • This result in a shortened globin chain containing only 39 instead of the normal 146 amino acids in the -globin protein chain. • This protein is functionally useless and is equivalent to absence of -globin gives clinical symptoms of  thalassaemia, SDK 2012

  29.  Thalassaemia SDK 2012

  30.  Thalassemias • Beta thalassaemia is a genetic disorder in which there is lack of beta globin. It may be the result of: • Deletion of the whole gene so that beta globin cannot not produced (designated  o ) • Deletion of the promoter region so that transcription cannot occur (designated  o ) • Deletion of a large part of the gene resulting in a grossly abnormal or reduced synthesis functional protein (designated  + ) SDK 2012

  31. Clinical Features of  -Thalassaemia • Haemoglobin A (α2  2) cannot be produced • Hb F (α2 g2) is produced even in adults • Hb A2 (α 2 d2) formation is increased • Eerythrocytes are microcytic (small) due to lack of normal haemoglobin • Erythrocytes rupture easily causing severe haemolytic anaemia, requiring repeated blood transfusions • The bone marrow expands trying to compensate by increasing haemopoiesis. SDK 2012

  32. Clinical Features of  -Thalassaemia • The bones of the face and skull are thickened causing a characteristic facial appearance • The spleen and liver enlarge because haemopoietic tissue forms in them • Excess iron accumulates in the blood and is deposited in the heart, liver, pancreas and other organs (this is because of repeated transfusions while no blood is actually lost from the body) • Children have delayed growth and development and are prone to repeated infections SDK 2012

  33. Point Mutation In alpha-Globin Gene “Elongated α Globin Chain, Haemoglobin Constant spring \Wayne Hb” • Here the stop codon UAA at position 142 in the alpha (-) globin gene was substituted by the codon for glutamine. • Translation of the protein thus continued until a stop codon was encountered at codon 173. • The -globin was considerably elongated, resulting in a variant of haemoglobin termed Haemoglobin Constant spring\ Wayne Hb. SDK 2012

  34. Elongated α Globin Chain Or Haemoglobin Wayne SDK 2012

  35. Insertion • Insertions are mutations in which extra base pairs are inserted into a new place in the DNA. CTGGAG CTGGCCTAG SDK 2012

  36. Insertion Mutations Normal DNA: CGA – TGC – ATC Alanine – Threonine – stop Mutated DNA: CGA – TAG – CAT – C Alanine – Isoleucine – Valine What will happen to the amino acids? This is an insertion mutation, also a type of frameshift mutation. An adenine was inserted thereby pushing all the other bases over a frame. What has happenedto the DNA? SDK 2012

  37. Insertion Mutations The cca tat eth era t The cat ate the rat SDK 2012

  38. Haemophilia A • An X-linked recessive disorder in which blood clotting does not occur due to deficiency of clotting factor VIII. • In most cases the mutation is the result of insertion of a large segment, consisting of about 3800 bp, in the coding region of the factor VIII. • This results in total inactivation of the protein. SDK 2012

  39. Haemophilia A • Inherited as a sex linked recessive trait with bleeding manifestations only in males. • Genes which control factor VIII and IX production are located on the x chromosome • Affected male marries a normal female: none of sons will be affected, all daughters will be carriers • Female carrier marries normal male: 50% chance sons will be affected and 50% chance daughters will be carriers SDK 2012

  40. Deletion • Deletions involve removal of one or more base pairs. • They vary greatly in size from deletion of a single base to deletion of a whole gene. • The clinical effects often depend on the size and location of the deleted part of the gene. CTGGAG CT AG SDK 2012

  41. Deletion Mutations Normal DNA: CGA – TGC – ATC Alanine – Threonine – stop Mutated DNA: CGA – TCA- TC Alanine – Serine What has happenedto the DNA? A guanine was deleted, thereby pushing all the bases down a frame. What will happen to the amino acids? This is called a deletion mutation, also a type of frameshift mutation. SDK 2012

  42. Muscular Dystrophy Deletions of Dystrophin Gene • Dystrophin is a protein that is an important component of skeletal muscle. • The dystrophin gene is located on the p arm of the X chromosome (Xp21.2). • It is a very large gene spanning 2.5 million bp of genomic DNA and consists of 79 exons coding for a protein of approximately 3600 amino acids (11kb). SDK 2012

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  45. Muscular Dystrophy Deletions of Dystrophin Gene • Deletion of the whole or most of the dystrophin gene • Dystrophin may not be produced at all • Or produce in in abnormal forms, • Resulting in Duchenne muscular dystrophy • This is a severe X-linked recessive disorder that affects boys and is transmitted by carrier females. X linked Recessive disorder. • In affected boys there is almost complete lack of dystrophin, muscle weakness beginning in childhood and increasing progressively in severity so that the individual is wheel-chair bound at the age of about 15 years. • Death usually ensues in the early twenties due to respiratory muscle involvement. SDK 2012

  46. X linked Recessive disorder SDK 2012

  47. Becker Muscular Dystrophy(BMD) • Deletions involving a small non-critical part of the gene result in altered dystrophin. • This causes the clinical condition of Becker muscular dystrophy • in BMD muscle weakness begins in adolescence and is very slowly progressive, and affected individuals may lead an almost normal life. SDK 2012

  48. Cystic Fibrosis. • Cystic fibrosis (CF) is a genetic condition that affects many organs in the body: especially the lungs, pancreas and sweat glands. • Cystic fibrosis is caused by a mutation in the Cystic Fibrosis Trans-membrane Regulator (CFTR) gene, that is located on chromosome 7. • This gene Produces a trans-membrane protein that regulates the flow of chloride ions into the cells. • The most common mutation is termed the ∆508 mutation, which is a deletion of a single codon at position number 508 in exon 10 of the CFTR gene. • Homozygotes(both parents need to be the carriers of the defective gene) for a ∆508 mutation have cystic fibrosis disease.

  49. Gene That Encodes CFTR • The gene that encodes the CFTR protein is found on the human chromosome 7, on the long arm at position q31.2. • Mutations consist of replacements, duplications, deletions or shortenings in the CFTR gene. • This may result in proteins that may not function, work less effectively, are more quickly degraded, or are present in inadequate numbers SDK 2012

  50. Cystic Fibrosis. • The defective gene produces a defective protein leading to a blockage in the transportation of the salt, thus leading to production of thick, sticky mucus. SDK 2012