--- Types of Mutations in ARBHN (Amino Acid Replacement Based on Hemoglobin to Product Form) 1. Nonsense Mutation Occurs when a point mutation introduces a prem
--- Types of Mutations in ARBHN (Amino Acid Replacement Based on Hemoglobin to Product Form) 1. Nonsense Mutation Occurs when a point mutation introduces a premature stop codon into the sequence. This abruptly terminates protein synthesis , leading to a truncated (shortened) protein . If the mutation occurs at the end of the gene , the protein may retain some partial function . Example: Duchenne Muscular Dystrophy (DMD) – Caused by nonsense mutations in the dystrophin gene. 2. Missense Mutation Definition: A substitution mutation that results in the replacement of one amino acid with another of different properties . Type: Usually occurs as a transversion substitution mutation . Example: Sickle Cell Anemia (SCA) – A genetic disorder affecting hemoglobin structure. Sickle Cell Anemia: A Case of Missense Mutation Molecular Basis - Hemoglobin (Hb) is composed of four polypeptide chains (two α-chains and two β-chains). - The β-globin chain consists of 146 amino acids . - In a normal hemoglobin (HbA) molecule, the 6th amino acid in the β-globin chain is glutamate (Glu). - In sickle cell hemoglobin (HbS) , a point mutation occurs in the gene, leading to substitution of glutamate (Glu) with valine (Val) at position 6 . Genetic Comparison Between Normal and Sickle Cell Patients Effect on Red Blood Cells 🩸 Normal Hemoglobin (HbA): - Glutamate is hydrophilic ( water-attracting ) and interacts with water , maintaining the biconcave shape of red blood cells (RBCs). - RBCs remain flexible and pass easily through capillaries. 🩸 Sickle Cell Hemoglobin (HbS): - Valine is hydrophobic ( water-repelling ) and hides from water , leading to distorted sickle-shaped RBCs . - Effects: Reduced flexibility – RBCs cannot pass through capillaries easily , causing blockages . Lower oxygen-carrying capacity – Leads to anemia . Increased risk of sickle cell crisis – Painful episodes due to blocked blood flow. 3. Frameshift Mutation Occurs when insertions, deletions, or duplications alter the reading frame of the genetic code. This shifts the grouping of codons , causing a completely different amino acid sequence . Can be highly detrimental , often leading to nonfunctional proteins . Example: Thymine Dimer Formation - UV radiation causes thymine dimers , disrupting DNA structure. - If NERM (Nucleotide Excision Repair Mechanism) is defective, it leads to xeroderma pigmentosum (XP) . Example: HIV Mutation Effects - HIV viral insertion into host DNA can cause frameshift mutations , altering gene function. 4. Silent Mutation A mutation that does not affect the organism’s phenotype . Occurs in the wobble position (3rd base of the codon), where multiple codons can code for the same amino acid . Even if an amino acid is changed, the new amino acid may have similar properties to the original, resulting in no functional difference . Example: Codon redundancy in genetic code. Example Codon Table - Mutation of GCU → GCC would not change the amino acid , leading to a silent mutation . Key Takeaways Missense mutations can be harmful, as seen in sickle cell disease. Nonsense mutations cause truncated proteins, leading to severe effects. Frameshift mutations disrupt the entire reading frame, often making the protein nonfunctional. Silent mutations do not affect protein function, as they occur in wobble positions. Here’s a well-structured and refined version of your notes on Mutation, DNA Repair, and Associated Disorders : Mutation, DNA Repair, and Associated Disorders 1. Transversion Mutations & Polymorphism Transversion mutations occur when a purine (A/G) is replaced by a pyrimidine (C/T) or vice versa. Most transversion mutations occur at the wobble position (third base of a codon). When transversion mutations exceed 1% of the population , they are classified as polymorphisms . 