Unit 6: Genetics 6.1 Structure of DNA

DNA Discovery Scientists originally believed proteins, not DNA, carried genetic information, due to the large number of possible amino acid combinations. A series of experiments found that DNA was in fact the carrier of genetic information. Griffith’s Experiment- 1928 The Hershey-Case Experiment- 1952

DNA Discovery- 1953 Rosalind Franklin used x-ray diffraction to show that DNA is a double-helix.

DNA Discovery- 1953 Watson and Crick used clues from the x-ray to build a model that explained the specific structure and properties of DNA. https://www.youtube.com/watch?v=VegLVn_1oCE https://www.youtube.com/watch?v=JiME-W58KpU

What is DNA? Stands for Deoxyribonucleic Acid Carries our genetic material from one generation to the next in all living things DNA is arranged in a double helix-two strands are twisted around each other like a winding staircase. If you unwind a single human chromosome of DNA, it will be about 5 feet long!

What does our DNA do? Only about 8% of our genetic sequence is made up of our genes Up until recently, scientists believed that the rest of our DNA was noncoding-they thought these sequences were “junk”!

However, a research project called Encode studied these sections and found: Parts of the DNA that don’t code for genes contain about 4 million gene switches that have important roles: Determine when the gene turns on and off Determine how much protein to make Switches for a variety of diseases Affects every cell, but can do so at different times in our lifetime Examples: Instructions for the cells to be different types of cells (brain cell vs liver cell) Instructions for pancreas cell to make insulin after a meal Instructions for cells to reproduce to replace dead or damaged cells (like skin cells replacing those that sloughed off)

Structure Nucleic Acids: Macromolecules, made up of nucleotides linked together (monomer) Nucleotides are made up of 3 things: A phosphate group (same on all nucleotides) 5 Carbon sugar molecule (same on all nucleotides) Nitrogen containing base (Different on each nucleotide) Adenine (Purines: 2 rings of Guanine Carbon and Nitrogen) Thymine (Pyrimidines: single ring of Cytosine Carbon and Nitrogen)

Nucleotides

Structure The phosphate of one nucleotide is attached to the sugar of the next nucleotide with a covalent bond. The base pairs on each strand of DNA are bound together with hydrogen bonds.

Staircase Analogy Staircase Analogy: Sugar and phosphate backbones are the handrails Nitrogen bases are the rungs of the ladder Nitrogen bases on one side are held together with Nitrogen bases on the other side by weak Hydrogen bonds.

Structure

Base Pairing Purines are always paired with Pyrimidines Adenine always pairs with Thymine (forms 2 hydrogen bonds) Guanine always pairs with Cytosine (forms 3 hydrogen bonds) This is called complementary base pairing-the sequence on one strand determines the sequence on the following pair. DNA has a direction-it is polar! It goes from the 5’ end to the 3’ end. The opposite strand also goes from the 5’ to 3’ end, but in the opposite direction!