1. DNA Replication
2.7 & 7.1

2. Essential Idea: Genetic information in DNA can be accurately copied and can be translated to make the proteins needed by the cell.
2.7
DNA replication, transcription and translation
Understandings:
The replication of DNA is semi-conservative and depends on complementary base pairing
Helicase unwinds the double helix and separates the two strands by breaking hydrogen bonds
DNA polymerase links nucleotides together to form a new strand, using the pre-existing strand as a template
Applicationss:
Use Taq DNA polymerase to produce multiple copies of DNA rapidly by the polymerase chain reaction (PCR)
Skills:
Analyze Meselson &Stahl’s results to obtain support for the theory of semi-conservative replication of DNA

3. Essential Idea: The structure of DNA is ideally suited to its function.
7.1
DNA Structure and Replication
Understandings:
DNA structure suggested a mechanism for DNA replication
DNA polymerases can only add nucleotides to the 3’ end of a primer
DNA replication is continuous on the leading strand and discontinuous on the lagging strand
DNA replication is carried out by a complex system of enzymes
Some regions of DNA do not code for proteins but have other important functions
Applications:
Use of nucleotides containing dideoxyribonucleic acid to stop DNA replication in preparation of samples for base sequencing
Tandem repeats are used in DNA profiling

4. I. Nature of Science
Meselson & Stahl - Made parent strand with “heavy” N, added free floating DNA nucleotides with “light” N
1st generation – new strands were 50% light, 50% heavy
2nd generation – half of the strands were 50/50, other half were all light

5. B. Watson & Crick’s structure indicated a mechanism for replication because of the complimentary base pairing

6. II. Semi-Conservative Replication

7. II. Semi-Conservative Replication
New strands are composed of 1 strand of parental DNA and 1 strand of newly formed DNA
B. Free-floating nucleotides
Can be DNA or RNA
Triphosphates – reactions to remove extra two phosphates are exergonic – provide the energy to build the new strand

8. III. Replication Enzymes
Function

9. III. Replication Enzymes
Function
Helicase
Unzips & unwinds DNA
Gyrase
Relieves strain of unwound DNA
SSBs
Help hold DNA open and stabilize it
Primase
Builds RNA Primer
DNA Polymerase III
Builds new DNA strand
DNA Polymerase I
Replaces RNA primer with DNA
DNA Ligase
Joins Okazaki fragments together

10. IV. The Process
A. The Steps

11. IV. The Process A. The Steps
Helicase unzips DNA (breaks hydrogen bonds) creating a replication bubble at the origin of replication
Multiple origins per chromosome in eukaryotes
Each side of bubble has replication fork
Bubble enlarges as replication proceeds until bubbles meet

13. 2. Primase builds a short primer of RNA nucleotides (5 – 10 bases)

15. DNA Polymerase III builds the complimentary strand of DNA in the 5’  3’ direction
a. Free-floating DNA nucleotides move in to match up with parent strand, DNA polymerase III moves along and binds them together

17. 4. DNA Polymerase I replaces RNA primer with DNA nucleotides

19. B. Leading and Lagging Strands
New nucleotides must be added on to the 3’ end
Leading strand – Bases easily added as DNA is unzipped

21. Lagging Strand – has a delay
a. Section unzips, then strand is built back towards origin
b. Results in chunks called Okazaki fragments
c. DNA ligase bonds Okazaki fragments together after primer is replaced

22. V. Speed and Accuracy

23. V. Speed and Accuracy
~4000 nucleotides per second
Few errors, but also have proof-reading mechanisms – enzymes (exonucleases) cut out mistake and replace with correct base (also activated by mutations such as by uv radiation, x-rays, etc.)

24. VI. Non-Coding DNA

25. VI. Non-Coding DNA
Coding sequences code for proteins = GENES (only about 1.5% of DNA in humans)
Non-coding regions
1. Produce tRNA & rRNA
2. Regulate gene expression – can act as enhancers or silencers

27. Repetitive Sequences (moderately or highly)
% of genome (over 50% in eukaryotes)
2. Telomeres (ends of chromosomes) – protect coding regions - during replication, enzymes can’t get all the way to the end of the chromosome  lose a bit of DNA each time