The Structure of DNA (Chapter 13.1)

DNA: The Genetic Material Genes are made up of small segments of deoxyribonucleic acid or “DNA” DNA is the primary material that causes recognizable, inheritable characteristics in related groups of organisms.

Searching for the Genetic Material 3 major experiments lead to the conclusion that DNA is the genetic material in cells.

1. Frederick Griffith Worked with 2 types of bacteria in mice (S bacteria & R bacteria) Discovered “transformation” – the transfer of genetic material in the form of DNA fragments from one cell to another.

Conclusion of Griffith’s Experiment: Something is present in the Smooth bacteria that is capable of transforming the Rough into Smooth This “mystery substance” is capable of changing the genetics of the organism. Hmmm…what could this mystery substance be?

2. Avery, McCarty & MacLeod 1944—Wanted to determine whether the transforming agent in Griffith’s experiments was a carbohydrate, lipid, protein, or DNA. Used enzymes to destroy each of these molecules in the Smooth (disease- causing) bacteria.

Avery, McCarty & MacLeod’s Experiment, Step by Step: Step 1: Using enzymes, they took the heat-killed smooth bacteria and removed the carbohydrates. After adding this to the Live Rough bacteria, they injected mice. The mice died of pneumonia. This means the Live Rough still turned into Smooth.

Avery, McCarty & MacLeod’s Experiment, Step by Step: Step 2: This time, they used enzymes to destroy the lipids in the heat-killed smooth. After adding it to the Live Rough, they injected mice. The mice died of pneumonia. This means the Live Rough still turned into Smooth, even without the lipids.

Avery, McCarty & MacLeod’s Experiment, Step by Step: Step 3: This time, they used enzymes to destroy the proteins in the heat-killed smooth. After adding it to the Live Rough, they injected mice. The mice died of pneumonia. This means the Live Rough still turned into Smooth, even without the proteins.

Avery, McCarty & MacLeod’s Experiment, Step by Step: Step 4: This time, they used enzymes to destroy the DNA in the heat-killed smooth. After adding it to the Live Rough, they injected mice. The mice lived! This means the Live Rough stayed Rough! Without the DNA from the smooth, the Rough did not turn into smooth.

Conclusion of Avery’s Experiment: Concluded that DNA is responsible for transformation in bacteria Were their findings accepted by the scientific community?

3. Alfred Hershey & Martha Chase Worked with bacteriophages (infectious viruses) made up of proteins and DNA. Found that DNA is the hereditary material in viruses.

Life Cycle of a Virus Thanks to the Hershey- Chase Experiment, we now know what part of the virus is injected into the cell.

Hershey-Chase Experiment:

Conclusion of the Hershey/Chase Experiment: DNA, (and only DNA) is the genetic material of the cell.

The Race is On…… In the 1950s, we knew genes were made of DNA but knew nothing about its structure. Scientists around the world were actively searching for more information about DNA

James Watson & Francis Crick Received his doctoral degree at age 22 in Zoology, with an emphasis on genetics. He also did post-doctoral research in biochemistry. Francis Crick, 36, a physicist working at Cambridge University in England James Watson Francis Crick

James Watson & Francis Crick Created the “double helix” model that is composed of 2 nucleotide chains. The chains are twisted around a central axis and are held together by weak hydrogen bonds

How did Watson & Crick determine this structure? (3 contributions) 1. In 1949, Erwin Chargaff found that there was always the same amount of adenine as thymine and the same amount of cytosine as guanine.

2. In 1952, Rosalind Franklin & Maurice Wilkins developed high quality x-ray images of strands of DNA. Images resembled a tightly coiled helix of 2 chains.

3. In 1953, Watson & Crick used the findings of Chargaff & Franklin to create a 3-D model of DNA.

Nucleotide Subunits (Review – 3 parts)

The 4 Nitrogen Bases:

Nitrogenous Bases Purines – Have 2 rings of carbon & nitrogen atoms per base. A – adenine G – guanine Pyrimidines – Have 1 ring of C & N atom per base T – thymine C – cytosine

Base- Pairing Rules A purine on one strand of DNA is always paired with a pyrimidine on the other A always with T G always with C

DNA Structure