DNA is made up of nucleotides that form a double helix structure. In the 1940s, Avery discovered that DNA, not proteins, is responsible for heredity. Chargaff determined that DNA bases pair up with equal ratios. Franklin's X-ray images helped Watson and Crick deduce the double helix structure in 1953. The structure consists of a sugar-phosphate backbone with nitrogen bases projecting into the helix. DNA replicates semi-conservatively, with each original strand serving as a template to create a new complementary strand. Genes are segments of DNA that code for polypeptides.

2. Nucleic acids are polymers of nucleotides.

3. DNA was discovered in 1868, but the significance of what it was used for was unknown.

5. Nucleotides:

6. There are four nitrogen bases: Pyrimidineshave one ring. Purineshave two rings

7. Erwin Chargaff (1905-2002) Studied the chemistry of DNA structure. Discovered: % Adenine = % Thymine % Guanine = % Cytosine (1:1 purines:pyrimidines) At the time, the structure of DNA was unknown, so the significance of this discovery was minimal at the time.

8. Maurice Wilkens& Rosalind Franklin In the early 1950’s, Wilkens hired Franklin to work in his lab at King’s College in London. Her work was to do X-ray crystallography on DNA in order to figure out it’s structure. She made many images, the most famous being “Photo 51”

9. Francis Crick & James Watson Were both working at Cambridge University in 1951. Using information from Avery, Chargaff, and Franklin, they eventually built a model of DNA that combined the ideas of all the other scientists working on the structure of DNA. Crick, Watson, and Wilkens shared the Nobel Prize in Medicine in 1962.

10. Their model is still on display in London

11. Here’s what the structure is like: Sugar – Phosphate “backbone” Nitrogen bases pairing in the middle Entire molecule is twisted into a double helix

12. The nitrogen bases follow “Chargaff’s Rules” Adenine bonds to Thymine Cytosine bonds to Guanine Because of this, if we know the sequence of bases on one strand of DNA, we can predict the sequence of bases on the complementary strand. A C G G C T A C T A C T G C C G A T G A T G

13. Now…. about the strands: They are running “anti-parallel”…Each deoxyribose looks like this: The carbons are numbered 1-5 The 1’ carbon bonds to a Nitrogen base The 5’ and 3’ end attach to phosphate groups

14. eukaryotic DNA has “ends” The 5’ end will attach to a phosphate group and then end. The 3’ end will attach to an -OH group (no phosphate) and then end. On one end there will be a 3’ and a 5’ end The other end will be a 5’ and a 3’ end

15. A picture to make it make sense: These arecalled “Anti-parallel” strands

16. DNA REPLICATION Begins at “origins of replication” – numerous places all at the same time. Helicase (an enzyme) untwists the DNA at each origin of replication. (there’s a lot of other steps in here I’m leaving out for the sake of time and complexity…) “free” nitrogen bases form new hydrogen bonds DNA polymerase (an enzyme) will finish new nucleotidesbut only to the 3’ end of both strands. Since nucleotides are only added to the 3’ end, what happens at the 5’ end???

18. End result of replication: How do we know both new strands are identical?

19. Genes A gene is a segment of DNA that codes for the production of a polypeptide. Genes are not always continuous (introns and exons) Genes may overlap You may have multiple copies (multigene families) Some genes are silent, some are turned on or off Some genes can “jump” from one chromosome to another (transposons) Some genes cause apoptosis (self-destruction) of cells that are damaged or mutated