A. Summary of knowledge in 1940's

1. chromosomes carry genetic information
2. chromosomes are made of proteins and DNA
3. most researchers thought proteins carried the genetic information

B. Hershey and Chase (1952)

1. used phages and bacteria
2. labeled the phages' DNA with a radioisotope of phosphorus
3. labeled the proteins of a separate group of phages with a radioisotope of sulfur
4. DNA must be the genetic material in the phages

C. Chargaff (1947)

1. DNA composition is species specific.
2. example - humans
a. A - 30.9%
b. T - 29.4%
c. C - 19.9%
d. G - 19.8%

3. Chargaff's rules
a. A=T
b. C=G
c. unexplained at that time

D. Watson and Crick (1953)

1. described the structure of DNA
a. double helix
b. purines (A,G) pair with pyrimidines (T,C)
2. used model building
3. used critical data from Rosalind Franklin

E. DNA stucture

1. monomers are called nucleotides
a. deoxyribose
b. phosphate group
c. nitrogenous base
2. sugar - phosphate backbone
3. complementary base pairs (A-T, G-C)
4. terminology
a. base pairs (humans ca. 6 billion)
b. nucleotide sequence
5. DNA strands are antiparallel
a. 5' end - phosphate group
b. 3' end - hydroxyl group

F. DNA replication

1. When does it occur?
2. semiconservative model
a. the two strands of DNA molecule separate
b. each strand copied
c. end up with two new DNA molecules, each with one strand from the original molecule
3. very fast, very accurate (ca. 1 error per billion nucleotides)
4. involves many enzymes and other proteins
5. origins of replication - specific nucleotide sequences starting points for replication
6. replication bubble
7. replication forks
8. DNA polymerases - enzymes that catalyze the addition of new nucleotides
9. leading strand - nucleotides added continuously in 5' to 3' direction
10. lagging strand - Okazaki fragments
11. DNA ligase - joins Okazaki fragments together to form a continuous strand of DNA
12. DNA polymerase - can only attach nucletotides to other nucleotides of a strand that has already been started
13. primer - short strand of RNA (about 10 nucleotides long) that allows new strands of DNA to form
14. primase - the enzyme involved in assembling the primer
15. another DNA polymerase later replicates the primer nucleotide with DNA fragments
16. each Okazaki fragment must be primed
17. hellicase - the enzyme that unwinds the double helix a the replication fork
18. single-strand binding protein - help stabilize unwound DNA until the new complementary strand is formed

G. DNA repair and proofreading

1. over 50 DNA repair enzymes have been identified
2. mismatch repair - occurs during replication
3. excision repair

H. Protein Synthesis

1. one gene - one enzyme hypothesis
2. one gene - one protein hypothesis
3. one gene - one polypeptide hypothesis
4. overview
a. transcription
- DNA --> pre-mRNA
- occurs in nucleus
b. RNA processing
- pre-mRNA --> mRNA
- occurs in nucleus
c. translation
- mRNA --> polypeptide
- tRNA
- ribosomes
- occurs in cytosol
5. RNA
a. monomers are nucleotides
- ribose
- phosphate group
- nitrogenous base
b. complementary pairs (A-U, G-C)
c. can pair with a single strand of a DNA molecule
d. can pair with another RNA molecule
6. the genetic code
a. 4 nucleotides
b. 20 amino acids
c. triplet code
d. 4³ = 64
e. 61 of the 64 different nucleotide triplets code for specific amino acids
f. the other 3 triplets are stop codes
g. codon - a triplet of mRNA
h. anticodon - a triplet of tRNA
i. reading frame - the code does not overlap
j. code is redundant but not ambiguous
k. code is nearly universal among all species that have been studied