DNA Replication
Tags
Biology
Cegep/1
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