Early Evidence for DNA as Genetic Material

Study Notes

Early scientists understood chromosomes carried genetic information but debated the exact component involved: DNA or protein.
Frederick Griffith's experiments with Streptococcus pneumoniae provided early evidence. He observed that injecting mice with a mixture of heat-killed pathogenic "S strain" bacteria and living non-pathogenic "R strain" bacteria resulted in the mice's death. This suggested a "transforming factor" from the dead S strain could convert the harmless R strain to a deadly form.
Oswald Avery's experiments aimed to isolate the "transforming factor." Avery used enzymes to degrade specific components within cell extracts from the S strain and tested their ability to transform the R strain into the S strain. Destroying DNA prevented transformation, while degrading RNA or protein had no effect, concluding that DNA was the transforming factor and thus carried genetic information.

Today, transferring specific DNA into cells is possible, utilized for research, agriculture, and potential medical applications.

Transgenic organisms are generated when DNA is added to a fertilized egg cell, resulting in the entire organism carrying the introduced genes.
This process enables modifications like enhancing crop yields, disease resistance, and nutrient content, and allows for the creation of transgenic lab animals that are vital for understanding genes linked to diseases and developmental disorders.

DNA consists of nucleotides, which are comprised of:
- a sugar (deoxyribose)
- a phosphate group
- one of four nitrogenous bases (Adenine (A), Cytosine (C), Thymine (T), or Guanine (G)).
Erwin Chargaff determined that the bases exist in specific ratios within DNA: Amount of Adenine = Amount of Thymine, and Amount of Guanine = Amount of Cytosine.

Rosalind Franklin's X-ray diffraction studies of crystallized DNA revealed a double helix structure with bases situated in the middle and a sugar-phosphate backbone on the outside.

James Watson and Francis Crick developed a physical model based on Franklin's findings, confirming the double helix structure with nitrogenous bases pairing in the middle.
They established the rules of base pairing: Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G).
The two strands run anti-parallel, meaning they are oriented in opposite directions for base pairing to occur correctly.

DNA is composed of a double-stranded helix with a sugar–phosphate backbone and inward-facing bases connected by hydrogen bonds.
Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G) through hydrogen bonds.
The strands run anti-parallel, meaning they have opposite orientations, crucial for proper replication.