1. Discovery of DNA
1850’s Gregor Mendel discovered hereditary “factors”, but did not know about genes or chromosomes or DNA.
1928…Frederick Griffith showed genes were responsible for heredity
DNA from dead bacteria was picked up by live bacteria (transformation) and killed mice

2. 1928…Griffiths

3. Discovery of DNA
1944…Oswald Avery showed DNA was responsible for heredity.
Repeated experiment, but destroyed molecules trying to find what it was that caused the transformation…what caused the dead bacteria to kill the mice
Destroyed proteins, lipids, carbohydrates, RNA and still transformation occurred, so must be DNA
Did again and destroyed DNA, no transformation occurred

4. Discovery of DNA 1952…Alfred Hershey
and Martha Chase backed up Avery’s experiments
Used bacteriophages viruses that attack bacteria
Has a protein coat and DNA core

5. 1952 Hershey and Chase
Used sulfur and phosphorus … proteins have sulfur, DNA has phosphorus
Found phosphorus so must be DNA

6. Discovery of DNA 1949 Edwin Chargaff noticed that
A&T and G&C appear almost equally in DNA, no matter what the organism!
Early 1950’s…Rosalind Franklin took first X-ray of DNA molecule
1953 James Watson and
Francis Crick announced
double helix shape
(based on RF’s Xray).

7. DNA Function in cell DNA represents a code
That code will be copied (transcription)
The copy will leave the nucleus
The copy will be decoded to make a protein (translation)
The protein will run some function in the cell or body
That’s how DNA in the nucleus is the ‘control center’ of the cell

8. DNA Structure Deoxyribonucleic Acid Makes up chromosomes
Chromosomes have small areas called genes
Genes are a section of DNA that is a code for a protein that does some function in your body (more about this in next unit)

9. DNA Structure

10. DNA Structure
Double helix like a twisted ladder made of subunits called nucleotides. Has three parts:
deoxyribose (sugar)
phosphate group
nitrogen base
4 different bases:
adenine guanine
cytosine thymine
Bases match each other (complementary), A matches with T, G matches with C
Order of bases called the base sequence

11. DNA Replication
DNA Copies itself during the S phase of the cell cycle, so daughter cells have complete copy of all genes.
DNA Helicase (an enzyme) “unzips” the DNA molecule by breaking the hydrogen (weak) bonds between nitrogen bases.
Area where “unzips” is called replication fork.
DNA polymerase (another enzyme) matches base pairs with their complement (A with T, G with C…Chargaff’s Rule) on both strands at once. The base pairs are already in the nucleus.

12. DNA Replication

13. DNA Replication
Because one strand is upside down (antiparallel), one strand is copied as one whole piece (leading strand) and one is copied in chunks (Okazaki fragments) and pieced together (lagging strand) by DNA ligase
DNA polymerase continues until whole strand is copied. Since two new strands have 1 original and 1 new, it’s called semi-conservative replication.

14. Prokaryotic DNA In Prokaryotes (bacteria), DNA is a ring and
replication starts at one
point and goes around
both ways until it’s done.
Thus there will be only two
replication forks.
In eukaryotes, replication will start in many places, so there are multiple replication forks.

15. Mistakes
Mistakes in copying DNA can be (and often are) made, but are usually caught by DNA polymerase and fixed.
If not fixed, called mutation, and the base sequence changes.
Sometimes this is good and leads to a new adaptation (evolution)
Sometimes its bad (cancer, other genetic disorders)

16. Mistakes
There are also mutations to chromosomes, but we’ll talk about all of this in the next unit.