DNA: Chemical Nature of Genetic Material

  • Structure of DNA
    • Described as a long, twisty, rope-ladder-like structure.
    • Historical Context:
    • In 1953, the structure of the DNA molecule was first described by James Watson and Francis Crick.

Requirements of Genetic Material

  1. Complex Information: Must contain the information necessary to create an organism.
  2. Faithful Replication: Must be capable of replicating accurately during cell division.
  3. Encodes Phenotype: Must encode the traits or phenotype of an organism.
  4. Capacity to Vary: Must have some level of variability to allow for evolution and adaptation.

Historical Understanding of DNA

  • Early Findings:
    • Identification of the nucleus and the acidic substances within it (nucleic acids).
    • Eventually identified as deoxyribonucleic acid (DNA).
    • Observations of chromosomes were made.

Debates Surrounding Inheritable Material

  • Before the importance of DNA was established, there was a debate regarding whether DNA or proteins served as inheritable materials.

Identification of the Genetic Material

Griffith's Experiment (1928)

  • Objective: Can extracts from dead bacterial cells genetically transform living cells?
  • Methods:
    1. Inject type IIIS (virulent) bacteria into a mouse.
    2. Inject type IIR (nonvirulent) bacteria into a mouse.
    3. Inject heat-killed type IIIS bacteria into a mouse.
    4. Inject a mixture of type IIR and heat-killed type IIIS bacteria into a mouse.
  • Results:
    • Mouse injected with heated type IIIS bacteria lives.
    • Mouse injected with type IIR bacteria lives.
    • Mouse injected with the mixture dies, and type IIIS bacteria are recovered.
  • Conclusion: A substance from heat-killed virulent bacteria genetically transformed nonvirulent bacteria into live, virulent type IIIS bacteria.

Avery, MacLeod, and McCarty's Experiment (1944)

  • Objective: What is the chemical nature of the transforming substance?
  • Methods:
    1. Kill virulent type IIIS bacteria by heat, homogenize and filter to obtain a filtrate.
    2. Treat samples with enzymes that destroy proteins (Protease), RNA (RNase), or DNA (DNase).
    3. Add treated samples to cultures of type IIR bacteria.
  • Results:
    • Cultures treated with Protease or RNase show transformed type IIIS bacteria.
    • Culture treated with DNase did not show transformation.
  • Conclusion: The transforming substance is DNA, as only DNase destroyed it.

Hershey-Chase Experiment (1952)

  • Objective: Which part of the phage serves as genetic material?
  • Methods:
    1. Infect E. coli with T2 phage grown in a medium containing 35S (sulfur, marking proteins).
    2. Infect another E. coli with T2 phage grown in a medium containing 32P (phosphorus, marking DNA).
  • Results:
    • After centrifugation, 35S was found only in the remaining protein, indicating protein was not transmitted to progeny.
    • 32P was found in the bacteria, indicating DNA was transmitted.
  • Conclusion: DNA, not protein, is the genetic material in bacteriophages.

Composition and Structure of Nucleic Acids

Nucleotides

  • Components:
    • Composed of three parts:
    • Phosphate group
    • Sugar (deoxyribose in DNA and ribose in RNA)
    • Nitrogenous base

Sugar Structure

  • Difference Between DNA and RNA:
    • DNA (Deoxyribonucleic Acid): Lacks a hydroxyl group on the 2'-carbon atom of its sugar.
    • RNA (Ribonucleic Acid): Contains a hydroxyl group on the 2'-carbon atom of its sugar.

Nitrogenous Bases

  • DNA Bases:
    • Adenine (A)
    • Thymine (T)
    • Guanine (G)
    • Cytosine (C)
    • Base Pairing: A ↔ T, C ↔ G.
  • RNA Bases:
    • Adenine (A)
    • Guanine (G)
    • Cytosine (C)
    • Uracil (U) replaces Thymine.

Base Grouping

  • Purines (two-ring structure):
    • Adenine (A)
    • Guanine (G)
  • Pyrimidines (single-ring structure):
    • Cytosine (C)
    • Thymine (T) (DNA)
    • Uracil (U) (RNA)

Phosphate Group

  • Phosphate is linked to the sugar of nucleotides; it is critical for the formation of nucleic acids.

Polynucleotide Strand

  • Composed of nucleotides linked by phosphodiester bonds.
  • Phosphodiester Bond: Strong covalent bond that joins the 5'-phosphate group of one nucleotide to the 3'-hydroxyl group of the next.

DNA's Secondary Structure

Double Helix

  • Nature of DNA:
    • DNA exists as a double-stranded helix, consisting of two complementary antiparallel strands wound around each other.
    • Base Pairing: A-T pairs have two hydrogen bonds; C-G pairs have three hydrogen bonds.
    • Antiparallel Nature: Strands run in opposite directions.
  • Major Grooves and Minor Grooves:
    • Major groove allows access to proteins that regulate DNA.
    • Minor groove is less accessible.

Chargaff's Rules

  • Developed by Erwin Chargaff, observing ratios of bases in DNA.
    • States that the total amount of adenine equals thymine, and guanine equals cytosine.

Differences Between DNA and RNA

  • DNA:
    1. Uses thymine.
    2. Usually double-stranded.
    3. Contains deoxyribose.
    4. Has a higher fidelity for encoding a stable genetic material.
  • RNA:
    1. Uses uracil.
    2. Usually single-stranded.
    3. Contains ribose.

Secondary Structures of RNA

Hairpin Structure

  • Formed when sequences of nucleotides on the same polynucleotide strand are complementary and can pair with each other, creating loops and stems.

Storage and Packaging of DNA

Compaction of DNA

  • Human cells comprise more than 6 billion base pairs of DNA, which would measure over 2 meters when stretched.
  • DNA must be tightly packed to fit within the nucleus.

Chromatin

  • Chromatin Definition: Complex of DNA and proteins (histones) that packages DNA into a smaller volume.
  • Structure: DNA wraps around histone proteins to form nucleosomes, reducing accessibility for enzymes.
  • Nucleosome: Basic unit of DNA packaging, consisting of DNA wrapped around a core of histone proteins.

Types of Chromatin

  • Euchromatin: Less condensed, active in transcription, present on chromosome arms.
  • Heterochromatin: More condensed, remains in a compacted state throughout the cell cycle, located at centromeres and telomeres.

Epigenetics

  • Definition: Stable alterations of chromatin structure that can be inherited.
  • Processes include methylation, phosphorylation, and acetylation, all changing chromatin structures.

Centromeres and Telomeres

  • Centromeres: Contains repetitive DNA; replaced with centromeric histone proteins during cell division.
  • Telomeres: Composed of short repeated sequences protecting chromosome ends from degradation, involving a multiprotein complex called shelterin.

Summary of Fundamental Concepts

  • DNA functions as the inheritable genetic material.
  • Nucleotides form the building blocks of DNA and RNA.
  • Primary differences between DNA and RNA lie in their bases, sugars, and structural forms.
  • Chromatin structure is essential for DNA protection and regulation of gene expression, with modifications playing a significant role in epigenetics.
  • The unique structure of centromeres and telomeres is crucial in chromosome stability and cellular division.