13.1 Discovery of DNA Structure and Function

The discovery of DNA's structure and function revolutionized our understanding of genetics. Scientists like Griffith, Avery, and Hershey-Chase proved DNA was the genetic material. Their work laid the foundation for Watson and Crick's groundbreaking double helix model.

The double helix structure explained how genetic information is stored, replicated, and passed on. This discovery sparked the DNA revolution, leading to advances in molecular biology and biotechnology that continue to shape our world today.

DNA as Genetic Material

Experiments Demonstrating DNA as Genetic Material

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• In the 1920s, Frederick Griffith's experiments with Streptococcus pneumoniae demonstrated the existence of a "transforming principle" that could convert harmless bacteria into virulent strains
  • Suggested that genetic information could be transferred between organisms
• Avery, MacLeod, and McCarty's experiments in the 1940s identified DNA as the "transforming principle" responsible for the changes observed in Griffith's experiments
  • Provided the first evidence that DNA carries genetic information
• Hershey and Chase's experiments in 1952 used radioactive labeling to show that DNA, not protein, enters bacterial cells during viral infection
  • Confirmed that DNA is the genetic material

Chargaff's Rules and DNA Composition

• Chargaff's rules, developed in the late 1940s, stated that in DNA:
  • The amount of adenine (A) always equals the amount of thymine (T)
  • The amount of guanine (G) always equals the amount of cytosine (C)
• Provided key insights into the structure of DNA
  • Suggested a complementary relationship between the nitrogenous bases
  • Hinted at the potential for base pairing in the DNA structure

DNA's Double Helix Structure

Composition and Arrangement of DNA

• DNA is composed of two antiparallel polynucleotide strands that wind around each other to form a right-handed double helix
• The sugar-phosphate backbones of the two strands are on the outside of the helix, while the nitrogenous bases face the interior
• The nitrogenous bases form complementary base pairs through hydrogen bonds:
  • Adenine (A) pairs with thymine (T)
  • Guanine (G) pairs with cytosine (C)

Significance of the Double Helix Structure

• The double helix structure provides a mechanism for the precise replication of genetic information during cell division
  • Each strand can serve as a template for the synthesis of a new complementary strand
• The complementary base pairing in the double helix allows for the faithful transmission of genetic information from one generation to the next
  • Ensures that genetic information is accurately copied and passed on to daughter cells
• The structure also helps protect the genetic information from damage
  • The interior location of the nitrogenous bases shields them from potential chemical or physical damage

DNA's Role in Genetics

DNA as a Blueprint for Life

• DNA serves as the blueprint for an organism's structure, function, and development
  • Encodes the instructions for the synthesis of proteins, which carry out most cellular functions
• The genetic information in DNA is stored in the sequence of nucleotides along the polynucleotide strands
  • Each set of three nucleotides (a codon) specifies a particular amino acid

Central Dogma of Molecular Biology

• During transcription, the genetic information in DNA is copied into a complementary RNA molecule called messenger RNA (mRNA)
  • Allows the genetic information to be transported out of the nucleus
• The mRNA carries the genetic information from the nucleus to the ribosomes in the cytoplasm, where translation occurs
  • The amino acids specified by the codons are assembled into proteins
• DNA replication ensures that each daughter cell receives an identical copy of the genetic information during cell division
  • Allows for the faithful transmission of traits from parent to offspring (heredity)

Discoverers of DNA's Structure

Watson and Crick's Double Helix Model

• James Watson and Francis Crick proposed the double helix model of DNA structure in 1953
  • Based on X-ray crystallography data and the known chemical properties of DNA
• Watson and Crick's model explained how DNA could replicate and how genetic information could be stored and transmitted
  • Revolutionized the field of molecular biology
• Awarded the Nobel Prize in Physiology or Medicine in 1962 for their work on DNA structure

Contributions of Franklin and Wilkins

• Rosalind Franklin's X-ray diffraction images, particularly the famous "Photo 51," provided crucial evidence for the helical structure of DNA
  • Also revealed the spacing between the nucleotide bases
• Maurice Wilkins, Franklin's colleague, shared her X-ray crystallography data with Watson and Crick
  • Helped them refine their model of DNA structure
• Franklin's contributions were not fully recognized until after her death in 1958
  • Her work was essential to the discovery of the double helix structure