Chapter 1 - DNA & Replication

Properties of Genetic Material

-Must encode information

-Transmission; Information must be transferred between generations

-Replication

-Variation; Able to undergo changes in order to adapt to the environment

Phoebus Levine’s Discoveries

American biochemist who studied DNA/RNA, discovered;

-Presence and structure of nitrogenous bases

-Structure of nucleic acids (phosphate group and pentose sugars)

-Linkages of nucleotides forming a polynucleotide chain

Nitrogenous Bases in DNA/RNA

-DNA contains 4 nitrogenous bases (Adenine, Guanine, Thymine, Cytosine)

• RNA replaces Thymine with Uracil

-Nitrogenous bases split into two categories based on structure;

• Purines: Double-ringed (bicyclic), consists of Adenine and Guanine

• Pyrimidines: Single-ringed, consists of Thymine, Uracil and Cytosine

Pentose Sugar

5-Carbon sugar present in DNA/RNA nucleotides

-DNA’s deoxyribose sugar lacks an oxygen in the 2’ Carbon position

-RNA’s ribose sugar has a hydroxyl group present in the 2’ Carbon position

Naming/Structure of a Nucleotide

-Nitrogenous base is linked to the 1’ Carbon position of the pentose sugar

-The phosphate group is linked to the 5’ Carbon position of the pentose sugar

Naming is based on the nitrogenous base present and if it's DNA/RNA

• DNA: deoxy(nitrogenousbase)monophosphate

• RNA: (nitrogenousbase)monophosphate

Example; deoxyadenosinemonophosphate (dAMP)

Nucleoside

-Contains a nitrogenous base and the sugar, no phosphate group

Polynucleotide Chain

-More than one nucleotide linked together forms a polynucleotide chain

-Each nucleotide forms a phosphodiester bond at the 3’ Carbon position and the 5’ Carbon position

-This creates polarity in the chain

Rosalind Franklin

-British x-ray crystallographer

-Determined that DNA had a helical structure

-based on the image, the diameter required 2 polynucleotide chains

• Forms a “Double-Helix”

-Determined that there was 10 nucleotides for each 360-degree turn

Erwin Chargaff

American biochemist, studied both prokaryotes and eukaryotes

-Observed a pattern; amount of Adenine = Thymine, amount of Cytosine = Guanine

-This became known as “Chargaff’s Rules”, the idea of complementary base pairing in equal ratios

Models for DNA Replication

Conservative Model

-Both original strands remain together, while the new strands remain together

Semi-Conservative Model

-Original strand attaches to a new strand

Dispersive Model

-Every chain has some old strand and new strand, constant mixing

Meselson-Stahl Experiment

-Attempted to identify the method of DNA Replication

-Used E-coli due to its simple chromosome and fast growth

-Some grew in N14 (light Nitrogen) while others grew in N15 (heavy nitrogen)

-After growth, they were placed in CsCl gradient centrifugation

• DNA with heavy nitrogen moves to the bottom, DNA with light nitrogen closer to the top

• DNA with one heavy strand and one light strand would be found in the middle