Mutation PPT PDF

# MUTATION ## What are mutations? - Any changes in the DNA sequence of an organism is a mutation. - DNA is made of a long sequence of smaller units strung together. There are four basic types of unit: A, T, G, and C. - Some parts of DNA are control centers for turning genes on and off. - Some part...

# MUTATION ## What are mutations? - Any changes in the DNA sequence of an organism is a mutation. - DNA is made of a long sequence of smaller units strung together. There are four basic types of unit: A, T, G, and C. - Some parts of DNA are control centers for turning genes on and off. - Some parts have no function. - And some parts have a function that we don't understand yet. - Organisms have mechanisms such as DNA repair to remove mutations. A diagram shows a double helix of DNA, where each base pair is labeled as Adenine, Thymine, Guanine, and Cytosine. ## Types of mutations A diagram displays the different types of mutations, with the main category of mutations branching into: * **Point mutation** * Transition * Transversions * **Frame shift Mutation** * Insertions * Deletions ## Common defects in DNA and their origins | Type of Defect | How Does This Type of Change Arise? | |---|---| | 1. Incorrect base in one strand cannot hydrogen bond with corresponding base in the opposite strand | Normal base tautomerizes (i.e., isomerizes in such a way that it is capable of an alternative form of hydrogen bonding); base substitution occurs during subsequent DNA replication | | 2. Missing bases | Depurination: N-glycosylic bond joining purine base to deoxyribose is spontaneously broken without breaking DNA backbone | | 3. Altered bases | Alkylating agents add methyl or ethyl groups to existing bases | | 4. Addition or deletion of one or more bases | May occur spontaneously, or be induced by chemical mutagens (intercalating agents) or biological agents (transposable elements) | | 5. Single-strand breaks | Phosphodiester bond is broken as a result of exposure chemical agents or ionizing radiation | | 6. Double-strand breaks | Phosphodiester bonds on opposite DNA strands are broken as a result of exposure to high doses of chemical agents or ionizing radiation | | 7. Cross-linking of complementary DNA strands | Certain antibiotics (mitomycin-C) or reagents (nitrite ions) form covalent bonds between two bases on complementary DNA strands, preventing strand separation during DNA replication | The rest of the document shows different diagrams of chemical structures and several examples of chemical mutations. ## Types of DNA Damage: BASE LOSS AND BASE MODIFICATION A diagram showing the chemical structures of: * Thymine * Adenine * Cytosine * Guanine * AP Site And a diagram illustrating the effect of a "photodamage thymine dimer" on DNA. The document then explains: * **Chemical Modification:** Changes in the chemical structure of a base * **Depurination:** The loss of a purine base from DNA * **Deamination:** The loss of an amino group from cytosine * **Chemical Modification by O₂ free radicals:** The effect on DNA resulting from the presence of free radicals ## Common and Rare Tautomeric Forms Several diagrams displaying the common and rare tautomeric forms of: * Thymine * Cytosine * Adenine * Guanine ## Tautomeric Shift: Transition Mutation A diagram shows how a tautomeric shift in the DNA sequence leads to a transition mutation after semiconservative replication. ## Causes of Mutations - **Spontaneous mutations:** Tautomeric shifts - mutations can cause a resulting mismatch that could be repaired. - Mutations may result if repair systems fail. A diagram shows the formation of a mismatch during DNA replication due to a tautomeric shift in an adenine base. ## Depurination A diagram shows how depurination removes a guanine base from DNA ## Examples of Types of Point Mutations | Type of Mutation | Result at Molecular Level | Example | |---|---|---| | **A. Base Substitution Mutations** | | | | 1. Transition | One purine replaced by a different purine; or one pyrimidine replaced by a different pyrimidine. | AT – GC | | 2. Transversion | A purine replaced by a pyrimidine or vice versa. | A – T → T – A | | **Changes in protein** | | | | 1. Silent mutation | Altered codon codes for same amino acid. | GAG – GAA | | 2. Neutral mutation| Altered codon codes for a different but functionally similar amino acid. (Protein may be functional.) | GAG – GAU | | 3. Missense mutation | Altered codon codes for a different, dissimilar amino acid. (Protein often nonfunctional.)