Simplest Model To Replicate a DNA
Notes from the coursera course.
- 2 complimentary DNA strands running in opposite directions around a circular chromosome unravel, starting at ori. They create two replication forks.
- These replication forks expand in both directions around the chromosome until the strands completely separate.
- The points at which the strands completely separate is called the replication terminus, and is roughly situated opposite the ori.
- DNA polymerase does not wait for the parent strands to completely separate before starting replication. It starts copying while the strands are unravelling.
- To start replication, a DNA polymerase needs a primer. A primer is a very short complementary segment that binds to the parent strand and jump starts the DNA polymerase.
- As the replication proceeds, it will finally reach terminal (ter) from both sides or 5 DNA polymerase, at which point the two strands will completely separate. Then the cell is ready to divide.
- Replication Origin = ori, Replication Terminus = ter. How unimaginative!
- DNA polymerase can only move in the reverse direction (3' -> 5'). Therefore, the replication on the forward half-strands happens backwards towards ori in small delayed intervals. Thus the overall replication is asymmetric.
- These small replication fragments formed on the forward half strand are called Okazaki fragments.
- Once the replication is nearly complete (and has reached ter), gaps remain between disconnected Okazaki fragments.
- Okazaki fragments are sewn together by an enzyme called DNA ligase, resulting in two intact daughter chromosomes. Each consists of a parent strand and a newly synthesised daughter strand. This sewing process starts as the replication is happening.