In this tutorial, you will learn about the process of DNA replication, including the enzymes involved and the differences between prokaryotic and eukaryotic replication. You will explore the semi-conservative model of DNA replication, the role of plasmid DNA and extrachromosomal replicons, as well as the key differences between semi-conservative and conservative models of DNA replication.
Contents:
- What is DNA Replication?
- Enzymes Involved in DNA Replication
- Semi-conservative Model of DNA Replication
- Prokaryotic DNA Replication
- Eukaryotic DNA Replication
- Differences Between Eukaryotic and Prokaryotic DNA
- Plasmid DNA
- Extrachromosomal Replicons
- Differences Between Semi-Conservative and Conservative Models of DNA Replication
What is DNA Replication?
DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules. Each new DNA molecule consists of one original (parental) strand and one newly synthesized strand, a mechanism known as semiconservative replication.
Enzymes Involved in DNA Replication
- Helicase: Unwinds the DNA double helix.
- Single-Strand Binding Proteins (SSBs): Prevent the unwound DNA strands from re-annealing.
- Topoisomerase: Relieves tension in the DNA molecule ahead of the replication fork.
- Primase: Synthesizes RNA primers to initiate DNA synthesis.
- DNA Polymerase: Responsible for adding nucleotides to the growing DNA strand (e.g., DNA Pol III in prokaryotes, DNA Pol α and δ in eukaryotes).
- DNA Ligase: Seals nicks between Okazaki fragments and the newly synthesized DNA.
Semi-conservative Model of DNA Replication
Semi-conservative model of DNA replication is a process where unwinding of DNA strands occurs, and the new strands acts as a template for synthesizing complementary strands.
- Each’s DNA strands integrity is conserved here for imparting template nature of them and act as template.
- But two parents’ strands are separated by breakage of H-bonds between them and individually they take part in formation of complementary strands.
- Two daughter molecules of DNA are formed, and it marks the end of replication. The daughters are identical to parent DNA molecule.
- Out of two strand, one strand is a newly formed DNA strand and other one is an intact template parental strand.
- This semi-conservative model of DNA replication is widely accepted as the best model of DNA replication. This model of replication includes two replication cycles over a period.
Prokaryotic DNA Replication
Prokaryotic DNA replication is the process by which prokaryotes duplicate their DNA to pass genetic material to the next generation.
Mechanism:
- Initiation: At the origin of replication, the DNA helix unwinds, aided by the helicase enzyme, which opens the replication fork and separates the strands.
- Stabilization: Single-strand binding (SSB) proteins coat the unwound DNA to prevent re-annealing.
- Topoisomerase Function: Topoisomerase enzymes relieve the strain of over-winding ahead of the replication fork.
- Primer Synthesis: RNA primers are synthesized by the primase enzyme, providing a starting point for DNA synthesis.
- Nucleotide Addition: DNA polymerase III extends the RNA primers by adding nucleotides in the 5’ to 3’ direction.
- Strand Elongation: Both leading and lagging strands are synthesized. The lagging strand is formed in short segments called Okazaki fragments.
- Completion: RNA primers are removed, and gaps are filled in by DNA polymerase III. Finally, DNA ligase seals the nicks between DNA fragments.
Eukaryotic DNA Replication
Eukaryotic DNA replication differs significantly from prokaryotic replication, primarily due to the complexity of eukaryotic cells.
Mechanism:
- Multiple Origins: Eukaryotic replication begins at multiple origins of replication (ORI), creating replication bubbles with forks moving in opposite directions.
- Helicase Action: The helicase enzyme unwinds the double helix, forming replication forks stabilized by SSB proteins and topoisomerases.
- Primer Synthesis: Alpha DNA polymerase synthesizes RNA primers to initiate lagging strand synthesis.
- Okazaki Fragment Formation: Discontinuous synthesis occurs on the lagging strand, creating Okazaki fragments.
- Leading Strand Synthesis: The leading strand is synthesized continuously by delta DNA polymerase in the 5’ to 3’ direction.
- Completion: DNA ligase seals the breaks between Okazaki fragments and around primers, ensuring a continuous DNA strand.
Differences Between Eukaryotic and Prokaryotic DNA
The following table highlights the key differences between eukaryotic and prokaryotic DNA:
| Properties | Eukaryotic DNA | Prokaryotic DNA |
|---|---|---|
| Shape | Linear in shape | Circular in shape |
| Replication Type | Slow | Fast |
| Genome Content | Multiple genome | Singular genome |
| Number of Genes | Very high in number | Less in number |
| Replication Occurrence | In nucleus | In cytoplasm |
| Histone Present/Absent | Histone present | Histone absent |
Plasmid DNA
Plasmids are small, circular, extrachromosomal DNA elements found primarily in bacteria. They play crucial roles in self-replication and various genetic processes.
- Plasmids can replicate independently of chromosomal DNA, making them useful in molecular biology.
- Plasmid DNA is instrumental in recombinant DNA technology, gene transfer, and gene therapy.
- They serve as vectors for studying gene expression and producing genetically modified organisms (GMOs).
- Plasmids are used for the production of proteins and therapeutic drugs.
Extrachromosomal Replicons
Extrachromosomal replicons are DNA molecules present in addition to the main chromosomes, primarily found in bacteria.
Types:
- Plasmids: Circular DNA that can replicate independently within a bacterial cell.
- Bacteriophages: Viruses that infect bacteria and can exist as independent genetic units, classified into lysogenic and episomal forms based on their lifecycle and integration into the host genome.
Differences Between Semi-Conservative and Conservative Models of DNA Replication
The following table summarizes the differences between the semi-conservative and conservative models of DNA replication:
| Properties | Semiconservative Model | Conservative Model |
|---|---|---|
| Helices | Two helices (Each contain contains one new and one old strand) |
Two helices (One contains old strands and other contain new strands) |
| DNA Unwinding | DNA unwinding occurs | DNA unwinding does not occur |
| Template Strand | Helps in synthesis of new strands | Function not clear |
| Significance | Biologically significant | Biologically insignificant |
| Validity | Valid and accepted till now | Not valid |
Key Points to Remember
Here is the list of key points we need to remember about “DNA Replication”.
- DNA replication produces two identical molecules, each containing one parental and one newly synthesized strand, following the semiconservative model.
- Helicase unwinds DNA, primase lays RNA primers, DNA polymerase adds nucleotides, and DNA ligase seals gaps between Okazaki fragments.
- Prokaryotic DNA replication is faster and occurs in the cytoplasm, while eukaryotic replication is slower, more complex, and happens in the nucleus.
- Plasmids are circular, extrachromosomal DNA elements, crucial for genetic research, gene therapy, and protein production.
- The semiconservative model is biologically significant and widely accepted, unlike the conservative model.
