Topic #4

DNA Structure, Replication and the Central Dogma: In-Depth Notes

Structure of DNA

  • DNA Building Blocks: Nucleotides

  • Composed of three parts:

    • Sugar: Deoxyribose (in DNA)

    • Phosphate Group

    • Nitrogenous Base

DNA Structure Features

  • Double Helix

  • The characteristic shape of DNA, resembling a twisted ladder.

  • Phosphodiester Bonds

  • Connects nucleotides in DNA, linking the sugar of one nucleotide to the phosphate of the next.

  • Grooves

  • Major and minor grooves serve as binding sites for proteins involved in processes like transcription and replication.

Storing Information in DNA

  1. Copying DNA During Cell Division

  • Errors must be minimized to ensure each daughter cell inherits complete genetic information.

  1. Translation of Genetic Information

  • Cells must access DNA instructions to synthesize specific proteins accurately, ensuring proteins are correctly made at the right times and locations.

Genetic Information

  • The sequence of nucleotide bases (A, T, C, G) encodes instructions for life.

  • Complementary Base Pairing:

  • A pairs with T, C pairs with G; hydrogen bonding stabilizes the helix.

DNA to RNA

  1. Genes

  • Segments of DNA that contain instructions for functional products, primarily proteins.

  • Some genes code for RNA molecules like tRNA and rRNA, which are essential for protein synthesis.

  1. Transcription

  • The process where genes are copied into messenger RNA (mRNA).

Chromosomes

  • Prokaryotes: Generally have a single circular chromosome in the nucleoid.

  • Eukaryotes: Have multiple linear chromosomes contained in a nucleus. DNA is wrapped around histones to form chromatin that compacts into chromosomes.

History of DNA

  • Friedrich Miescher: First to isolate DNA; called it nuclein.

  • Frederick Griffith: Demonstrated DNA as a transforming principle in bacteria.

  • Avery, MacLeod, McCarty: Proved DNA is necessary for bacterial transformation.

  • Hershey-Chase Experiment: Established DNA as the genetic material using bacteriophage T2.

  • Chargaff: Found ratios A=T and C=G, species-specific base composition.

  • Rosalind Franklin: Used x-ray crystallography to reveal DNA's double helix structure.

DNA Replication Models

  • Three Models:

  • Conservative

  • Dispersive

  • Semi-conservative (preferred): Each original strand serves as a template for a new strand.

Key Enzymes in DNA Replication

  • Topoisomerase: Relaxes super-coiled DNA.

  • DNA Helicase: Unwinds the double helix.

  • Primase: Synthesizes RNA primers to start replication.

  • DNA Polymerase: Synthesizes new DNA strands and proofreads for accuracy.

  • DNA Ligase: Joins Okazaki fragments on the lagging strand.

DNA Proofreading and Repair

  • DNA Polymerase: Corrects errors during replication.

  • Mismatch Repair: Detects and fixes bases added incorrectly.

  • Nucleotide Excision Repair: Fixes thymine dimers caused by UV exposure.

Telomeres

  • Located at chromosome ends; protect the chromosome.

  • Telomerase: Enzyme that extends telomeres to prevent loss during replication, potentially related to aging.

DNA Mutations

  • Changes in DNA sequence caused by copying errors or environmental factors (e.g., UV light).

  • Types of mutations: Substitution, Deletion, Insertion, Translocation.

  • Mutations can be hereditary (in every cell) or acquired (in specific cells).

Mosaicism

  • Somatic mutations occurring early in development can result in mosaic patterns in tissue, affecting cell groups differently.

Central Dogma of Molecular Biology

  • Information flow: DNA → RNA → Protein

  1. Transcription: DNA is transcribed into mRNA in the nucleus.

  2. Translation: mRNA is translated into a polypeptide (protein) in the cytoplasm.

Gene Expression Regulation

  • Five levels:

  • Pretranscriptional (Nucleus)

  • Transcriptional (Nucleus)

  • Posttranscriptional (Nucleus)

  • Translational (Cytoplasm)

  • Posttranslational (Cytoplasm)

Summary Questions

  • Review topics such as how DNA stores genetic information, the role of base pairing in replication, the types and impacts of mutations, and the steps of transcription and translation.