MUTATIONS FINAL.kinds of mutation,causes

Mutation type cause

1. Mutations Navya b no: 16

2. What is mutation? Mutations are sudden, inheritable alterations of the genetic material of individuals. It can occur due accidental errors during replication or by exposure to Mutagens. The altered sequence becomes stably incorporated in the genome, and affect the protein encoded by the affected genes.

3. Classification of mutations

4. 1. Based on the kinds of cells in which they occur A. Somatic Mutation It is acquired mutation, occurs in somatic tissue, at any time of life. Does not get inherited and has no role in Evolution. B. Germinal mutation It plays an important role in evolution through natural selection. It hereditary mutation, occurs during gametogenesis.

5. 2. Based on the type of chromosome in which they occur A. Sex-Linked Mutation Mutations that occur in the sex chromosomes. They exhibit sex-linked inheritance. Unequal chances of inheritance by male and female offsprings. The inheritance can be Sex-Linked Dominant (Rett Syndrome) or Sex-Linked Recessive (Hemophilia). Inheritance can be easily studied using a family tree .

6. Mutations that occur in the autosomal chromosomes. They exhibit autosomal inheritance. Males and females are affected equally. The inheritance may be Autosomal Dominant (Huntington’s Disease) or Autosomal Recessive (Sickle Cell Anemia). Study of mode of inheritance is difficult. B. Autosomal Mutation

7. 3. Based on the mode of origin A. Spontaneous Mutation Mutations that occur naturally without the involvement of any external agents (Mutagens) It is infrequent and random, the low frequency is due to cellular repair mechanisms. Caused by natural errors in DNA replication, depurination, deamination and transposons. Eg ; Sickle cell anemia in humans B. Induced Mutation It is produced by exposure with a physical or chemical agent. Occurs due to the base mispairings, base damage, etc. Eg ; Change of sex cell gene by the exposure of gonads to mutagens.

8. 4. Based on their direction A. Forward Mutation Mutations that alters the wild-type allele into the deleterious allele (mutant allele). The mutant formed shows different phenotype than the wild phenotype. The errors in the genotype formed cannot be corrected by repair mechanism. eg ; A mutation that inactivates the LacZ gene of the bacterium E.coli, makes them unable to grow on medium with lactose as the sole carbon source. B. Reverse Mutation Mutation that alters the mutant type allele into the wild type allele. It restores normal nucleotide sequence and normal phenotype is formed. Eg ; Suppose gene A is not able to produce protein A due to a mutation. A reverse mutation can result in the production of protein A, thereby reversing the mutant in gene A.

9. 5. Based on phenotypic expression of mutated genes A. Dominant Mutation Mutations that occur in the dominant alleles. It is expressed even in heterozygous condition. It leads to gain of function and forms new phenotypic variation. Eg ; Huntington’s Disease, for instance, is a dominant mutation where, if one allele is carrying that version of the Huntington’s gene, it will result in disease, regardless of what that person’s other allele is. B. Recessive Mutation Mutation that occur in the recessive allele. It is expressed only in homozygous condition. It leads to loss of function. It leads into lethality of organisms, hence called Lethal Mutation. Eg ; Cystic fibrosis, Sickle cell anemia and Tay-Sach’s disease.

10. Mutant types

11. Lethal mutations It is a mutation which can result in death or reduce significantly the expected longevity of an organism carrying the mutation. Depending on the genes involved, fatal genes can be dominant or recessive. The death of the organism can occur at any moment, with the most prevalent occurrence being during the early phases of development.

12. Recessive lethal mutation Example- ALBINISM Albinism results from inheritance of recessive gene alleles and is known to affect all vertebrates including humans. It is due to the absence or defect of Tyrosinase, a copper-containing enzyme involved in the production of melanin. It is characterized by the absence of melanin in the eyes, skin, hair, scales, feathers or cuticle. Albinism is associated with a number of vision defects, such as Photophobia, Nystagmus (rapid back and forth movement of eyes that can’t be controlled) and Amblyopia (type of poor eyesight). Lack of skin pigmentation aids to increased susceptibility to Sunburn and Skin cancer.

13. Dominant lethal MUTATION Example- HUNTINGTON’S DISEASE It is a neurodegenerative disorder, caused due to a single defective gene on chromosome 4. It is fatal genetic disorder that causes the progressive breakdown of nerve cells in the brain. Because of the late onset of the disease (ages of 30 to 50), individuals carrying the allele can pass it on to their offsprings.

14. EARLY STAGE Changes in coordination, Involuntary movements (Chorea), Difficulty in thinking through problems, Depressed or irritable mood, Less able to work. MIDDLE STAGE Severe movement disorder, Diminished speech, Difficulty in swallowing. LATE STAGE Patients totally dependent on others, Chorea may be severe or it may cease, Do not walk or speak but retains an awareness of family and friends, Finally death.

