DNA Structure, Replication, and Protein Synthesis

Structure of DNA

• Components of DNA:

  • Sugar
  • Phosphate group
  • Nitrogen-containing base

• Nitrogen Bases:

  • Four types: adenine (A), thymine (T), cytosine (C), guanine (G)
  • Complementary base pairing:
  • A pairs with T
  • C pairs with G

• Historical Context:

  • Discoverers: James Watson and Francis Crick
  • They described complementary base pairing.

Gene Theory

• Defines genetic information:

  • Coded information in the sequence of nitrogen bases
  • This dictates form, function, and behavior of organisms

• DNA Code:

  • Sequence of bases contains the information to make proteins
  • Example: Humans have approximately 3.2 billion bases in their chromosomes

DNA Replication

• Process Overview:

  • Occurs during stage S of the cell cycle
  • DNA strands separate due to hydrogen bonds breaking
  • Enzymes involved:
  • DNA Polymerase:
    • Uses old strands as templates
    • Lays down nucleotides one at a time based on complementary base pairing

• Outcome:

  • Two identical double-stranded DNA molecules
  • Form sister chromatids that separate during anaphase of mitosis

• Key Concept:

  • Importance of complementary base pairing in DNA replication

From DNA to Protein

• Initiating Protein Synthesis:

  • DNA stored in the nucleus contains codes for proteins
  • Transcription:
  • Process of making messenger RNA (mRNA) from DNA
  • RNA polymerase adds RNA nucleotides based on DNA sequence
  • U replaces T in RNA

• Definition of a Gene:

  • Segment of DNA that codes for a protein

• mRNA Function:

  • Carries the genetic code from the nucleus to ribosomes for protein synthesis
  • Codons (three bases on mRNA) represent specific amino acids

Translation

• Process:

  • Ribosomes read mRNA codons and assemble corresponding amino acids into proteins
  • Start Codon:
  • AUG (codes for methionine)
  • Stop Codons:
  • UAA, UGA, UAG

• Mutations:

  • Changes in the DNA base sequence that may affect protein synthesis
  • Not all mutations result in changes to the amino acid sequence (silent mutations)
  • Impact of mutations varies depending on the proteins affected

Recap & Next Steps

• Key Takeaways:
  • Complementary base pairing is crucial in DNA replication and transcription
  • The flow of genetic information: DNA ➜ mRNA ➜ Protein
  • The complexity of gene expression and regulation will be discussed further in chapter 13
• Next Discussion:
  • Further exploration of translation and protein synthesis
  • Relationship between mutations and protein functionality