DNA replication

Question 1

Initiation of replication: (E.coli)

Answer 1

6 Proteins involved: DnaA, DnaB, DnaC, HU, Gyrase, SSB
DnaA is an initiator protein. When bound to ATP it is activated. This DnaA-ATP compled binds to the fully methilated oriC (first high affinitiy, then low) It twitst and melts the helix with the help of HU. Two DnaB/DnaC complex then bind. DnaB is an ATP hydrolysis-dependent 5’-3’ helicase. DnaC is a chaperon. Together they from two replication forks. Gyrase help relaxes the DNA supercoils. SSB stabilized DNA and keeps the replication bubble open

Question 2

Joining of adjacent Okazaki fragments in E.coli

Answer 2

DNA polymerase III has only 3’-5’ exonulease activity. It stops synthesizing DNA when it finds a primer. DNA polymerase I has both 3’-5’ and 5’-3’ exonulcease activities. Degrades the primer with the 5’-3’exonuclease activity and synthesizes new complementary DNA simultaneously. DNA ligase ligates the adjacent fragments

Question 3

Joining of Okazaki adjecent fragments in eukaryotes

Answer 3

DNA polymerase o and helicase displace the primer, creating a 5’ flap. Simultaneously the DNA polymerase fills the gap. Flap endonuclease I (FEN1) cleaves the flap removing the primer. DNA ligase ligates the adjecent fragments

Question 4

In E.coli the progession of the replication fork is maintained by these enzymes:

Answer 4

Helicase, Gyrase, SSB, Primase, DNA polymerase I and III, Ligase

Question 5

Telomere functions:

Answer 5

1. Cell disinguis telomeres from other free ends caused bu damage
2. Telomeres can be extended to avoid short of chromosomes
3. Telemeres form a T-toop that stabilized the end of the chromosome by invasion of G-rich 3’-end in an upstream region of the telomere.