1. DNA & RNA

2. Before We Knew about DNA
Gregor Mendel – the “father of genetics” was a monk who, in the 1800’s, was the first person to hypothesize “factors” which were passed down, or inherited, from generation to generation.

3. Before We Knew about DNA
But what were these “factors”?
Many scientists were on the case.
In the first half of the 1900’s, scientists Griffith, Avery, Hershey and Chase all conducted experiments contributing to the isolation of the DNA molecule.

4. Before We Knew about DNA
In 1950, Erwin Chargaff conducted chemical experiments and concluded there was the same amount of guanine as of cytosine in DNA.
In addition, there is the same amount of adenine as thymine in DNA.
Thus establishing the pairing of the nitrogenous bases.
This became known as Chargaff’s rule, and from this we know that:
adenine always pairs with thymine, and that cytosine always pairs with guanine
A=T and C=G

5. Before We Knew about DNA
In the early 1950’s, Rosalind Franklin worked with X-ray photography to take the famous photo, showing the structure of DNA for the first time.
Sadly, she died of radiation poisoning

6. Before We Knew about DNA
Then in 1953, James Watson and Francis Crick took the work of all these scientists before them and built a model of the DNA molecule.
Think “Whats in the Crick?”

7. Discovering DNA So how long have we known about DNA?
If you count from Watson and Crick, only a little over a half century….
But Mendel had a pretty good idea long before that.

8. The Structure of DNA DNA: deoxyribonucleic acid
Genetic material that contains the instructions for the functions of the cell
Found in the nucleus of eukaryotic cells
In prokaryotes, shaped as a ring
DNA is a very long, thin molecule
Shape is double helix – like a twisted ladder

9. Structure of DNA
“railing” or “backbone” of ladder consists of 2 alternating parts :
sugars, deoxyribose,
and phosphate molecules
Steps, or “rungs” of ladder consist of pairs of nitrogen bases:
Adenine & Thymine
Guanine & Cytosine

10. Structure of DNA
Each nitrogen base “step” is connected to the railing at the sugar molecule
Each sugar molecule is connected to its own phosphate molecule
One nitrogen base, together with its sugar and phosphate backbone make one unit called a nucleotide
Nucleotides are made up of three components:
1. deoxyribose (sugar)
2. phosphate
3. nitrogenous base

11. DNA structure Nitrogen bases:
1. Purines: molecules whose shape is 2 ringed
-Adenine and Guanine
2. Pyrimidines: 1 ring
-Thymine and Cytosine
Adenine only bonds with thymine and guanine only bonds with cytosine
Remember Chargaff’s rule?
A=T and C=G

12. DNA structure This bonding is called base pairing
The nitrogen bases are held together with loose hydrogen bonds
These are usually shown as dotted lines

13. DNA Structure There are millions of “steps”
in each of our 46 DNA molecules,
in each of our cells’ nuclei.
If you unraveled one single DNA molecule all the way and stretched it out,
It would be about 5 cm long – that’s about 2 inches!
How does all that DNA fit inside a nucleus?

14. DNA Structure
Histones are proteins that the DNA molecule wraps itself around to make a smaller compact unit to fit in the nucleus.

15. DNA Replication p 334 DNA must replicate before cell can reproduce.
Each new copy must be exactly the same as the original

16. DNA Replication – Big Picture
Two complete strands will result, these will be exactly alike
Each resulting strand has one “old” half and one “new” half

17. DNA Replication p 334
First step: DNA unwinds and enzymes “unzip” DNA into two separate strands by breaking the hydrogen bonds, exposing unpaired nucleotides

18. DNA Replication – p 334
Because nitrogen bases can only match in specific pairs, this allows exact copies to be made from each open half

19. DNA Replication
Second step: free nucleotides join onto open strands to form two new strands of DNA, aided by enzymes called DNA polymerese.
DNA polymerese: an enzyme that helps new nucleotides bond onto open strand of DNA during replication.

20. DNA Replication
When DNA replicates, each strand works in opposite directions, from 5 prime to 3 prime ends.

21. The leading edge replicates in one smooth motion.
The lagging strand adds nucleotides in pieces, working toward open end.

22. DNA Replication
The segments produced on the lagging end are called Okazaki segments
DNA ligase helps connect the Okazaki segments

23. RNA RNA: ribonucleic acid
Nucleic acid which carries out protein synthesis
Differences from DNA: different sugar (ribose), single strand, different base (no thymine, uracil instead)