Journey from Mendel to DNA: Unraveling the Genetic Code

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)