DNA Introduction Honors

A gene is a segment of DNA that encodes a protein Genes vary in size, from just a few thousand pairs of nucleotides to over 2 million! You have about 6 billion nucleotides in each of your cells. This means in your DNA there is roughly 30,000 genes!

Hershey-Chase Experiment (1952) Back in the day, the question was: is the genetic material of organisms made of DNA or proteins? Hershey-Chase Experiment (1952) Proved that the genetic material that infects bacteria is found in the viral DNA Bacteriophage “phage”: virus that infects bacteria. Composed of DNA and protein Use of Radioactive Trackers Injected radioactive markers in the virus…markers in phosphorus (not in protein) and sulfur (not in DNA) If 32 P were found in bacteria (DNA infects the bacteria) If 35 S were found in bacteria (protein infects the bacteria

Animation Radioactivity in liquid Radioactivity in pellet 1 Mix radioactively labeled phages with bacteria. The phages infect the bacterial cells. 2 2 Agitate in a blender to separate phages outside the bacteria from the cells and their contents. 3 3 Centrifuge the mixture so bacteria form a pellet at the bottom of the test tube. 4 Measure the radioactivity in the pellet and liquid. Radioactive protein Empty protein shell Radioactivity in liquid Phage Bacterium Phage DNA DNA Batch 1 Radioactive protein Centrifuge Pellet Hershey Chase wanted to discover which part of the virus, the protein coat or the DNA core infects cells (were genes made of protein or DNA) Injected radioactive markers in the virus…markers in phosphorus (not in protein) and sulfur (not in DNA) If 32 P were found in bacteria (DNA infects the bacteria) If 35 S were found in bacteria (protein infects the bacteria) Measured the radioactivity in the PELLETS (which were the bacteria when centrifuged and blended) Radioactive DNA Batch 2 Radioactive DNA Radioactivity in pellet Pellet

Overall Conclusion: Genetic information is stored in and passed to subsequent generations through DNA molecules

Structure of DNA

Rosalind Franklin: Photo 51 The X in the center of photo 51 was caused by the helical shape of the DNA molecules in the sample.

DNA Structure History James Watson and Francis Crick worked out the three-dimensional structure of DNA, based on work by Rosalind Franklin

DNA is a Nucleic Acid Made up of long chains (polymers) of nucleotides (monomers)

Basic Structure of a Nucleotide 3 components to all nucleotides Pentose sugar Deoxyribose or ribose Phosphate group Nitrogenous Bases Adenine (A) Thymine (T) Cytosine (C) Guanine (G) Uracil (U) Nucleic acid: nuclei (within the nucleus of cells) Deoxyribose- sugar that is different from ribose because it lacks an oxygen (deoxy) Phosphate group is acidic component. Nucleic ACID

Back Bone of DNA Sugar and Phosphate groups make up the backbone of DNA. They are covalently held together via Phosphodiester bonds The nitrogenous bases are on the inside and hold the two strands of the double helix together via Hydrogen bonds Each phosphate group links the 3’ carbon of one sugar to the 5’ carbon of the next sugar along the backbone Pairs of complementary bases from hydrogen bonds that hold the two strands of the DNA double helix together

Nucleotides 4 nucleotides differ only in their nitrogenous bases Adenine (A) and Guanine (G) are purines Thymine (T) and Cytosine (C) are pyrimidines Nucleotides are joined together y covalent bonds between sugar of one and the phosphate of the next = sugar-phosphate backbone. Thymine (T) Cytosine (C) Adenine (A) Guanine (G) Pyrimidines Purines

Hydrogen bonds hold together the nitrogenous bases in the DNA double helix Always pyrimidine with purine If not, the diameter of double helix would not be equal An A-A pair would be twice as wide as a C-C pair Each base has chemical side groups that can best form hydrogen bonds with one appropriate partner Which pair do you think is stronger? C-G have 3 hydrogen bonds and are therefore stronger than A-T

Chargaff’s Rules Adenine and Guanine are purines = 2 ringed Thymine and Cytosine are pyrimidines = 1 ring Chargaff’s Rules

Amount of adenine always equal to the amount of thymine If 30% of the nucleotide bases are thymine. How many nucleotide bases are guanine? If there are 30% T then there are 30% A = 60% 100%-60% = 40% 40%= C + G G = 20%

DNA Strand Orientation Each carbon in the nucleotide is numbered Nitrogenous base is on 1’ Phosphate group is on 5’ 3’ binds to phosphate group of another nucleotide Complementary strands are antiparallel Phosphate end = 5’ end Sugar (-OH) end = 3’ end One strand runs 5’3’, the other 3’5’

Structure of DNA Nucleotide Hydrogen bonds Sugar-phosphate backbone Key Adenine (A) Thymine (T) Cytosine (C) Guanine (G)