The History of DNA

1.Griffith- experiment showed that live uncoated bacteria acquired the ability to make coats from dead coated bacteria. He called the process transformation. The study suggested that DNA was the genetic material.

Disease-causing bacteria (smooth colonies) Harmless bacteria (rough colonies) Heat-killed, disease- causing bacteria (smooth colonies) Control (no growth) Heat-killed, disease- causing bacteria (smooth colonies) Harmless bacteria (rough colonies) Dies of pneumonia LivesLives Live, disease-causing bacteria (smooth colonies) Dies of pneumonia Section 12-1 Figure 12–2 Griffith’s Experiment Go to Section:

Smooth bacteria → mouse = dies of pneumonia Rough bacteria → mouse = lives Heat-killed smooth bacteria → mouse = lives Heat-killed smooth bacteria + Live rough bacteria → mouse = dies of pneumonia Experiments Frederick Griffith

Experiments Oswald Avery Question: Which molecule caused the transformation of rough to smooth bacteria? Avery extracted the heat killed bacteria and mixed them with macromolecules 1. Lipids 2. Proteins 3. nucleic acids 4. carbohydrates He exposed the rough cells to the macromolecules one at a time and each time no transformation occurred, except when exposed to the nucleic acid DNA.

2. Avery- discovered that DNA is the nucleic acid that stores and transmits the genetic information from one generation to the next. 3. Hershey & Chase- used radioactive labeling to identify DNA. They showed that DNA, not protein, is the genetic material of a bacteriophage (virus).

The Hershey-Chase Experiment Alfred Hershey & Martha Chase worked with a bacteriophage: A virus that invades bacteria. It consists of a DNA core and a protein coat DNA Protein coat

4. Erwin Chargaff showed the amounts of the 4 bases on DNA 1950 Chargaff ’ s Rule: Equal amounts of Adenine and Thymine, and equal amounts of Guanine and Cytosine Erwin Chargaff

5. Rosalind Franklin took x-ray diffraction photographs of DNA Rosalind Franklin X-Ray diffraction image of DNA taken by Franklin in 1951

6.Watson and Crick built the first model of DNA using Franklin’s x-rays James Watson Francis Crick The 3-D model was called a double helix

DNA Structure DNA is made up of monomers called nucleotides. A Nucleotide consists of : –5 carbon sugar called deoxyribose –Phosphate group –Nitrogen base

DNA is like a ladder that has been twisted to form a double helix. The sugar and phosphate groups alternate and make up the sides of the DNA ladder.

The nitrogen bases make up the rungs of the DNA ladder. The nitrogen bases are: –Adenine –Guanine –Cytosine –Thymine The bases are held together by weak hydrogen bonds

Purines have a double ring structure Pyrimidines have a single ring structure A Purine will always pair with a Pyrimidine

DNA Replication 1. Before a cell divides DNA is copied (replicated) during Synthesis of Interphase 2. During DNA replication, the DNA molecule separates into 2 strands. Each new strand will hook up with it’s complementary base partner, making 2 new complementary strands.

The strands follow Chargaff’s rule on base pairing. 3. The sites where separation and replication occur are called replication forks. 4. The replication is carried out by enzymes that “unzip” the DNA called helicases.

5. DNA polymerases are enzymes that create DNA molecules by assembling nucleotides, the building blocks of DNA. These enzymes are essential to DNA replication and usually work in pairs to create two identical DNA strands from a single original DNA molecule.

DNA to Chromosome DNA coils around histones to form nucleosomes, which coil to form chromatin fibers. The chromatin fibers super coil to form chromosomes that are visible in the metaphase stage of mitosis