DNA

The Genetic Code

Chapter 8 in your textbook

Discovery of DNA

• Experiments that identified DNA as the genetic material & determined its structure:

1. Griffith2. Avery3. Hershey – Chase 4. Chargaff5. Franklin & Wilkins6. Watson & Crick

Griffith’s Experiment

• Used mice and pneumonia bacteria– One strain, “S”, was harmful

(pathogenic)

– Other strain, “R”, was not harmful (non-pathogenic)

• Discovered transformation – one bacteria was genetically changed by another bacteria

Griffith, cont’dAnimation

• The harmless strain (R) mixed with the heat-killed pathogenic strain (S) killed the mice.

Griffith, cont’d

• Some “factor” was transferred from the S to the R bacteria, turning the harmless R into something deadly

• At this point, scientists weren’t sure whether the transforming factor was DNA or protein

Avery’s Experiment

• Piggy-backed off of Griffith’s experiment to figure out what the “transforming factor” was

• Used enzymes to isolate various components of the bacteria: DNA, proteins, lipids, and carbohydrates

• Transformation only took place when the harmless bacteria was treated with DNA from the harmful bacteria

Avery, cont’d• Discovered that DNA stores and

transmits genetic information

Hershey & Chase’s Experiment

• Used viruses that infect bacteria (bacteriophages)

• Used radioactive phosphorus to mark the DNA

• Used radioactive sulfur to mark the proteins

• This allowed them to trace where these molecules went during the viral infection process

• Demonstrated that DNA was the genetic material -- video (2:05)

Hershey / Chase, cont’d

Chargaff

• Chargaff determined that the amount of adenine is the same as the amount of thymine in all the organisms he studied (and that C = G)

Franklin & Wilkins

• Franklin & Wilkins did X-ray crystallography pictures of the DNA molecule that showed it to be a helical structure with at least 2 helices

Watson & Crick

• Using Chargaff’s rules, they knew that A=T and C=G

• Using the X-ray pictures from Franklin & Wilkins, they knew it was a helical shape

• They knew the structure had to provide a way to replicate very accurately

• Video (4:04)

Watson & Crick, cont’d• Determined the structure of DNA in 1953

– Double helix with bases on the inside and sugar and phosphate alternating on the outside

DNA Basics

• Deoxyribonucleic Acid

• Double helix (twisted ladder)

• Located in the nucleus of eukaryotic cells– Prokaryotes lack a nucleus, but

still have DNA in their cells; prokaryotic (bacteria) DNA hangs out in the nucleoid region

DNA is a Nucleic Acid

• Organic compound made up of carbon, hydrogen, nitrogen, oxygen, and phosphorus.

• Made of repeating subunits (monomers) called nucleotides

The Nucleotide

• Made of a sugar (deoxyribose), a phosphate group and one of 4 nitrogen bases

Types of Nitrogen Bases

Pyrimidines (1 ring)• Cytosine• Thymine

Purines (2 rings)• Adenine • Guanine

DNA Structure

• Sides of the ladder are the sugar and phosphate group

• The rungs (part you’d step on) are 2 of the nitrogen bases

• Each rung is one purine paired with one pyrimidine – always C-G and A-T

DNA Structure

DNA Structure

DNA Replication

• The process of making identical copies of DNA

• It is semiconservative – in each molecule, there is one old and one new strand

• Occurs during the S phase of the cell cycle

• It is necessary for cell division

Steps of DNA Replication

• Step 1: DNA helicase (an enzyme) unwinds the molecule

• Step 2: DNA polymerase binds free-floating nucleotides to the exposed bases on the original strands

• Step 3: The 2 new molecules wind up again

• Animation