DNA Replication ​
Tags
Cegep1
Biology
Word count
280 words
Reading time
2 minutes
Process during S phase in which DNA makes a copy of itself
Semiconservative (one original strand is preserved in each copy)
Enzymes ​
- Helicase: unwinds DNA strands by breaking H-bonds
- Primase
- DNA polymerase
- Ligase: repairs phosphodiester bonds
Steps ​
Initiation ​
- Helicases bind to origins of replication and begin unwinding DNA to form a replication bubble with two replication forks, each fork with two strands.
- Single-stranded binding proteins bind to the template DNA to keep them separated.
Elongation ​
Synthesis of the complementary strands
Different for the two strands
Leading strand ​
5' to 3' continuous DNA synthesis in the same direction of DNA unwinding
One RNA primer
- Primase makes a complementary RNA primer on the template strand.
- DNA polymerase binds to RNA primer and synthesizes new strand.
- DNA polymerase replaces RNA primer with DNA.
- Ligase repairs link between replaced primer and rest of strand.
Lagging strand ​
3' to 5' discontinuous DNA synthesis in the opposite direction of DNA unwinding
Many RNA primers
- Repeat many times:
- Primase makes a complementary RNA primer on the template strand.
- DNA polymerase binds to the RNA primer and synthesizes short strand called Okazaki fragment.
- DNA polymerase replaces RNA primers with DNA.
- Ligase repairs link between replaced primers and Okazaki fragments.
Termination ​
In eukaryote DNA replication, removal of primer at beginning of lagging strand leaves a gap that cannot be filled by DNA polymerase.
=> shortening of telomere & deletion of genes => limits lifespan
Not a problem for prokaryotes because they have circular DNA.
Ajitani Hifumi