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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