Molecular Biology: DNA and Experiments

Griffith's Experiment

• Griffith discovered a transforming substance from dead virulent bacterial cells that could transform non-virulent cells into pathogenic cells.

Hershey-Chase Experiment

• Hershey and Chase, using bacteriophages, determined that the genetic material of the virus is DNA.

Watson and Crick's Model

• Watson and Crick proposed the double helix model of DNA.
• Key components include a sugar-phosphate backbone (the "handrails"), phosphate bridges, and hydrogen bonds between nitrogenous bases.

DNA Structure

• DNA comprises a phosphate group, a 5-carbon sugar, and nitrogenous bases.
• Two types of bases:
  • Purines (double ring): Adenine (A) and Guanine (G)
  • Pyrimidines (single ring): Thymine (T) and Cytosine (C)
  • Base pairing rules: A=T (2 hydrogen bonds), G=C (3 hydrogen bonds - requiring more energy to break)

DNA Replication

• DNA replication is the process of creating two identical DNA molecules from one.
• Occurs during interphase.
• Three models of replication:
  • Conservative: One molecule with original strands, one with new strands.
  • Semi-conservative: Each strand serves as a template for a new strand, resulting in two hybrid molecules.
  • Dispersive: Each strand is a mix of old and new DNA, resulting in hybrid molecules.
• The model accepted is Semi-conservative.

DNA Replication Phases

• Initiation:
  • Begins at the origin of replication.
  • Helicase unwinds the DNA by breaking hydrogen bonds.
  • Single-strand binding proteins prevent the separated strands from re-joining.
  • Topoisomerase relieves the strain on the DNA caused by unwinding.
• Elongation:
  • Primase creates RNA primers, which are used by DNA polymerase (the main copying enzyme) to start building new complementary strands.
  • DNA polymerase III adds nucleotides to create complementary strands.
  • Leading strand is built continuously towards the replication fork.
  • Lagging strand is built in short segments (Okazaki fragments) away from the replication fork.
• Termination:
  • Occurs when replication of new strands is complete.

Central Dogma

• Information flow: DNA (transcription) → mRNA (translation) → protein.

Gene Expression (Transcription and Translation)

• Transcription: DNA sequence serves as a template for RNA synthesis in the nucleus.
  • Steps:
    • Initiation: RNA polymerase binds to the promoter region (before the gene), unwinding DNA.
    • Elongation: RNA polymerase moves along the template strand, synthesizing mRNA.
    • Termination: RNA polymerase reaches termination sequence, detaching from DNA.
• Translation: mRNA sequence serves as a template for protein synthesis in the cytoplasm.
  • Components:
    • mRNA (messenger RNA): carries genetic code.
    • tRNA (transfer RNA): carries amino acids to the ribosome.
    • Ribosomes: site of protein synthesis.
  • Codons: three-nucleotide sequences on mRNA that code for amino acids or signals.
  • Start codon: AUG (methionine)
  • Stop codons: signal translation termination.
  • Redundancy: Multiple codons can code for the same amino acid.
  • Introns (non-coding) and exons (coding) are involved
• Ribosomes have three sites (A, P, E) for tRNA binding.
• Anticodons (on tRNA) are complementary to mRNA codons.

Transcription Stages:

• Initiation: RNA polymerase binds to the promoter region.
• Elongation: RNA polymerase synthesizes mRNA.
• Termination: RNA polymerase reaches a termination signal.

Translation Components and Function

• mRNA: carries genetic code
• tRNA: carries amino acids; anticodon base pairs with codon.
• Ribosomes: site of protein synthesis

Wobble Hypothesis

• The 3rd base (wobble position) of the codon is less critical in determining the amino acid.

Central Dogma Summary

• DNA carries genetic information
• Replication copies the DNA
• Transcription produces mRNA from DNA
• Translation uses mRNA to synthesize proteins