15. Conditional mutation Conditional mutation is a mutation that has a wild type phenotype under certain “Permissive” environmental conditions and a mutant phenotype under certain ‘‘Restrictive’’ conditions. One example is the Temperature sensitive pigmentation of Siamese cats. Siamese cats have temperature sensitive fur color; their fur appears unpigmented (light colored) in warmer areas of the body. The hair appears pigmented (dark) where is cooler temperature such as ears, nose, tail and paws. The mutated version of Tyrosinase protein is the reason for this. It is involved in the production of Melanin at temperatures below 36*C, but not above 39*C.

16. ALL KINDS OF MUTATION INVOLVES; PLOIDY CHANGES CHROMOSOMAL ABBERATIONS GENE MUTATION OR POINT MUTATION

17. Changes in the number of chromosomes Ploidy is the measure of number of chromosomes in a cell. The variation in chromosome numbers are due to error in meiosis or mitosis. Ploidy change result in mutations. EUPLOIDY is the condition of a cell, tissue or organism that has the chromosome number, which is the multiple of the basic chromosome set. 1. Monoploidy (n) 2. Diploidy (2n) 3. Polyploidy (3 or more n) Auto polyploidy Allo polyploidy

18. AUTOPOLYPLOIDY Having more than two sets of chromosomes, all derived from the same species. It generally occurs due to non-disjunction of chromosome, resulting in diploid cells. When this diploid gametes fuse with haploid or diploid, it forms triploid or tetraploid cells. Auto polyploids are commonly found in fruits and vegetables. Eg ; Chrysanthemum and Daylilies (tetraploid). ALLOPOLYPLOIDY Having more than two sets of chromosomes, all derived from two different species. It occurs when 2 species mate to produce a hybrid species, which differ from the parent species.

19. ANEUPLOIDY The unbalanced chromosome numbers are called aneuploid. It caused due to non-disjunction of chromosomes during meiosis. In humans, the rate of non-disjunction rises rapidly with the age of the mother. A. HYPOPLOIDY Monosomy: When there is one chromosome less in the cells ( eg ; Turner’s Syndrome- XO) Nullisomy : When there is loss of a pair of chromosomes(2n-2). Often, they do not survive. B. HYPERPLOIDY Trisomy: When there is an extra chromosome is in the cell ( eg ; Klinefelter Syndrome- XXY, Down syndrome-Trisomy 21) Tetrasomy: Additional pair of chromosomes (2n+2) Disomy : When there is an additional chromosome in the monoploid organism (n+1) Mosaic Variegated Aneuploidy: Mosaicism occurs when some of the cells contain an abnormal number of chromosomes.

20. Chromosomal aberrations Chromosomal aberrations or abnormalities, are changes to the structure of chromosomes, which are strands of condensed genetic material. Humans typically have 23 pairs of chromosomes, of which 22 pairs are autosomal and the last pair of chromosomes are sex chromosomes (XX or XY). A change to any of the chromosomes, in number or structure, creates a chromosomal aberration and may cause medical disorders. 4 basic types; DELETION : A portion of the chromosome is deleted, or taken out, which can make the chromosome less functional. DUPLICATION : Part of the chromosome is duplicated, resulting in extra genetic material. INVERSION : It happens when the genetic material is inverted, or flipped in the opposite direction. TRANSLOCATION : When a piece of one chromosome has broken off from its original location and attached to another chromosome.

21. Point mutation A point mutation occurs in a genome, when a single base pair is added, deleted or changed. While most point mutations are benign, they can also have various functional consequences, including changes in gene expression or alterations in encoded proteins. It may be due to copying errors during transcription, or by influence of chemical or physical factors, etc. Example: SICKLE CELL ANEMIA It is caused by a single point mutation in the seventh codon of the beta-goblin gene of the hemoglobin (Glutamic acid is replaced by Valine). Characterized by anemia and severe acute painful crises, limiting the average life span to just 36 to 40 years of age. HbS formed instead of HbA ; will now cause the abnormal sharpening of RBCs to crescent or sickle shaped. It is of Autosomal Recessive inheritance.

22. Frameshift mutation It is a genetic mutation caused by indels (insertions or deletions) of a number of nucleotides in a DNA sequence that is not divisible by 3. Due to the triplet nature of gene expression by codons , the insertion or deletion can change the reading frame, resulting in a completely different translation from the original. The polypeptide being created could be abnormally short or abnormally long, and will most likely not be functional.

23. TAY-SACH’S DISEASE It is a genetic disorder that results in the destruction of nerve cells in the brain and spinal cord. It is caused by a frameshift mutation in the HEXA gene on Chromosome 15. Most common form is infantile Tay-Sach’s Disease, which becomes apparent around the age of 3 to 6 months, with the baby losing the ability to turn over, sit or crawl. This is then followed by seizures, hearing loss, and inability to move, with death usually occurring by age of 3 to 5 .

24. Environmental Factors; which induce mutations IONIZING RADIATIONS : Nuclear Radiations, UV Light, X-Rays, Gamma Rays, etc. can induce mutations. CHEMICAL SUBSTANCES : Benzene, Formaldehyde, CCl4, etc. VIRUSES & MICROORGANISMS : They integrate into human chromosome and upset genes. ALCOHOL & DIET : High alcohol intake increases the risk of some cancers. Diet high in fat and those containing burned or highly preserved meat.

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