Term 3 Review: DNA Replication, Transcription & Translation

Term 3 Review: Part 1 **1. Central Dogma Overview** - Describes the flow of genetic information:\ **DNA → RNA → Protein** - Includes three main processes: **Replication, Transcription, and Translation** **2. DNA Replication (Occurs in the Nucleus)** **Purpose:** - Produces an exa...

Term 3 Review: Part 1 **1. Central Dogma Overview** - Describes the flow of genetic information:\ **DNA → RNA → Protein** - Includes three main processes: **Replication, Transcription, and Translation** **2. DNA Replication (Occurs in the Nucleus)** **Purpose:** - Produces an exact copy of DNA before cell division. **Key Enzymes & Proteins:** - **Helicase** -- Unzips the DNA strands by breaking hydrogen bonds. - **Topoisomerase** -- Relieves strain ahead of the replication fork. - **Single-Strand Binding Proteins (SSBs)** -- Prevent DNA strands from reannealing. - **Primase** -- Adds RNA primers to initiate replication. - **DNA Polymerase** -- Synthesizes new DNA strands (adds nucleotides in the **5\' to 3\'** direction). - **Ligase** -- Seals gaps in the lagging strand (Okazaki fragments). **Key Features:** - **Leading strand** -- Synthesized continuously. - **Lagging strand** -- Synthesized discontinuously (Okazaki fragments). - **Semi-conservative replication** -- Each new DNA molecule has one old and one new strand. **3. Transcription (DNA → mRNA, Occurs in the Nucleus)** **Purpose:** - Converts DNA into messenger RNA (mRNA). **Key Enzymes & Steps:** 1. **Initiation:** RNA polymerase binds to the **promoter** region (e.g., TATA box). 2. **Elongation:** RNA polymerase synthesizes pre-mRNA in the **5\' to 3\'** direction. 3. **Termination:** Transcription stops at the terminator sequence. **Post-Transcriptional Modifications (in Eukaryotes):** - **5\' Cap (Modified Guanine)** -- Protects mRNA from degradation. - **Poly-A Tail (3\' End)** -- Stabilizes mRNA for transport. - **Splicing (by Spliceosome)** -- Removes **introns** (non-coding regions) and joins **exons** (coding regions). **4. Translation (mRNA → Protein, Occurs in the Ribosome)** **Purpose:** - Converts mRNA into a polypeptide (protein). **Key Components:** - **mRNA** -- Carries codons (3-base sequences). - **tRNA** -- Carries amino acids and has anticodons that match mRNA codons. - **Ribosome (rRNA + Proteins)** -- Site of protein synthesis. **Stages of Translation:** 1. **Initiation:** - Ribosome assembles around mRNA. - Start codon **AUG** (Methionine) is recognized. 2. **Elongation:** - tRNA brings amino acids. - Peptide bonds form between amino acids. 3. **Termination:** - Stop codon (UAA, UAG, UGA) is reached. - Release factors free the polypeptide. **Post-Translational Modifications:** - Folding (by chaperone proteins). - Cleavage of signal peptides. - Addition of functional groups (phosphorylation, glycosylation). **5. Genetic Code Features** - **Triplet Code** -- Three nucleotide bases (codon) = one amino acid. - **Redundant/Degenerate** -- Multiple codons can code for the same amino acid. - **Universal** -- Used by almost all living organisms. - **Start Codon** -- **AUG (Methionine)**. - **Stop Codons** -- **UAA, UAG, UGA** (No amino acid added). **6. Mutations and Their Effects** **Types of Mutations:** - **Point Mutation** (Single nucleotide change) - **Silent** -- No change in amino acid. - **Missense** -- Changes one amino acid. - **Nonsense** -- Creates a stop codon (shortens protein). - **Frameshift Mutation** (Insertion/Deletion) -- Alters reading frame, leading to drastic changes in the protein. ### **1. Genes & Alleles** - **Gene** -- A segment of DNA that codes for a protein or trait. - **Allele** -- Different versions of a gene (e.g., dominant **A** or recessive **a**). - **Locus** -- The specific location of a gene on a chromosome. - **Homozygous** -- Two identical alleles (**AA** or **aa**). - **Heterozygous** -- Two different alleles (**Aa**). ### **2. Genetic Inheritance** - **Genotype** -- Genetic makeup (e.g., AA, Aa, or aa). - **Phenotype** -- Physical expression of a trait (e.g., blue eyes, tall height). - **Dominant Allele** -- Expressed even if only one copy is present (**A**). - **Recessive Allele** -- Expressed only if two copies are present (**aa**). ### **3. Mendelian Inheritance (Gregor Mendel's Laws)** 🔹 **Law of Segregation** - Each individual has **two alleles** for a trait, but only one is passed to offspring. - Occurs during **meiosis (gamete formation)**. 🔹 **Law of Independent Assortment** - Genes for different traits are inherited **independently** if they are on different chromosomes. 🔹 **Law of Dominance** - If an individual has a **dominant allele**, it will be expressed over the recessive allele. ### **4. Types of Genetic Crosses** **Cross Type** **Genotypic Ratio** **Phenotypic Ratio** ---------------------------------- ----------------------------------------- -------------------------- **Monohybrid Cross** (Aa × Aa) 1 AA : 2 Aa : 1 aa 3 Dominant : 1 Recessive **Dihybrid Cross** (AaBb × AaBb) 9:3:3:1 (if genes assort independently) 9:3:3:1 ### **5. Types of Non-Mendelian Inheritance** - **Incomplete Dominance** -- Blending of traits (e.g., red × white → pink). - **Codominance** -- Both alleles are expressed equally (e.g., blood type AB). - **Multiple Alleles** -- More than two alleles exist (e.g., ABO blood type). - **Polygenic Inheritance** -- Traits controlled by multiple genes (e.g., height, skin color). - **Sex-Linked Traits** -- Genes located on **X or Y chromosomes** (e.g., hemophilia, color blindness).