Intro to DNA Notes

1/5/09 DNA and RNA Obj: Investigate the structure of the DNA. Warm-up: How is it possible to fit approximately1 meter of human DNA into each of your cells? HW: Part 1 of DNA Notes WS

History of DNA Scientists have long understood that offspring inherit traits from their parents Example Scientist call what causes a certain trait a gene So what makes up a gene? http://www.virginmedia.com/images/simpsons-jessica-gal-siblings.jpg

Salmon

Bases on the chromosome DNA and Chromosomes Chromosomes: Threadlike structures in the nucleus of eukaryote cells and in the cytoplasm of prokaryote cells that contain genetic information Usually 1 chromosome in prokaryotic cells Varies in Eukaryotic cells 46 chromosomes in human cells 22 chromosomes in Sequoia tree cells 78 in dog cells Chromosome Bases on the chromosome

DNA Length DNA is a long molecule… 4,639,221 base pairs in E-coli 1.6 mm long, An E-coli cell is only .0016 mm long Human DNA is over 1 meter long! (almost 2!)

Eukaryotic DNA In a cell, DNA is wrapped around proteins called histones, shown here in green. The whole structure is called a nucleosome When the histones squeeze the DNA tightly, they "hide" that section of genetic material from the cell. Nucleosomes have a lot of info in a very small space DNA/Chromosome analogy- why is it convenient to wind the DNA like this on histones? (The whole structure makes up a chromosome)- what happens during mitosis?

How are the terms GENES, CHROMOSOMES, and DNA related? Genes- sections of DNA that codes for a protein DNA- molecule made of coding genes and “junk” sequences Chromosome- structure made up of large amounts of DNA in a condensed form DNA makes up GENES which makes up CHROMOSOMES!

Bill Nye- Understanding DNA This video will give you background on the: DISCOVERY of DNA DISCOVERY of DNA’s structure. In your notes: Describe the Hershey/Chase bacteriophage experiment. Why is this experiment considered a “breakthrough” in the identity of DNA? How does this experiment show the scientific process? What were Watson and Crick (along with other competition) trying to solve? What were TWO things already known about DNA? When and in what journal did Watson and Crick reveal their discovery to the world? What WAS their discovery? Who was Rosalind Franklin? What was her “unknowing” contribution to Watson and Crick’s discovery

Hershey- Chase Experiment What was the chemical base of genes? DNA or Protein? Used viruses, which are basically DNA or RNA with a protein “coat” Phage infects bacterium Bacteriophage with phosphorus-32 in DNA Radioactivity inside bacterium Phage infects bacterium No radioactivity inside bacterium Bacteriophage with sulfur-35 in protein coat

Rosalind Franklin’s X-Ray Crystallography Took x-ray photographs of crystallized DNA Discovered that the shape of DNA must be a helix, because of the x-ray pattern Died of cancer (too many x-rays?)

An example of X-Ray Crystallography

Watson and Crick Credited for discovering the structure of DNA in 1953 Used Chargaff and Franklin’s discoveries along with toy models to discover the structure

Structure of DNA DNA is a very long molecule Made of three basic parts A sugar called deoxyribose A phosphate group A nitrogen containing base Purines Pyrimidines Adenine Guanine Cytosine Thymine Phosphate group Deoxyribose

History of the Shape? Edwin Chargaff- Noticed that amount of: guanine (G) = cytosine (C), and adenine (A) as Thymine (T). Rosalind Franklin- developed the technique for looking at DNA. (X-Ray Crystallography) James Watson and Francis Crick- figured out the structure of the double helix!

Shape of DNA Double helix- like a winding staircase DNA’s two backbones (sides of ladder) each consists of: deoxyribose (5-sided sugar) phosphate The “rungs” of the DNA ladder are base pairs.

Base Pair Rules Watson and Crick also could explain Chargaff’s findings A = T in the rungs of a ladder and C = G So if one side of the ladder had an A the other had to have a T

Example C --  The other side would be? A -- G -- T -- Which 2 are purines? Which 2 are pyrimidines?

But why not A with C and G with T? What can you tell about the number of bonds each molecule can make? T/A: Only two opportunities to bond C/G: 3 places to bond The answer: only with A & T and with C & G are there opportunities to establish hydrogen bonds (shown here as dotted lines) between them (two between A & T; three between C & G). The ability to form hydrogen bonds makes the base pairs more stable structurally. These base pair relationships are often called Chargaff's rules of DNA base pairing, named after the Columbia University scientists who observed that there are equal molar concentration of A & T, as well as G & C in most DNA molecules. The rules of base pairing tell us that if we can "read" the sequence of nucleotides on one strand of DNA, we can immediately deduce the complementary sequence on the other strand.

Adenine Nucleotide

Cytosine Nucleotide

Guanine Nucleotide

Now you Replicate some DNA! Go to: http://www.nobelprize.org/educational/medicine/dna_double_helix/dnahelix.html Read the instructions and play the game. Fastest + Most Accurate = Playoffs?

Chargaff’s Rule If the individual strands of a DNA duplex are isolated and their base compositions determined, then: %A = %T, and %C = %G The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same (Chargaff's rule). Similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same. (Rudner et al., 1968).

Example: A chicken’s DNA is made up of approximately 29% THYMINE Example: A chicken’s DNA is made up of approximately 29% THYMINE. What is the % of: Adenine? Cytosine? Guanine?

Your turn: Humans: 20% cytosine A? T? G?

Relative Proportions (%) of Bases in DNA Base Pair Rules     Relative Proportions (%) of Bases in DNA Organism A T G C Human 30.9 29.4 19.9 19.8 Chicken 28.8 29.2 20.5 21.5 Grasshopper 29.3 20.7  Sea Urchin 32.8 32.1 17.7 17.3 Wheat 27.3 27.1 22.7 22.8 Yeast 31.3 32.9 18.7 17.1 E. coli 24.7 23.6 26.0 25.7