Molecular Biology,Molecular Genetics & Microbial-Biochemistry LAQs & SAQs

**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** an