It is crucial to thoroughly understand the basic concepts of DNA Replication within this section before advancing to the other Genetics units. Please take extra time in visualizing and understanding the replication fork.
Mechanism of DNA Replication
1. Separation of Double Stranded DNA
- DNA Gyrase uncoils the DNA strand that is about to be replicated ahead of the replication fork.
- Helicase unwinds the DNA
- SSB (single-strand binding proteins) stabilize the DNA and keep it unwound.
2. Synthesis of Complementary DNA
- Primase adds short complementary RNA strands to the DNA.
- DNA Polymerase I is now able to recognize the RNA primers and begin synthesizing the new strand in the 5′ to 3′ direction ONLY.
- DNA Polymerase III works much faster and takes over as the main synthesizing polymerase.
- Due to the 5′ to 3′ nature of DNA polymerase and the anti-paralell nature of DNA, there is a leading strand and lagging strand (see above figure for clarification).
- The lagging strand is synthesizing away from the replication fork and therefore needs to be synthesized in short fragments called Okazaki Fragments.
3. Final Touches
- RNA primers are replaced by a new DNA Polymerase.
- The Okazaki Fragments are then stitched together by DNA Ligase.
- DNA Polyermase III utilizes its proofreading function to correct errors it made during the process.
- Note: DNA replication is bidirectional. These processes are occuring in both directions.
Semi-Conservative Nature of Replication
DNA Replication is semi-conservative. This means that after replication, the new DNA has:
- One original parent strand.
- One newly synthesised daughter strand.
The figure below depicts this phenomenon:
Origin of Replication – Prokaryotes
- Prokaryotes have one chromosome thus only one origin.
- Prokaryotic chromosomes are circular and therefore replication is classified as theta replication.
The figure below represents this concept of theta replication:
Origin of Replication – Eukaryotes
- Several origins due to the extremely large chromosomes.
- Creation of replication bubbles
- When these bubbles meet, the daughter strands are then ligated together via DNA Ligase.
The figure below represents this concept:
Due to the 5′ to 3′ nature of DNA polymerase and its need for primers, the end of the chromosome will have no space on the lagging strand to place more primers. This causes DNA replication to halt at the end of chromosomes. The solution to this problem is telomeres.
- Telomeres are short repeats at the end of the chromosome that are disposable. Therefore after every replication cycle, the DNA shortens slightly.
- Once telomeres are depleted, this is sensed and the cell either halts division or undergoes apoptosis (cell death).
- In humans, the number of times a cell can divide until telemeres are depleted is called the Hayflick limit.
- Telomerase is an enzyme that creates telomeres to extend the DNA. This is usually only expressed in germ line, embryonic stem cells, and some white blood cells.
It is important to note that telomeres are actively being studied due to their connection to age-related diseases and ageing. Cancer cells are also able to express telomerase which contributes to their immortal nature.