DNA Structure & Replication - Vocabulary Flashcards

Topic 1: Proving DNA is the Genetic Material

• Historical Context

  • Early 1900s: Scientists sought to identify the molecule containing genes for inheritance.
  • Gregor Mendel: Found that "heritable factors" were passed from parents to offspring (mid-1800s).
  • Thomas Hunt Morgan (1910): Showed that genes are carried on chromosomes, narrowing potential genetic materials to proteins and DNA.

• Griffith’s Experiment (1928)

  • Frederick Griffith worked with pneumonia-causing bacteria, using pathogenic (S strain) and nonpathogenic (R strain) variants.
  • Key Finding: Heat-killed pathogenic bacteria mixed with live nonpathogenic bacteria transformed the nonpathogenic bacteria into pathogenic ones.
  • Transfiguration Definition: Change in genotype and phenotype due to assimilation of foreign DNA.

• Avery-McCarty-Macleod Experiment (1944)

  • Identified DNA as the transforming principle from Griffith’s experiment.
  • Experiment involved degradation of components in heat-killed S cells to determine which allowed R strain transformation.
  • Controversy: Skepticism from the scientific community; many believed proteins were the genetic material.

• Hershey-Chase Experiment (1952)

  • Researchers used bacteriophage T2 to show that DNA, not protein, is the genetic material.
  • Protocol included radioactively labeling DNA and proteins to track their transfer into bacteria during infection.

Topic 2: Structure of DNA

• Discovery of DNA's Structure

  • By the 1950s, DNA identified as a polymer of nucleotides but its specific shape was unknown.
  • Composed of three parts: phosphate group, deoxyribose sugar, and nitrogenous base (adenine, thymine, cytosine, guanine).

• Base Pairing and Helical Structure

  • Chargaff’s Rule: Adenine pairs with thymine (2 hydrogen bonds) and guanine pairs with cytosine (3 hydrogen bonds).
  • Rosalind Franklin: Used X-ray crystallography to provide evidence of helical structure; two sugar-phosphate backbones with nitrogenous bases inside.
  • Watson and Crick: Developed the double helix model using Franklin’s data.

• Polynucleotide Formation

  • Nucleotides linked by phosphodiester bonds between the 3' hydroxyl group of one nucleotide and the 5' phosphate of another, forming a strand with 3' and 5' ends.

Topic 3: DNA Replication

• Principle of Semiconservative Replication

  • Each strand of DNA serves as a template for the synthesis of a new complementary strand.
  • Meselson-Stahl Experiment: Used isotopes of nitrogen to confirm the semiconservative replication model in bacteria.

• Key Enzymes in DNA Replication

  • Helicase: Unwinds DNA strands at replication forks.
  • DNA Polymerases: Synthesizes new DNA strands. Can only add nucleotides to the 3' end of a growing strand.
  • Primase: Synthesizes RNA primers needed for DNA polymerase to initiate synthesis.
  • Ligase: Joins Okazaki fragments on the lagging strand.

• Leading vs Lagging Strands

  • Leading strand synthesized continuously while lagging strand synthesized in segments called Okazaki fragments.
  • Synthesis direction (5' to 3') results in opposite copying directions for the two strands.

Topic 4: Structure and Function of RNA

• Differences between RNA and DNA

  • RNA is single-stranded; DNA is double-stranded; RNA contains ribose, DNA contains deoxyribose.
  • Nitrogenous bases: RNA has uracil instead of thymine.

• RNA Types and Functions

  • mRNA: Carries genetic information from DNA to ribosomes. Codons determine amino acid sequences.
  • tRNA: Transfers amino acids to ribosomes during protein synthesis. Contains an anticodon for translation.
  • rRNA: Combines with proteins to form ribosomes, the site of protein synthesis.
  • Other Types: miRNA regulates gene expression; ribozymes catalyze specific reactions.