The Structure of DNA

• Contributions to Discovery:

  • James D. Watson & Francis Crick: Developed the first correct model of DNA structure using X-ray data.
  • Rosalind Franklin & Maurice Wilkins: Provided critical X-ray crystallography images of DNA which revealed key structural features.

• Structure of DNA:

  • Monomer: DNA nucleotide composed of:
  • Phosphate group
  • Nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
  • 09Deoxyribose (5-carbon sugar)
  • Polymer: DNA consisting of multiple nucleotides.
  • Double Helix Structure:
  • Two strands of nucleotides twisted around each other.
  • Sugar-phosphate backbone of each strand is on the outside with nitrogenous bases paired inside.
  • Base Pairs: A pairs with T and G pairs with C, held together by hydrogen bonds.

Arrangement of DNA in Cells

• Prokaryotic DNA:

  • Single, circular chromosome arranged in loops supercoiled at the center of the bacterial cell.

• Eukaryotic DNA:

  • Multiple linear chromosomes that are tightly packaged with proteins (histones) to form nucleosomes.
  • Nucleosomes coil to form higher-order structures and supercoils for organization.

DNA Replication

• Semiconservative Model:

  • Each new DNA double helix consists of one old strand and one new strand.
  • Complementary strands: A=T, G=C. The double helix unwinds during replication, allowing each strand to act as a template.

• Key Players in DNA Replication:

  • Helicase: Unwinds the DNA double helix at the origin of replication.
  • DNA Polymerase: Adds nucleotides to the growing strand using RNA primers.
  • DNA Ligase: Connects DNA fragments by sealing gaps.

• Replicative Process:

  1. Helicase unwinds the double helix.
  2. Primase synthesizes short RNA primers.
  3. DNA Polymerase elongates the new DNA strand.
  4. Primers are replaced, and strands are sealed by DNA ligase.

• DNA Repair Mechanisms:

  • Proofreading: DNA polymerase checks and corrects errors during replication.
  • Mismatch Repair: Recognizes and fixes incorrect base pairings.
  • Nucleotide Excision Repair: Removes damaged DNA nucleotides and replaces them with the correct bases using the template strand.

Transcription

• Central Dogma:

  • The flow of genetic information: DNA → mRNA → Protein.
  • Genes are transcribed into mRNA, which is then translated into proteins.

• Transcription Stages:

  1. Initiation: RNA polymerase binds to the promoter and unwinds DNA.
  2. Elongation: RNA polymerase synthesizes mRNA by adding complementary nucleotides to the growing chain.
  3. Termination: RNA polymerase reaches the terminator region, and mRNA is released.

• Eukaryotic RNA Processing:

  • Capping: A 5' cap is added for protection.
  • Splicing: Introns are removed and exons joined together.
  • Polyadenylation: A poly-A tail is added to the 3' end to enhance stability and exit from the nucleus.

Translation

• Genetic Code & tRNA:

  • Each mRNA codon corresponds to a specific amino acid.
  • tRNA: Transfers the correct amino acids to the ribosome; has an anticodon that pairs with the mRNA codon.
  • Start Codon: AUG signals the beginning of translation. Stop Codons: UAA, UAG signal termination of translation.

• Steps of Translation:

  1. Initiation: Ribosome assembles at the start codon; tRNA brings the first amino acid.
  2. Elongation: Ribosome moves along the mRNA, and amino acids are linked by peptide bonds.
  3. Termination: Stop codon is reached; the polypeptide chain is released, and the ribosome disassembles.

Summary of Key Concepts

• Structure of DNA: Double-helix with complementary base pairing.
• DNA Replication: Semiconservative, yielding two identical helices from one original DNA strand.
• Central Dogma: Information flows from DNA to RNA to proteins, with processes for transcription and translation in eukaryotes and prokaryotes.