1. DNA Replication

2. How DNA Replication Occurs
DNA replication is the process by which DNA is copied in a cell before a cell divides by mitosis, meiosis, or binary fission
One copy moves to each new cell
Since the two strands are
complementary, each strand
serves as a template to make a
new strand

3. Steps of DNA Replication
Helicases separate the DNA Strands
These move along the DNA molecule breaking the hydrogen bonds between the complementary bases
This allows the two strands to separate
The Y shaped region where the strands are separating is called the replication fork (not spoon)

4. Steps of DNA Replication
DNA polymerases add complementary nucleotides to each of the original strands
Nucleotides float freely in the nucleus
As nucleotides are added, covalent bonds form between the sugar of one nucleotide and the phosphate group of the next
Hydrogen bonds form between the bases

5. Steps of DNA Replication
DNA polymerases are finished and fall off
- Result is two molecules of DNA identical to the original
- Molecules are ready to move into new cells during division
- Called semi-conservative replication because each of the new DNA molecules has kept (or conserved) one of the two (or semi) original DNA strands.

6. Steps of DNA Replication
Click picture for video

7. Action at the Replication Fork
DNA synthesis occurs in different directions on each strand
As the replication fork moves along the original DNA, synthesis follows the movement of the replication fork
Synthesis on the other strand moves in the opposite direction, away from the replication fork
This leaves gaps in the newly synthesized strand, which are later joined by an enzyme DNA ligase

8. Prokaryotic and Eukaryotic Replication
Circular chromosome
Replication begins at one place along the chromosomes
Two replication forks are formed and proceed in opposite directions
Replication continues along each fork until they meet an the whole molecule is copied

9. Prokaryotic and Eukaryotic Replication
Each chromosome is long, but not circular
At the rate a polymerase adds nucleotides (50/sec), it would take 53 days to replicate the largest human chromosome
Instead, replication begins at many points or origins along the DNA, and two forks move in opposite directions
In a fruit fly, replication begins simultaneously at about 3500 sites in a DNA molecule

10. Errors in DNA Replication
DNA replication usually occurs with great accuracy (only one error for every billion nucleotides added)
How is this done???
Polymerases have repair or “proofreading” functions in the same way the computer checks for spelling errors
For example, if A pairs with C, the polymerase removes the C and replaces it with T
Click picture for video

11. Errors in DNA Replication
When mistakes do occur, the base sequence of the newly formed DNA differs from the original
A change in the nucleotide sequence of a DNA molecule is called a mutation
These can have serious effects on the function of a gene – therefore disrupting cell function

12. DNA Replication and Cancer
Mutations can arise and lead to altered cells and organisms
The changes allow individuals to survive and reproduce better
Sometimes non-repaired mutations can cause cancer
Tumors are caused by mutations that affect cell division

13. How is the exact replication of DNA ensured?
By complementary base pairing and proofreading by DNA polymerases

14. What are replication forks?
Areas of DNA where the double helix separates prior to replication