GOOD LUCK - Gene structure: Eukaryotic genes are composed of exons (coding sequences) and introns (non-coding sequences), with introns being removed during RNA
GOOD LUCK - Gene structure: Eukaryotic genes are composed of exons (coding sequences) and introns (non-coding sequences), with introns being removed during RNA splicing. - Promoter region: Upstream of the coding sequence lies a promoter containing specific sequences (e.g., the TATA box ) essential for the initiation of transcription by RNA polymerase II. - Regulatory elements: Eukaryotic genes are also controlled by enhancers , silencers , and insulators , which regulate the level, timing, and tissue-specific expression of genes. - Chromatin packaging: DNA is wrapped around histone proteins to form nucleosomes , enabling compaction into chromatin; gene accessibility depends on the degree of chromatin condensation (euchromatin vs. heterochromatin). - Post-transcriptional modification: After transcription, primary RNA undergoes capping , polyadenylation , and splicing to form mature mRNA before translation. - Ingestion of oocysts: Consuming food or water contaminated with mature oocysts from cat feces. - Ingestion of tissue cysts: Eating undercooked or raw meat containing tissue cysts (especially from pigs, sheep, or goats). - Congenital transmission: Vertical transmission from an infected mother to her fetus across the placenta during pregnancy. - Mutation: Spontaneous changes in DNA introduce new alleles into a population, altering genotype frequencies. - Natural selection: Differential survival and reproduction favor certain genotypes over others, leading to changes in allele frequencies. - Genetic drift: Random fluctuations in allele frequencies, especially in small populations, can lead to loss or fixation of alleles. - Gene flow (migration): Movement of individuals between populations introduces new genetic material, affecting allele frequencies. - Non-random mating: Mating preferences (such as assortative or disassortative mating) can alter genotype distributions by increasing homozygosity or heterozygosity. - Denaturation (94–98°C): Double-stranded DNA is heated to separate into two single strands. - Annealing (50–65°C): Short DNA primers bind (anneal) to complementary sequences on the single-stranded templates. - Extension (72°C): Taq DNA polymerase synthesizes new DNA strands by adding nucleotides to the primers. - Importance: Taq DNA polymerase is thermostable , meaning it remains active at the high temperatures used in PCR, allowing continuous DNA synthesis without denaturation of the enzyme. - Source: It is derived from Thermus aquaticus , a thermophilic bacterium found in hot springs. - Mediating virus entry: Virus receptors on host cells enable attachment and entry of viruses, initiating infection. - Tropism determination: The type and distribution of receptors determine the host range and tissue specificity of the virus (e.g., HIV uses CD4 receptors found on T-helper cells). - Disease pathogenesis: Viral binding to specific receptors can influence disease severity and clinical manifestations . - Therapeutic targets: Virus-receptor interactions are targeted in drug development (e.g., receptor blockers to prevent viral entry). - Vaccine development: Knowledge of receptors aids in vaccine design by focusing immune responses on blocking virus attachment. 1. Initiation - Chromatin remodeling opens DNA (via HATs and remodeling complexes). - Core promoter : Usually has a TATA box (~25 bp upstream). - Pre-Initiation Complex (PIC) : TFIID binds TATA (TBP subunit). - TFIIA , TFIIB , TFIIF (brings RNA Pol II), TFIIE , and TFIIH assemble. - TFIIH : Helicase : Unwinds DNA. - Kinase : Phosphorylates RNA Pol II CTD → activates transcription. - Promoter clearance : RNA polymerase II escapes the promoter and starts RNA synthesis. --- 2. Elongation - RNA polymerase II synthesizes RNA 5’ → 3’, reading DNA 3’ → 5’. - Uses ribonucleotide triphosphates (NTPs) . - Co-transcriptional processing : 5’ Capping (7-methylguanosine). - Splicing (introns removed, exons