| GAG – AAG | | 4. Nonsense mutation(= Chain termination mutation)| New codon is a termination codon. (Protein synthesis stops. Protein is nonfunctional.) | GAG – UAG | | **B. Frameshift Mutations: Addition or deletion of one or more base pairs will result in a shift in the reading frame of the resulting mRNA molecule, and lead to production of a nonfunctional protein.**| | | | 1. Wild type base sequence | ATG ACC AGG TC | | | 2. Base addition| ATG ACA CAG GTC | | | 3. Base deletion| ATG ACA GGT C | | ## Frameshift Mutation A diagram illustrates how a frameshift mutation can occur in a DNA sequence, altering the sequence of amino acids, and potentially producing a nonfunctional protein. ## Mutagens | Class | Mutagen | |---|---| | **Physical mutagens** | | | 1. Ionizing radiations | - Alpha, beta, fast neutrons, thermal neutrons | | - Particulate radiations | | | - Non-particulate radiations | - X-rays, gamma rays | | 2. Non-ionizing radiations | - UV rays | | **Chemical mutagens** | | | 1. Alkylating agents | - mustard gas, nitrogen mustard, EMS, MMS,EES. | | - Base analogues | -5-bromouracil, 2-amino purine | | - Acridine dyes | -acriflavin, proflavin, acridine orange. | | 2. Deamination agents | -nitrous acid (HN02) | | - Other chemical mutagens | -hydroxylamine, sodium azide. | ## Mutagenic Agents | PHYSICAL MUTAGENS | CHEMICAL MUTAGENS | BIOLOGICAL MUTAGENS | METALS | |---|---|---|---| | X-rays (ionizing radiation) | Reactive oxygen species (ROS) such as hydrogen peroxide and superoxide | Transposons (Transposons disrupt the functional elements of the gene when they are inserted into chromosomal DNA) | Arsenic | | Gamma rays (ionizing radiation) | Deaminating agents such as nitrous acid | Viruses (Viruses disrupt genetic function when inserted into the genome of a cell) | Chromium | | Alpha rays (ionizing radiation) | Alkylating agents such as nitrosamines and ethylnitrosourea | Bacteria (Some bacteria such as _Helicobacter pylori_ cause inflammation during which oxidative species are produced, causing DNA damage and reducing efficiency of DNA repair systems, thereby increasing mutation) | Cadmium | | Ultraviolet radiations | Aromatic amines and amides | Prions | Nickel | | Radioactive decay | Alkaloids from plants | | | | Cosmic rays | Bromine, benzene and sodium azide | | | ## Types of Chemical Mutagens and Their Mechanisms of Action - **Base analogs:** - Examples: 5-bromouracil, 2-aminopurine - **Acridines:** - Eg. proflavin - **Alkylating agents:** - Examples: Di-(2-chloroethyl)sulfide (sulfur mustard), Di-(2-chloroethyl)methylamine (Nitrogen mustard), ethylmethane sulfonate (EMS) - **Deaminating agents:** - Examples: Nitrous acid (HN02) - **Miscellaneous** - Examples: hydroxylamine, free radicals ## Some Commonly Used Chemical Mutagens and Their Mechanisms of Action | Type of Mutagenic Agent | Example of This Type of Agent | Mode of Action of This Agent | Diagram of Mode of Action | |---|---|---|---| | 1. Base analogue | 5-Bromouracil (base analogue of T) | Normally pairs with A, but can undergo a tautomeric shift and pair with G. This results in incorporation of C into the daughter DNA strands during subsequent rounds of DNA replication. | A diagram showing a cytosine base pairing with guanine and a bromouracil base pairing with guanine. | | 2. Nitrous acid | Structure: Nitrous acid | Converts amino groups to keto groups by oxidative deamination. C → uracil (U) (pairs with A) A → hypoxanthine (H) (pairs with C) G → xanthine (X) (pairs with C) Reacts with C and converts it to a modified base that pairs only with A. | A diagram showing the conversion of adenine to hypoxanthine, cytosine to uracil, and guanine to xanthine. | | 3. Hydroxylamine | Structure: hydroxylamine | | A diagram showing the conversion of cytosine to hydroxylaminocytosine. | | 4. Alkylating agents | EMS (ethylmethane sulfonate) and MMS (methylmethane sulfonate) | Add alkyl groups (ethyl or methyl) to the hydrogen-bonding oxygen of G and T, producing 0-6 alkylguanine (pairs with T) and O-4-alkylthymine (pairs with G). | A diagram showing the conversion of guanine to 0⁶-methylguanine and thymine to 0⁴-methylthymine. | |5. Intercalating agents: Planar, three-ringed molecules whose dimensions are roughly the same as those of a purine-pyrimidine base pair | proflavine, acridine orange | Insert between two adjacent base pairs in a DNA molecule, causing insertions or deletions. | A diagram showing an acridine molecule intercalating between two base pairs in DNA. | ## Deamination - Deamination involves the removal of an amino group from the cytosine base - The other bases are not readily deaminated A diagram illustrating the process of deamination, converting cytosine to uracil - Deamination is a common process in DNA and causes one common type of mutation, where cytosine is converted to uracil. Uracil can then pair with adenine during DNA replication, leading to a C to T point mutation. ## Hydroxylamine * Hydroxylamine is a very specific base modifying mutagen that adds a hydroxyl group to cytosine. * It converts cytosine into hydroxylaminocytosine. * This conversion increases the frequency of a rare tautomer. * The tautomer pairs with adenine instead of guanine and leads to CG:TA transitions. * Because hydroxylamine acts only on cytosine, it will not generate TA:CG transitions. A diagram shows how a hydroxylamine molecule modifies cytosine, leading to the formation of hydroxylaminocytosine. ## Induced Mutations Arise from DNA Damage Caused by Chemicals and Radiation * **Alkylating Agents:** - Mustard gas is an example of an alkylating agent that adds alkyl groups to the purine or pyrimidine of the nucleotide. A diagram showing the conversion of guanine into 6-ethylguanine caused by the alkylating agent EMS (ethylmethanesulfonate). - **Alkylating agents:** - Chemicals that donate alkyl groups. These agents include methyl (CH3) and ethyl (CH3-CH2) groups, which are added to nucleotide bases by some chemicals. - Example, ethylmethanesulfonate (EMS) adds an ethyl group to guanine, producing 6-ethylguanine, which pairs with thymine. A diagram illustrating the process of an ethyl group being added to guanine by EMS, resulting in 6-ethylguanine. ## Intercalating Agents - **Intercalating agents:** Substances whose dimensions are roughly the same as those of a purine-pyrimidine pair. In aqueous solutions, these substances form stacked arrays, and are also able to stack with a base-pair by insertion between two base-pairs. This may result in frameshift mutation. Two diagrams showing the structures of proflavine and acridine orange. - **Intercalating agents** - Intercalating agents produce mutations by sandwiching themselves (intercalating) between adjacent bases in DNA. - They distort the three-dimensional structure of the helix and causing single-nucleotide insertions and deletions in replication. - These insertions and deletions frequently produce frameshift mutations. - And so the mutagenic effects of intercalating agents are often severe. - Because intercalating agents generate both additions and deletions, they can reverse the effects of their mutations. - E.g.: proflavin, acridine orange, ethidium bromide, and dioxin Two diagrams showing the intercalation of proflavin and acridine orange between DNA base pairs - **Intercalating Agents:** - Intercalating agents such as Actinomycin D distort the structure of DNA A diagram showing the intercalation of Actinomycin D between DNA base pairs # THANK YOU