The Story of Our DNA.

Chapter 11, Section 1 Notes

Important Discoveries: DNA’s Function

• Frederick Griffith (1928) – Transformation

• -Turned non-lethal bacteria into lethal

• -Hypothesized that a gene was responsible (He did NOT know what biomolecule genes were made of)

Avery (1944) • DNA is the molecule

responsible for heredity

• a. Treated an extract from the heat-killed bacteria with enzymes to destroy everything except the nucleic acid.

• Resulted in an identical outcome to Griffith’s experiment

Hershey-Chase (1952) -Studied

bacteriophages

-Determined that the DNA not the protein coat was the source of genetic information

Now that scientists agree that DNA carries the information…

How is DNA organized?

Chargraff (1950)

Determined that nucleotides (the building blocks of DNA) occur in specific ratios

- Guanine = cytosine

- Adenine = thymine

Did not know why this was true, only that every organism followed this rule

Franklin (1950’s)

- Used x-ray diffraction

- Three important pieces of information

1) Each strand is coiled (helix)

2) Two strands of in the structure

3) The nitrogen bases are near the center

Watson and Crick (1953)

- Used Franklin’s experiment to determine the 3D structure of DNA-The double helix explained both how DNA could carry information and how that information could be copied

- Their model explained why A=T and C=G

What we know now about the structure of DNA

• Double helix • Made of a phosphate backbone • “Rungs” are the nucleotide bases • Bases attach in certain patterns

– C G – A T

• One side is called the 3’ side the other is called 5’• On the ends of the chromosomes in eukaryotes

are structures called telomeres that are repeating bases that protect the DNA

DNA Replication The process of duplicating DNA

Commonalities in all organisms

• One side of the double helix gives the instructions for the other

• DNA replication occurs in both directions

• The site where DNA separation and duplication occurs is called a fork

Prokaryotes • More simple

– DNA is not in nucleus – Plasmid – single, circular DNA molecule– Much less DNA than a eukaryote

• DNA replication begins at a single point

Eukaryotes • DNA is more complex

– DNA is in a membrane-bound nucleus– DNA is wrapped tightly around proteins to form

chromosomes– More DNA than prokaryotes

• DNA replication occurs at hundreds of points

How Replication Occurs

• Step 1: DNA is opened– DNA Helicase – reduces the strain caused by

the unwinding of the DNA

Step 2: New DNA is formed • DNA Polymerase – Helps the daughter strand

form by matching base pairs – Also proofreads the DNA and corrects mistakes

• Complimentary base pairs are attached to each side of the now single-sided DNA – Each strand acts as a template – New strands of DNA are ½ new and ½ original – DNA has a “north” and “south” – they are called 5’

and 3’ – Base pairs are copied following the 3’ to the 5’

direction of the parent strand

•Leading strand – the side of the DNA that is replicated faster – follows the 3’ 5’ of the parent strand

•Lagging strand – It is replicated more slowly

•Okazaki fragment – the pieces that are formed in the lagging strand, because replication can only occur in the 3’ to 5’ direction as the DNA is unwound.

Step 3: The pieces of DNA are connected

• DNA ligase attaches the Okazaki fragments together