DNASC STANDARD B-4: THE STUDENT WILL DEMONSTRATE AN UNDERSTANDING OF THE MOLECULAR BASIS OF GENETICS

Cornell Notes

Topic: DNA

EQ: How does the overall structure of DNA explain the mechanism of inheritance?

History Griffith’s Experiment

1928: Griffith Transformation :

◦Process by which 1 strain of bacteria is changed into a 2nd strain after being in the presence of the 2nd strain

DNA

Avery: 1944◦Identified transforming agent as DNA

Hershey-Chase Experiment

Bacteriophages infect bacteria and transfer their genetic material to the bacteria’s DNA. These experiments proved it was the genetic material not the protein that “transformed” the bacteria.

Parts of DNA

DNA

1. 5-carbon sugar: deoxyribose

2. Phosphate group3. Nitrogenous Base

1. Purines1. Adenine2. Guanine

2. Pyrimidines1. Cytosine2. Thymine

Chargaff’s Rules

In same sample of DNA:

cytosine = guanine ◦[ C ] = [ G ]

adenine = thymine◦[ A ] = [ T ]

X-RAY EVIDENCE

Rosalind Franklin

studied DNA using x-ray diffraction

The pattern on the left gave some important clues as to the structure of DNA◦ shaped in a helix◦ nitrogenous bases in

center

THE DOUBLE HELIX

Watson & Crick

Built 3-D models trying to find a structure that explained everything they knew DNA could do

When they saw Franklin’s x-rays the shape became clear to them

DNA

Had to be able to do 3 things:1. carry

information from 1 generation to next

2. use the information to produce traits

3. be easily copied

THE DOUBLE HELIX

DOUBLE HELIX

TWISTED LADDER WITH RUNGS OF LADDER MADE UP OF THE NITROGENOUS BASES HELD TOGETHER BY HYDROGEN BONDS

DOUBLE HELIX

BASE PAIRING EXPLAINED CHARGAFF’S RULES SINCE ADENINE ALWAYS PAIRED WITH THYMINE & GUANINE ALWAYS PAIRED WITH CYTOSINE