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 o 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 o Adenine only bonds with thymine and guanine only bonds with cytosine o Remember Chargaff’s rule? o A=T and C=G

12. DNA structure o This bonding is called base pairing o The nitrogen bases are held together with loose hydrogen bonds o 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 o RNA: ribonucleic acid o Nucleic acid which carries out protein synthesis o Differences from DNA: different sugar (ribose), single strand, different base (no thymine, uracil instead)