2. Replication Fidelity & Mutation Repair Mechanisms Importance of Replication Fidelity DNA replication accuracy is crucial to ensure genetic stability. Errors occur at a rate of 1 per 10⁹ bases incorporated during replication. Fidelity of DNA replication ensures the correct transfer of genetic information across generations. Rules for Maintaining Replication Fidelity - Watson-Crick Base Pairing Rules (A pairs with T, G pairs with C). - Proofreading Activity of DNA Polymerase DNA polymerase corrects mismatches in the 3' → 5' direction using exonuclease activity . - Errors that escape proofreading are corrected by the Mismatch Repair Mechanism . 3. DNA Repair Mechanisms A. Base Excision Repair (BER) Corrects small-scale errors such as: - Deamination of cytosine → Uracil conversion . - Deamination of 5-methylcytosine → Thymine conversion . Base Excision Repair Process: - Uracil-DNA Glycosylase removes uracil from DNA. - AP Endonuclease cleaves the DNA strand. - DNA Polymerase β synthesizes new nucleotides. - DNA Ligase seals the DNA strand. - Helicase restores DNA structure. B. Nucleotide Excision Repair (NER) Repairs large distortions in DNA, including thymine dimers caused by UV light. Nucleotide Excision Repair Process: - XPG & XPF Nucleases cut ~30 nucleotides around the thymine dimer. - DNA Polymerase β synthesizes new nucleotides. - DNA Ligase seals the repaired DNA strand. - Helicase unwinds the DNA to ensure stability. 4. Defects in DNA Repair & Their Associated Disorders Key Takeaways High-fidelity DNA replication reduces mutation rates. DNA polymerase proofreading and mismatch repair prevent errors. Base Excision Repair (BER) fixes small errors like deamination. Nucleotide Excision Repair (NER) fixes large-scale damage like thymine dimers. Defects in DNA repair mechanisms lead to serious genetic disorders and increased cancer risks. Prevision Equations (List the equations from your assignment as given.) Types of Point Mutations & Their Effects Mutations occur due to nucleotide changes in DNA. Point mutations affect only a single nucleotide or, at most, two nucleotides . These mutations have different effects on protein synthesis depending on the type of nucleotide change. 1. Substitution Mutations - A single nucleotide is replaced by another. - Two types: Transition Mutation → Purine (A ↔ G) or Pyrimidine (C ↔ T) substitution. Transversion Mutation → Purine ↔ Pyrimidine substitution (A/G ↔ C/T). - Effect : Transition mutations often have little or no impact since they occur at the wobble position (third base of codon). - Transversion mutations are more disruptive, altering amino acid sequences and affecting protein function. 2. Frameshift Mutations - Insertion or deletion of nucleotides, altering the reading frame . - Types: Insertion – Adds one or more nucleotides. Deletion – Removes nucleotides, altering protein structure. Duplication – A segment of DNA is copied and inserted. - Effect :Almost always results in a nonfunctional protein . - Can lead to premature stop codons . - Example: Tay-Sachs Disease (caused by a 4bp insertion in HEXA gene). 3. Nonsense Mutation - A substitution results in a stop codon (UAA, UGA, UAG) appearing too early . - Effect :Produces a short, truncated protein . - If the mutation occurs near the end of the gene , the protein might retain partial function . - Example: Duchenne Muscular Dystrophy . 4. Missense Mutation - A single base substitution changes one amino acid to another with different properties . - Effect :Can alter protein function , especially if the change occurs in a critical region. - Example: Sickle Cell Anemia (see below). 5. Silent Mutation - A base change does not alter the amino acid sequence due to the redundancy of the genetic code. - Effect :No change in protein structure or function. - Mostly harmless , though it can affect mRNA stability in rare cases. Types of Mutations in RHN & Effects on Protein Production Mutations affect protein production in different ways: - Nonsense Mutation → Produces a shortened protein that is usually non-functional. - Missense Mutation → Can either disrupt protein function or have no significant effect . - Frameshift Mutation →