Discovery of DNA: The Molecule of Heredity

Discovery of DNA: The Molecule of Heredity

• Charles Darwin proposed evolution from a common ancestor with natural selection as the driving force. However, he lacked an explanation for inheritance.
• Gregor Mendel's research focused on pea plants, identifying heritable characteristics masked in the first generation and predicting different character ratios later on. These "factors" were later termed as genes.
• Johann Friedrich Miescher isolated "nuclein" – DNA and proteins – in 1869 from the nucleus, containing phosphorus and nitrogen in a unique ratio.
• Albrecht Kossel identified the five nuclein components: Adenine, Cytosine, Guanine, Thymine, and Uracil, and coined the term "Deoxyribonucleic acid" (DNA).
• Thomas Hunt Morgan's observations on Drosophila melanogaster (fruit flies) demonstrated the inheritance of eye color, following Mendelian principles with a 3:1 ratio (red:white) in the F2 generation.
• Morgan's experiments revealed that all F2 females had red eyes, while males showed a 50% red and 50% white eye distribution, further supporting the link between chromosomes and inheritance.
• Frederick Griffith's research with Streptococcus pneumoniae strains demonstrated the transformative nature of genetic material, suggesting it wasn't protein due to heat denaturing.
• Oswald Avery confirmed DNA as the transformative substance.
• Alfred Hershey and Martha Chase further solidified DNA as the inheritance molecule through experiments with bacteriophages.
• Edwin Chargraff's work showed that: a) DNA base composition varies across species, indicating it wasn't a tetranucleotide with equal proportions of bases, and b) the amount of A = T and G = C (Chargaff's rules).
• Maurice Wilkins and Rosalind Franklin utilized X-ray diffraction to demonstrate DNA's structure as a long helical rod with a diameter of 2 nm and structural regularity at 0.34 nm and 3.4 nm intervals.
• James Watson and Francis Crick, using existing data, proposed the double helix structure of DNA, a groundbreaking discovery published in Nature in 1953.
• Watson and Crick received the Nobel Prize in Physiology or Medicine in 1962, along with Maurice Wilkins.

Nucleic Acids and their Structure

• Nucleic acids like DNA are polymers composed of repeating subcomponents called nucleotides.
• A nucleotide consists of a phosphate group, a sugar (deoxyribose in DNA), and a nitrogenous base.
• The phosphate group connects nucleotides via phosphodiester bonds, forming the backbone of polynucleotides.
• The sugar links the phosphate backbone to the base.
• The nitrogenous base determines the specific nucleotide (Guanine, Thymine, Adenine, Cytosine).
• Adenine and Guanine are purines, characterized by a double ring structure with a pyrimidine ring connected to an imidazole ring.
• Cytosine and Thymine are pyrimidines, consisting of a single 4-carbon ring with nitrogen atoms at positions 1 and 3.
• Adenine pairs with Thymine, and Guanine pairs with Cytosine.

DNA Structure and Replication

• DNA has directionality, with one strand running from 5' to 3' and the complementary strand running 3' to 5' in the opposite direction.
• The two strands are antiparallel, meaning they run in opposite directions.
• The base sequence on one strand dictates the complementary sequence on the other, ensuring a consistent structure.
• The double-helix model depicts the two DNA strands intertwined, forming a helical structure.
• The helix has a major groove and a minor groove.
• DNA replication is a semi-conservative process, resulting in two new DNA molecules, each containing one original strand and one newly synthesized strand.
• The Meselson and Stahl experiment provided direct experimental evidence for the semi-conservative replication of DNA.