joined). - Elongation factors help polymerase move efficiently. --- 3. Termination - Polyadenylation signal (AAUAAA) triggers termination. - Cleavage of RNA transcript downstream. - Poly-A tail (~200 As) added at 3’ end. - RNA polymerase II disengages from DNA. --- 4. Post-Transcriptional Modifications - 5’ cap : Protects RNA, aids translation. - Splicing : Removes introns; can allow alternative splicing . - Poly-A tail : Stabilizes RNA, helps export from nucleus. --- QUICK MEMORY TIP: "CRP → PIC → TFIIH → Cap-Spin-Tail" (Chromatin Remodeling → Pre-Initiation Complex → TFIIH activation → Capping, Splicing, Tail addition) --- --- Protein Translation (8 Marks) Definition: Translation is the process of protein synthesis where the ribosome reads the mRNA codon sequence and assembles amino acids into a polypeptide chain. --- Stages of Translation 1. Initiation - mRNA binds to the small ribosomal subunit . - Initiator tRNA (carrying methionine) binds to the start codon (AUG) . - Large ribosomal subunit joins → functional ribosome formed. 2. Elongation - Ribosome reads mRNA one codon at a time . - tRNA delivers correct amino acids based on codon-anticodon matching. - Peptide bonds form between amino acids. - Ribosome shifts (translocates) along the mRNA. 3. Termination - Ribosome reaches a stop codon (UAA, UAG, UGA). - Release factors bind → polypeptide chain is released. - Ribosomal subunits dissociate. --- Role of tRNA - tRNA molecules bring amino acids to ribosome. - Each tRNA has: Anticodon loop (binds mRNA codon). - Amino acid attachment at 3′ end. --- Location - Prokaryotes : Cytoplasm (free ribosomes). - Eukaryotes : Cytoplasm and rough endoplasmic reticulum (RER). --- Summary Line: Translation = mRNA is decoded by ribosomes → tRNA delivers amino acids → polypeptide chain forms. --- Explanation: - Albinism is caused by a recessive allele ("a"). - Normal skin color is controlled by the dominant allele ("A"). - For two normal-skinned parents to have an albino child:Both parents must be heterozygous (Aa). - They carry one normal ("A") and one albino ("a") gene. - Their genotypes are Aa × Aa . - The albino child’s genotype must be aa (homozygous recessive). --- Punnett Square: --- Probability of Albino Child: - From the Punnett square:1/4 (25%) chance of aa (albino) per pregnancy. Thus, the probability that the fifth child will be albino = 25% or 1/4 . Key Points: - Each organism carries two alleles for each gene (one from each parent). - During gamete formation (meiosis), these two alleles separate (segregate) . - Each gamete receives only one allele for each gene. - Fertilization restores the pair of alleles (one from each parent). - Segregation is random — either allele is equally likely to go into any gamete. - Explains the 3:1 ratio seen in monohybrid crosses (dominant to recessive traits). --- Example: - A pea plant with genotype Tt (tall) will produce gametes:50% with T (tall allele). - 50% with t (short allele). --- Summary Line for Quick Memory: In Mendel’s Principle of Segregation, alleles separate during gamete formation , ensuring each gamete carries only one allele for each trait. Development of a Live-Attenuated Vaccine: - Isolate the Pathogen : Obtain the virus or bacterium causing the disease. - Attenuation : Weaken the pathogen through culturing in non-human cells or genetic modification to reduce virulence. - Safety Testing : Test in animals to ensure no disease is caused but immunity is stimulated. - Immune Response : The weakened pathogen stimulates the immune system to produce antibodies and memory cells. - Production : Grow the pathogen in large quantities, purify, and prepare the vaccine. - Clinical Trials : Conduct phased human trials for safety, efficacy, and optimal dosing. - Approval : Regulatory bodies approve the vaccine after successful trials. - Examples : MMR, Yellow Fever, Polio (oral). Summary : Live-attenuated vaccines are made by weakening a pathogen to stimulate immunity without causing disease. Mechanism Underlying the Immune Protective Ability of Live-Attenuated Vac