Introduction to DNA and Genetic Information

Introduction to DNA

• Understanding DNA is crucial to understand how genetic information is passed from one generation to the next
• Scientists needed to understand the nature of genes to determine how a cell becomes a specific organism (e.g., a mouse or a moose)

Bacterial Transformation

• Frederick Griffith discovered transformation in 1928 while studying bacteria that cause pneumonia
• He isolated two types of bacteria: disease-causing (S) and harmless (R) strains
• S strain produced smooth colonies, while R strain produced colonies with rough edges

Griffith's Experiment

• Injecting S type bacteria into mice resulted in death and pneumonia
• Injecting R type bacteria into mice resulted in survival and no pneumonia
• Mixing heat-killed S bacteria with live R bacteria and injecting into mice resulted in death and pneumonia

Transformation

• Griffith's experiment showed that a chemical factor transferred from heat-killed S bacteria to live R bacteria, changing the R bacteria permanently
• This process is called transformation

Avery's Experiment

• Dr. Avery repeated Griffith's experiment to determine which molecule was responsible for transformation
• He used enzymes to destroy different molecules and found that DNA was responsible for transformation

DNA as Genetic Material

• By observing bacterial transformation, Avery and other scientists discovered that DNA stores and transmits genetic information from one generation to the next
• Hershey and Chase confirmed Avery's results, showing that DNA is the genetic material found in genes, not just in viruses and bacteria, but in all living cells

Structure of DNA

• DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds
• Contains 4 nitrogenous bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T)
• Contains a 5-Carbon sugar called Deoxyribose

Chargaff's Rule

• Erwin Chargaff discovered that the percentage of A-T bases is equal to the percentage of C-G bases in all DNA

Double Helix Model

• The double helix model shows that the two strands of DNA run in opposite directions (antiparallel) with a 5' - 3' direction
• The sides of the DNA model are comprised of Deoxyribose sugar and phosphate groups
• The inside of the model is comprised of nitrogenous bases, with Adenine bonded with Thymine and Guanine bonded with Cytosine

Hydrogen Bonding

• Hydrogen bonds hold together nitrogenous bases, with A-T and C-G bonds
• These bonds are weak and can easily separate

Rosalind Franklin's Contribution

• Rosalind Franklin used x-ray diffraction to study the structure of DNA
• Her X-ray pattern showed that the strands are twisted around each other like the coils of a spring

Watson and Crick's Model

• Watson and Crick used Franklin's X-ray data to build a model that explained the specific structure and properties of DNA
• Their model showed that the nitrogenous bases are stacked at regular intervals near the center of the molecule

DNA Replication

• DNA can be copied, with one base on one strand pairing with one base on the opposite strand
• DNA is complementary, meaning one strand can be used to make the other strand
• DNA replication occurs during the S phase of cell division
• Ensures each resulting cell has the same complete set of DNA molecules

DNA Replication Process

• DNA is unzipped by helicase
• Two strands are formed, with one old strand and one new strand
• DNA polymerase joins individual nucleotides to produce a new strand of DNA and proofreads each strand
• Resulting strands are identical and semiconservative