DNA Replication and Structure Biology PDF

### **Griffith, Avery et al., and Hershey--Chase Experiments** - **Griffith (1928): Showed that a \"transforming principle\" could transfer genetic information between bacteria. Harmless bacteria became deadly when mixed with heat-killed deadly bacteria.** - **Avery et al. (1944): Prov...

### **Griffith, Avery et al., and Hershey--Chase Experiments** - **Griffith (1928): Showed that a \"transforming principle\" could transfer genetic information between bacteria. Harmless bacteria became deadly when mixed with heat-killed deadly bacteria.** - **Avery et al. (1944): Proved the \"transforming principle\" is DNA by isolating DNA from bacteria and showing it caused the transformation.** - **Hershey--Chase (1952): Confirmed DNA is the genetic material. Used viruses (phages) labeled with radioactive DNA or proteins to show only DNA entered bacteria during infection.** ### **Discovering DNA's Structure** - **Franklin & Gosling: Used X-ray crystallography to photograph DNA (Photo 51), revealing it is a helix.** - **Watson & Crick: Used Franklin\'s data to build the first accurate model of DNA, showing it as a double helix with complementary base pairing.** ### **Overall Structure of DNA** - **Components of a nucleotide:** - **A phosphate group (attached to the 5' carbon of sugar)** - **A deoxyribose sugar (5-carbon sugar)** - **A nitrogenous base (A, T, G, or C attached to the 1' carbon)** - **Antiparallel Strands: One strand runs 5' to 3', the other 3' to 5'.** - **Complementary Base Pairing:** - **Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)** - **Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)** - **Chargaff's Rule: %A = %T, %G = %C** ### **DNA Replication** - **Definition: Process of copying DNA. Each strand serves as a template to create a new complementary strand.** - **Semiconservative: Each new DNA molecule has one old strand and one new strand (proved by the Meselson--Stahl experiment using isotopes of nitrogen).** ### **DNA Polymerase III's Limitations** 1. **Cannot start replication on its own: Needs an RNA primer to begin.** 2. **Works only in the 5'-to-3' direction: Adds nucleotides to the 3' end of the growing strand.** ### **Key Terms in DNA Replication** - **Template Strand: The original strand used to make a complementary strand.** - **Leading Strand: Synthesized continuously in the 5'-to-3' direction.** - **Lagging Strand: Synthesized in fragments (Okazaki fragments) in the 5'-to-3' direction.** ### **Major Steps in DNA Replication** 1. **Helicase: Unwinds the DNA helix.** 2. **SSBPs: Keep the strands separated.** 3. **Primase: Adds RNA primers.** 4. **DNA Polymerase III: Synthesizes the new DNA strand (5' to 3').** 5. **DNA Polymerase I: Replaces RNA primers with DNA.** 6. **Ligase: Joins Okazaki fragments on the lagging strand.** ### **Importance of Telomerase** - **Telomerase prevents shortening of chromosomes by extending the telomeres (ends of chromosomes), especially in rapidly dividing cells.** ### **3 Types of DNA Repair Mechanisms** 1. **Mismatch Repair: Fixes errors made during replication.** 2. **Base Excision Repair: Removes and replaces damaged bases.** 3. **Nucleotide Excision Repair: Fixes bulky damage like thymine dimers caused by UV light.** ### **tRNA Charging** - **How: Aminoacyl-tRNA synthetase enzymes attach the correct amino acid to the tRNA's 3' end.** ### **Steps of Translation** 1. **Initiation:** - **The small ribosomal subunit binds mRNA.** - **The first tRNA (with methionine) binds to the start codon (AUG).** - **The large subunit attaches.** 2. **Elongation:** - **The ribosome moves along mRNA, matching tRNAs to codons.** - **Amino acids are joined by peptide bonds.** 3. **Termination:** - **A stop codon (UAA, UAG, or UGA) is reached.** - **Release factors disassemble the ribosome, releasing the polypeptide.**