1. DNA Replication
Lecture #23
Ms. Day Honors Biology

2. DNA Replication DNA Replication = DNA  DNA
Parent DNA makes 2 exact copies of DNA
Why??
Occurs in Cell Cycle (S phase)

3. DNA Replication: a closer look

4. DNA Replication Steps:
Initiation
involves assembly of replication fork (bubble) at origin of replication
sequence of DNA found at a specific site
Elongation
Parental strands unwind and daughter strands are synthesized.
the addition of bases by proteins
Termination:
the duplicated chromosomes separate from each other. Now, there are 2 IDENTICAL copies of DNA.

5. DNA Replication (Part 3A-initiation)

6. THE REPLICATION FORK and BUBBLE IS NOW CREATED!
Segments of single-stranded DNA are called template strands.
Copied strand is called the complement strand (think “c” for copy)
BEGINNING OF DNA REPLICATION
(INITIATION)
DNA helicase (think “helix”)
binds to the DNA at the replication fork
untwist (“unzips”) DNA using energy from ATP
Breaks hydrogen bonds between base pairs
THE REPLICATION FORK and BUBBLE IS NOW CREATED!

7. DNA is unzipped by helicase using ATP
base pairs 5’ 3’
DNA is unzipped by helicase using ATP
Helicase
ATP

8. DNA Replication (Elongation)
DNA polymerase
A DNA chain making enzyme
adds nucleotides to make daughter (new) DNA strands
Adds 1 nucleotide AT A TIME matches up to opposite nucleotide on template (parent) strand
base pairs 5’ 3’
Helicase
ATP
DNA Polymerase

9. DNA Replication: Elongation

10. DNA Replication (Termination)
1.) Make telomeres
Telomeres
Short repeats of “G” base found at END of LINEAR chromosomes in eukaryotes
protect ends of linear chromosomes
Telomerase is the enzyme that makes telomeres

11. Telomeres, Aging & Cancer
Telomeres get shorter as cell divides leads to aging???
Most cancers come from body cells
Cancer cells divide indefinitely.
Normal cells limited to ~50-75 divisions  stop making telomerase.
85–90% cancer cells continue to LOTS of telomerase & are able to prevent further shortening of their telomeres.
Leads to “immortality”

12. Mistakes Made during DNA Replication
Mutation
Change in DNA (genetic material)
2 Main Types of Mutations:
Frameshift(s)
extra or missing base(s).
Substitutions
when the wrong nucleotide is incorporated (mismatch mutation).

13. DNA Replication (Termination)
2.) Fix Mutations
Mismatch repair (wrong nucleotide used)
1. DNA polymerase
“Proofreads” new DNA as it makes it
If mistake is missed my DNA polymerase, excision (“cut out”) repair needs to happen
2. Nuclease does this

14. DNA Repair