Semi-Conservative DNA Replication
Biology: IB Diploma Higher Level, topic 1.2.3
DNA Replication
DNA replication copies DNA exactly. Each new molecule gets one old strand and one new strand.
Semi-conservative replication
- Each DNA strand acts as a template for a new, complementary strand.
- Each new DNA molecule has one original strand and one new strand.
- Replication is semi-conservative because one strand is always original.
Starting replication
- Replication begins like this:
- An origin of replication is recognised by proteins.
- Helicase breaks hydrogen bonds and unwinds the DNA.
- Two replication forks form and extend in opposite directions.
DNA polymerase and primers
- DNA polymerase can only extend DNA in the 5' to 3' direction.
- DNA primase first makes a short RNA primer with a free 3'-OH group.
- DNA polymerase extends the primer, adding complementary nucleotides.
Leading and lagging strands
- The leading strand forms continuously towards the fork.
- The lagging strand forms as short Okazaki fragments.
- Each fragment starts with its own RNA primer.
- DNA ligase joins the fragments together.
Test yourself on Semi-Conservative DNA Replication
What is true of new DNA strands formed during replication?
Which of the following statements about semi-conservative replication are true?
Which enzyme separates the two strands of DNA?
Why does the lagging strand form as a series of Okazaki fragments instead of one continuous strand?