EUKARYOTIC DNA REPLICATIO N Dr.MAHA

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Dr. MAHA SMAISM

EUKARYOTIC DNA REPLICATION


Eukaryotic DNA replication is not as well understood as bacterial replication

The two processes do have extensive similarities,

Nevertheless, DNA replication in eukaryotes is more complex
Large linear chromosomes
Tight packaging within nucleosomes
More complicated cell regulation

• Eukaryotes have long linear chromosomes

• They therefore require multiple origins of replication
• To ensure that the DNA can be replicated in a reasonable time
• DNA replication proceeds bidirectional in many origins of replication
Multiple Origins of Replication

Bidrectional DNA synthesis

Replication forks will merge

• Eukaryotic enzymes:
• Five common DNA polymerases from mammals.
• DNA Polymerase  (alpha): is responsible for synthesis of RNA primer
for both leading and lagging strands of DNA
2. DNA Polymerase  (beta): nuclear, DNA repair.

3. DNA Polymerase  (gamma): mitochondria, DNA repl., proofreading


4 . DNA Polymerase  (delta): nuclear, DNA replication on the leading strand, and proofreading function.
5. DNA Polymerase  (epsilon): nuclear , DNA synthesis on the lagging strand and proofreading function
• Different polymerases for the nucleus and mt DNA
• Some polymerases proofread; others do not.
• Some polymerases used for replication; others for repair.

Initiation of replication at a eukaryotic replication origin

1. Origin recognition complex (ORC) bind to an
origin of replication(ori) .
2. minichromosome maintenance proteins (MCM) must bind with ORC, Cdc6p is required for binding MCM &ORC .
3. The initiate complex is activated, and helicase activity opens the parental strands to form a very small bubble .
4. Replication protein A (RPA) bind to the exposed single strand

5. DNA pol a is the only polymerase to associate with primase

The DNA pol a/primase complex synthesizes a short RNA-DNA hybrid 10 RNA nucleotides followed by 20 to 30 DNA nucleotides

6. Then DNA pol a/primase complex leaves
Then DNA pol d or e will processes elongation of the leading and lagging strands

7. the primer is removed by Rnase H enzyme leaving a single ribonucleotide attached to DNA which removed by FEN1 (flap endonuclease 1)

• DNA polymerases also play a role in DNA repair

• DNA pol b is not involved in DNA replication
• It plays a role in base-excision repair
• Removal of incorrect bases from damaged DNA
• Recently, more DNA polymerases have been identified
• Lesion-replicating polymerases
• Involved in the replication of damaged DNA
• They can synthesize a complementary strand over the abnormal region